Understanding Fish Nutrition,Feeds, and Feeding

Understanding Fish Nutrition,Feeds, and Feeding

Steven Craig and Louis A. Helfrich*

Publication420-256

2002

VIRGINIA POLYTECHNIC INSTITUTEAND STATE UNIVERSITY VIRGINIA STATE UNIVERSITY

Virginia Cooperative Extension programs and employment are open to all, regardless of race, color, religion, sex, age, veteran status,

national origin, disability, or political affiliation. An equal opportunity/affirmative action employer. Issued in furtherance of Cooperative

Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department

of Agriculture cooperating. J. David Barrett, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg;

Lorenza W. Lyons, Administrator, 1890 Extension Program, Virginia State, Petersburg.

VT/028/0402//420256

IntroductionIntroductionGood nutrition in animal production systems is essential toeconomically produce a healthy, high quality product. Infish farming, nutrition is critical because feed represents40-50% of the production costs. Fish nutrition hasadvanced dramatically in recent years with the develop-ment of new, balanced commercial diets that promote opti-mal fish growth and health. The development of newspecies-specific diet formulations supports the aquaculture(fish farming) industry as it expands to satisfy increasingdemand for affordable, safe, and high-quality fish andseafood products.

PPrreparepared (ared (artificial)tificial)DietsDietsPrepared or artificial diets may be either complete or sup-plemental. Complete diets supply all the ingredients (pro-tein, carbohydrates, fats, vitamins, and minerals) necessaryfor the optimal growth and health of the fish. Most fishfarmers use complete diets, those containing all therequired protein (18-50%), lipid (10-25%), carbohydrate(15-20%), ash (< 8.5%), phosphorus (< 1.5%), water (<10%), and trace amounts of vitamins, and minerals. Whenfish are reared in high density indoor systems or confinedin cages and cannot forage freely on natural feeds, theymust be provided a complete diet.

In contrast, supplemental (incomplete, partial) diets areintended only to help support the natural food (insects,algae, small fish) normally available to fish in ponds oroutdoor raceways. Supplemental diets do not contain a fullcomplement of vitamins or minerals, but are used to helpfortify the naturally available diet with extra protein, carbo-hydrate and/or lipid.

Fish, especially when reared in high densities, require ahigh-quality, nutritionally complete, balanced diet to growrapidly and remain healthy.

ProteinProteinBecause protein is the most expensive part of fish feed, it isimportant to accurately determine the protein requirementsfor each species and size of cultured fish. Proteins areformed by linkages of individual amino acids. Althoughover 200 amino acids occur in nature, only about 20 aminoacids are common. Of these, 10 are essential (indispensa-ble) amino acids that cannot be synthesized by fish. The10 essential amino acids that must be supplied by the dietare: methionine, arginine, threonine, tryptophan, histidine,isoleucine, lysine, leucine, valine and phenylalanine. Ofthese, lysine and methionine are often the first limitingamino acids. Fish feeds prepared with plant (soybeanmeal) protein typically are low in methionine; therefore,extra methionine must be added to soybean-meal baseddiets in order to promote optimal growth and health. It isimportant to know and match the protein requirements andthe amino acid requirements of each fish species reared.

Protein levels in aquaculture feeds generally average 18-20% for marine shrimp, 28-32% for catfish, 32-38% fortilapia, 38-42% for hybrid striped bass. Protein require-ments usually are lower for herbivorous fish (plant eating)and omnivorous fish (plant-animal eaters) than they are forcarnivorous (flesh-eating) fish, and are higher for fishreared in high density (recirculating aquaculture) than lowdensity (pond aquaculture) systems.

Protein requirements generally are higher for smaller fish.As fish grow larger, their protein requirements usuallydecrease. Protein requirements also vary with rearing envi-ronment, water temperature and water quality, as well asthe genetic composition and feeding rates of the fish.Protein is used for fish growth if adequate levels of fats andcarbohydrates are present in the diet. If not, protein maybe used for energy and life support rather than growth.

Proteins are composed of carbon (50%), nitrogen (16%),oxygen (21.5%), and hydrogen (6.5%). Fish are capable ofusing a high protein diet, but as much as 65% of the protein

*Extension Specialists, Virginia-Maryland College of Veterinary Medicine, and Department of Fisheries and Wildlife Sciences, Virginia Tech, respectively

may be lost to the environment. Most nitrogen is excretedas ammonia (NH3) by the gills of fish, and only 10% is lostas solid wastes. Accelerated eutrophication (nutrientenrichment) of surface waters due to excess nitrogen fromfish farm effluents is a major water quality concern of fishfarmers. Effective feeding and waste management prac-tices are essential to protect downstream water quality.

Lipids (fats)Lipids (fats)Lipids (fats) are high-energy nutrients that can be utilizedto partially spare (substitute for) protein in aquaculturefeeds. Lipids supply about twice the energy as proteins andcarbohydrates. Lipids typically comprise about 15% of fishdiets, supply essential fatty acids (EFA) and serve as trans-porters for fat-soluble vitamins.

A recent trend in fish feeds is to use higher levels of lipidsin the diet. Although increasing dietary lipids can helpreduce the high costs of diets by partially sparing protein inthe feed, problems such as excessive fat deposition in theliver can decrease the health and market quality of fish.

Simple lipids include fatty acids and triacylglycerols. Fishtypically require fatty acids of the omega 3 and 6 (n-3 andn-6) families. Fatty acids can be: a) saturated fatty acids(SFA, no double bonds), b) polyunsaturated fatty acids(PUFA, >2 double bonds), or c) highly unsaturated fattyacids (HUFA; > 4 double bonds). Marine fish oils are nat-urally high (>30%) in omega 3 HUFA, and are excellentsources of lipids for the manufacture of fish diets. Lipidsfrom these marine oils also can have beneficial effects onhuman cardiovascular health.

Marine fish typically require n-3 HUFA for optimal growthand health, usually in quantities ranging from 0.5-2.0% ofdry diet. The two major EFA of this group are eicosapen-taenoic acid (EPA: 20:5n-3) and docosahexaenoic acid(DHA:22:6n-3). Freshwater fish do not require the longchain HUFA, but often require an 18 carbon n-3 fatty acid,linolenic acid (18:3-n-3), in quantities ranging from 0.5 to1.5% of dry diet. This fatty acid cannot be produced byfreshwater fish and must be supplied in the diet. Manyfreshwater fish can take this fatty acid, and through enzymesystems elongate (add carbon atoms) to the hydrocarbonchain, and then further desaturate (add double bonds) tothis longer hydrocarbon chain. Through these enzyme sys-tems, freshwater fish can manufacture the longer chain n-3HUFA, EPA and DHA, which are necessary for other meta-bolic functions and as cellular membrane components.Marine fish typically do not possess these elongation anddesaturation enzyme systems, and require long chain n-3HUFA in their diets. Other fish species, such as tilapia,require fatty acids of the n-6 family, while still others, suchas carp or eels, require a combination of n-3 and n-6 fattyacids

CarbohydratesCarbohydratesCarbohydrates (starches and sugars) are the most economi-cal and inexpensive sources of energy for fish diets.Although not essential, carbohydrates are included in aqua-culture diets to reduce feed costs and for their binding

activity during feed manufacturing. Dietary starches areuseful in the extrusion manufacture of floating feeds.Cooking starch during the extrusion process makes it morebiologically available to fish.

In fish, carbohydrates are stored as glycogen that can bemobilized to satisfy energy demands. They are a majorenergy source for mammals, but are not used efficiently byfish. For example, mammals can extract about 4 kcal ofenergy from 1 gram of carbohydrate, whereas fish can onlyextract about 1.6 kcal from the same amount of carbohy-drate. Up to about 20% of dietary carbohydrates can beused by fish.

VVitaminsitaminsVitamins are organic compounds necessary in the diet fornormal fish growth and health. They often are not synthe-sized by fish, and must be supplied in the diet.

The two groups of vitamins are water-soluble and fat-solu-ble. Water-soluble vitamins include: the B vitamins,choline, inositol, folic acid, pantothenic acid , biotin andascorbic acid (vitamin C). Of these, vitamin C probably isthe most important because it is a powerful antioxidant andhelps the immune system in fish.

The fat-soluble vitamins include A vitamins, retinols(responsible for vision); the D vitamins, cholecaciferols(bone integrity); E vitamins, the tocopherols (antioxidants);and K vitamins such as menadione (blood clotting, skinintegrity). Of these, vitamin E receives the most attentionfor its important role as an antioxidant. Deficiency of eachvitamin has certain specific symptoms, but reduced growthis the most common symptom of any vitamin deficiency.Scoliosis (bent backbone symptom) and dark colorationmay result from deficiencies of ascorbic acid and folic acidvitamins, respectively.

MineralsMineralsMinerals are inorganic elements necessary in the diet fornormal body functions. They can be divided into twogroups (macro-minerals and micro-minerals) based on thequantity required in the diet and the amount present in fish.Common macro-minerals are sodium, chloride, potassiumand phosphorous. These minerals regulate osmotic balanceand aid in bone formation and integrity.

Micro-minerals (trace minerals) are required in smallamounts as components in enzyme and hormone systems.Common trace minerals are copper, chromium, iodine, zincand selenium. Fish can absorb many minerals directlyfrom the water through their gills and skin, allowing themto compensate to some extent for mineral deficiencies intheir diet.

Energy and ProteinEnergy and ProteinDietary nutrients are essential for the construction of livingtissues. They also are a source of stored energy for fishdigestion, absorption, growth, reproduction and the otherlife processes. The nutritional value of a dietary ingredient

is in part dependant on its ability to supply energy.Physiological fuel values are used to calculate and balanceavailable energy values in prepared diets. They typicallyaverage 4, 4, and 9 kcal/g for protein, carbohydrate andlipid, respectively.

To create an optimum diet, the ratio of protein to energymust be determined separately for each fish species.Excess energy relative to protein content in the diet mayresult in high lipid deposition. Because fish feed to meettheir energy requirements, diets with excessive energy lev-els may result in decreased feed intake and reduced weightgain. Similarly, a diet with inadequate energy content canresult in reduced weight gain because the fish cannot eatenough feed to satisfy their energy requirements forgrowth. Properly formulated prepared feeds have a well-balanced energy to protein ratio.

Feed TFeed Types ypes Commercial fish diets are manufactured as either extruded(floating or buoyant) or pressure-pelleted (sinking) feeds.Both floating or sinking feed can produce satisfactorygrowth, but some fish species prefer floating, others sink-ing. Shrimp, for example, will not accept a floating feed,but most fish species can be trained to accept a floatingpellet.

Extruded feeds are more expensive due to the higher manu-facturing costs. Usually, it is advantageous to feed a float-ing (extruded) feed, because the farmer can directlyobserve the feeding intensity of his fish and adjust feedingrates accordingly. Determining whether feeding rates aretoo low or too high is important in maximizing fish growthand feed use efficiency.

Feed is available in a variety of sizes ranging from finecrumbles for small fish to large (1/2 inch or larger) pellets.The pellet size should be approximately 20-30% of thesize of the fish species mouth gape. Feeding too small apellet results in inefficient feeding because more energy isused in finding and eating more pellets. Conversely, pel-lets that are too large will depress feeding and, in theextreme, cause choking. Select the largest sized feed thefish will actively eat.

Feeding Rate,Feeding Rate,FrequencyFrequency, and, andTTimingimingFeeding rates and frequencies are in part a function of fishsize. Small larval fish and fry need to be fed a high proteindiet frequently and usually in excess. Small fish have ahigh energy demand and must eat nearly continuously andbe fed almost hourly. Feeding small fish in excess is not asmuch of a problem as overfeeding larger fish because smallfish require only a small amount of feed relative to the vol-ume of water in the culture system.

As fish grow, feeding rates and frequencies should be low-ered, and protein content reduced. However, rather than

switching to a lower protein diet, feeding less allows thegrower to use the same feed (protein level) throughout thegrow-out period, thereby simplifying feed inventory andstorage.

Feeding fish is labor-intensive and expensive. Feedingfrequency is dependent on labor availability, farm size, andthe fish species and sizes grown. Large catfish farms withmany ponds usually feed only once per day because of timeand labor limitations, while smaller farms may feed twiceper day. Generally, growth and feed conversion increasewith feeding frequency. In indoor, intensive fish culturesystems, fish may be fed as many as 5 times per day inorder to maximize growth at optimum temperatures.

Many factors affect the feeding rates of fish. These includetime of day, season, water temperature, dissolved oxygenlevels, and other water quality variables. For example,feeding fish grown in ponds early in the morning when thelowest dissolved oxygen levels occur is not advisable. Incontrast, in recirculating aquaculture systems where oxygenis continuously supplied, fish can be fed at nearly any time.During the winter and at low water temperatures, feedingrates of warmwater fishes in ponds decline and feedingrates should decrease proportionally.

Feed acceptability, palatability and digestibility vary withthe ingredients and feed quality. Fish farmers pay carefulattention to feeding activity in order to help determine feedacceptance, calculate feed conversion ratios and feed effi-ciencies, monitor feed costs, and track feed demandthroughout the year.

Published feeding rate tables are available for most com-monly cultured fish species. Farmers can calculate opti-mum feeding rates based on the average size in length orweight and the number of fish in the tank, raceway, or pond(see Hinshaw 1999, and Robinson et al. 1998). Farmedfish typically are fed 1-4% of their body weight per day.

Automatic FeedersAutomatic FeedersFish can be fed by hand, by automatic feeders, and bydemand feeders. Many fish farmers like to hand feed theirfish each day to assure that the fish are healthy, feedingvigorously, and exhibiting no problems. Large catfishfarms often drive feed trucks with compressed air blowersto distribute (toss) feed uniformly throughout the pond.

There are a variety of automatic (timed) feeders ranging indesign from belt feeders that work on wind-up springs, toelectric vibrating feeders, to timed feeders that can be pro-grammed to feed hourly and for extended periods.Demand feeders do not require electricity or batteries.They usually are suspended above fish tanks and racewaysand work by allowing the fish to trigger feed release bystriking a moving rod that extends into the water.Whenever a fish strikes the trigger, a small amount of feedis released into the tank. Automatic and demand feederssave time, labor and money, but at the expense of the vigi-lance that comes with hand feeding. Some growers usenight lights and bug zappers to attract and kill flyinginsects and bugs to provide a supplemental source of natu-ral food for their fish.

Feed ConversionFeed Conversionand and EfEfficiencyficiencyCalculations:Calculations:Because feed is expensive, feed conversion ratio (FCR) orfeed efficiency (FE) are important calculations for thegrower. They can be used to determine if feed is beingused as efficiently as possible.

FCR is calculated as the weight of the feed fed to the fishdivided by the weight of fish growth. For example, if fishare fed 10 pounds of feed and then exhibit a 5 poundweight gain, the FCR is 10/ 5 = 2.0. FCRs of 1.5-2.0 areconsidered ÒgoodÓ growth for most species.

FE is simply the reciprocal of FCRs (1/FCR). In theexample above, the FE is 5/10 = 50%. Or if fish are fed 12pounds of feed and exhibit a 4 pound weight gain, the FE =4/12 = 30%. FEs greater than 50% are considered ÒgoodÓgrowth.

Fish are not completely efficient (FEs of 100 %, FCRs of1.0). When fed 5 pounds of feed, fish cannot exhibit 5pounds of growth because they must use some of the ener-gy in feed for metabolic heat, digestive processing, respira-tion, nerve impulses, salt balance, swimming, and other liv-ing activities. Feed conversion ratios will vary amongspecies, sizes and activity levels of fish, environmentalparameters and the culture system used.

Feed Care andFeed Care andStorageStorageCommercial fish feed is usually purchased by large farmsas bulk feed in truckloads and stored in outside bins.Smaller farms often buy prepared feed in 50-pound bags.Bag feed should be kept out of direct sunlight and as coolas possible. Vitamins, proteins, and lipids are especiallyheat sensitive, and can be readily denatured by high storagetemperatures. High moisture stimulates mold growth andfeed decomposition. Avoid unnecessary handling and dam-age to the feed bags which may break the pellets and createÒfinesÓ which may not be consumed by fish.

Feed should not be stored longer than 90 to 100 days, andshould be inventoried regularly. Bags should not bestacked higher than 10 at a time. Older feed should be usedfirst, and all feed should be regularly inspected for moldprior to feeding. All moldy feed should be discardedimmediately. Mice, rats, roaches and other pests should bestrictly controlled in the feed storage area, because theyconsume and contaminate feed and transmit diseases.

Medicated FeedsMedicated FeedsWhen fish reduce or stop feeding, it is a signal to look forproblems. Off-feed behavior is the first signal of trouble

such as disease or water quality deterioration in the fishgrowing system. Relatively few therapeutic drugs areapproved for fish by FDA (see Helfrich and Smith 2001),but some medicated feeds for sick fish are available.Although using medicated feeds is one of the easiest waysto treat fish, they must be used early and quickly becausesick fish frequently will stop feeding.

Managing FishManaging FishWWastesastesThe most important rule in fish nutrition is to avoid over-feeding. Overfeeding is a waste of expensive feed. It alsoresults in water pollution, low dissolved oxygen levels,increased biological oxygen demand, and increased bacteri-al loads. Usually, fish should be fed only the amount offeed that they can consume quickly (less than 25 minutes).Many growers use floating (extruded) feeds in order toobserve feeding activity and to help judge if more or lessfeed should be fed.

Even with careful management, some feed ends up aswaste. For example, out of 100 units of feed fed to fish,typically about 10 units of feed are uneaten (wasted) and10 units of solid and 30 units of liquid waste (50% totalwastes) are produced by fish. Of the remaining feed, about25% is used for growth and another 25% is used for metab-olism (heat energy for life processes). These numbers mayvary greatly with species, sizes, activity, water temperature,and other environmental conditions.

Useful ReferencesUseful ReferencesFood Intake in Fish. 2001. Houlihan, D., Bouiard, T. andJobling, M., eds. Iowa State University Press. BlackwellScience Ltd. 418 pp.

Fish Kills: Their Causes and Prevention. 2001. Helfrich L.,and S. Smith. Viginia Cooperative Extension ServicePublication 420- 252. Website:http://www.ext.vt.edu/pubs/fisheries/420-252/420-252.html

Feeding Catfish in Commercial Ponds. 1998. E. Robinson,M. Li, and M. Brunson. Southern Regional AquacultureCenter, Fact Sheet # 181. Web Site:http://www.msstate.edu/dept/srac/fslist.htm

Nutrient Requirements of Fish. 1993. Committee onAnimal Nutrition. National Research Council. NationalAcademy Press. Washington D.C. 114 pp.

Nutrition and Feeding of Fish. 1989. Tom Lovell. VanNostrand Reinhold, New York. 260 pp.

Principles of Warmwater Aquaculture. 1979. Robert R.Stickney. John Wiley and Sons, New York. 375 pp.

Standard Methods for the Nutrition and Feeding of FarmedFish and Shrimp. 1990. Albert G.J. Tacon. Volume 1: TheEssential Nutrients. Volume 2:Nutrient Sources and

Larval fish culture is one of theriskiest phases of freshwater fishculture, but it can be one of themost profitable. Special planningis required to overcome the risk ofhigh mortality during fry culture.Producers must have a depend-able larvae supply, a facilityappropriate for fry and finger-lings, the right size fry, the rightkinds and quantity of food, andfry weaned from natural to pre-pared foods. They must also takespecial care in handling fish andpreparing the pond. This factsheet concentrates on the relation-ship between fry size and thetypes and sizes of zooplanktonfound in culture ponds.Zooplankton is required as a firstfood for many cultured fish; forothers it contributes to fastergrowth and higher survival.

Pond sizePonds for fry culture and smallfingerling production should besmaller than grow-out ponds.Ponds from 0.1 to 3 acres are idealbecause they are easier to harvestand will produce more naturalfood per unit area. There is ahigher ratio of pond bottom areato water volume in small pondsthan in large ponds, which

increases the availability of fertil-izing nutrients and resting zoo-plankton eggs. Increased shorelineto water volume increases thenumber of small insects beingblown into the pond, and theymay be a significant source offood for fingerlings. However,ponds with lots of shoreline mayhave more problems with preda-ceous wading birds.

Using many small ponds ratherthan a few large ponds mayensure that at least some finger-lings get to market. Smaller pondsallow the farmer to more easilycontrol the size of the fish bymanipulating nutrient (either fer-tilizer or feed) input. Small pondsalso allow the farmer to more eas-ily determine fish size and esti-mate survival rates because it iseasier to locate the fish. Withmany small ponds instead of afew large ones, farmers can growfingerlings of different sizes forvarious markets. Spreading differ-ent sizes of fingerlings among dif-ferent ponds also helps minimizecannibalism. Farmers can rotatethe harvest among many smallponds rather than harvesting thesame pond over and over; thisreduces stress.

Of course, the benefits of smallerponds must be balanced againstthe increased costs and decreasedpond area per acre of land thatresult when small ponds are used.

Fry sizeTiny fry eat only tiny prey, buttiny fry are preyed upon by manycreatures bigger than they are. Itis important to know the size ofthe fry you are stocking and tomake sure that the pond you areputting them into contains plank-ton of the size that will be theirprey and is also void of creaturesthat will prey on the fry.

The total length of cultured fishfry (Table 1) when they hatchvaries from 2 mm for sunshineand white bass to more than 15mm for muskellunge. In mostcases, fry are a few millimeterslonger than the values in Table 1when they are stocked into ponds.Suggested stocking times in thetable are based upon the size ofthe fry and the sizes and types ofzooplankton that show up at dif-ferent times in ponds that havebeen filled with well water andfertilized. It is safer to stock earlierthan the time listed than to stocklater. Stocking later increases thechance that predaceous zooplank-ton or insects will be present.

Plankton types and sizesPond plankton is composed oftiny plants called phytoplanktonand animals called zooplankton,as well as organisms that are noteasily classified into those twogroups (such as protozoans and

July 1999

SRAC Publication No. 700

Zooplankton Succession and Larval FishCulture in Freshwater Ponds

Gerald M. Ludwig*

*Stuttgart National Aquaculture ResearchCenter

bacteria). Planktonic organismsare suspended in the water andare so small that even slight cur-rents move them about. Fish fryeat zooplankton, phytoplankton,and tiny plants and animalsattached to objects on the pondbottom.

Most fish fry eat three main typesof zooplankton—rotifers, cope-pods and cladocerans. For thetiniest fish fry, such as the newlyhatched fry of sunshine bass orwhite bass, small rotifers may bethe only zooplankton smallenough to eat. For larger fry, the

smallest rotifers may not provideenough nutrients to make chasingand ingesting them worth theeffort. Copepod nauplii, which arejust-hatched copepods, are impor-tant first foods for larval fish, too.Protozoans may also be eaten, butlittle is known about their contri-bution to fry diets.

In general, the smallest of themain zooplankton groups arerotifers (Fig. 1). Body lengths ofrotifer species vary from 0.04 to2.5 mm. This diverse group of ani-mals obtained their name fromtheir “wheel organ,” a ring of cilia

that appears to rotate around themouth. They are often the earliestvisible zooplankton to appear inponds, hatching almost immedi-ately after ponds are filled.Rotifers reach maturity 2 to 8 daysafter hatching and some speciescan increase in number veryrapidly. Rotifers that show uplater in ponds, when larger zoo-plankton are present, are usuallymuch larger than the first thatappeared. The first rotifers hatchfrom “resting eggs” that survivedon the pond bottom duringinclement weather or while the

Table 1. Total lengths of fry for commonly cultured cool and warm water fish.

Common name Scientific name Fry size (mm)* When to stock fry**

Sunshine bass Morone chrysops X M. saxatilis 2-6 5White bass M. chrysops 3-4 5Black crappie Pomoxis nigromaculatus 3-5 5White crappie P. annularis 3-5 5Goldfish Carassius auratus 3-5 5Fathead minnow Pimephales promelas 4-6 5Rosy-red minnow P. promelas 4-6 5Sauger Stizostedion canadense 4-6 5Golden shiner Notemigonus chrysoleucas 4-7 5Common carp Cyprinus carpio 5-7 5Yellow perch Perca flavescens 5-7 5Largemouth bass Micropterus salmoides 6-7 7Walleye Stizostedion vitreum 6-9 7Grass carp Ctenopharyngdon idella 6-9 7Silver carp Hypophthalmichthys molitrix 6-9 7Bighead carp H. nobilis 7-8 7Striped bass M. saxatilis 7-10 10Palmetto bass M. saxatilis X M. chrysops 7-10 10Paddlefish Polyodon spathula 8-10 10Spotted sucker Minytrema melanops 8-10 11White sucker Catostomus commersoni 8-10 11Shovelnose sturgeon Scaphirhynchus platorynchus >10 12Channel catfish Ictalurus punctatus 10-12 13Muskellunge Esox masquinongy 11-15 14

*25.4 mm equals1 inch

**Estimated number of days after pond starts filling at 21 to 27o C (70 to 80o F) Sources of fry sizes for Table 1:

Becker, G. 1983. Fishes of Wisconsin. University of Wisconsin Press. Madison, Wisconsin.Carlander, K.D. 1977. Handbook of freshwater fishery biology. Vol. 2. The Iowa State University Press. Ames, Iowa.

Ludwig, G.M. 1996. Tank culture of sunshine bass Morone chrysops X M. saxatilis fry with freshwater rotifers Brachionus calyciflorus and salmon starter meal as first food sources. Journal of the World Aquaculture Society 25:337-341.

Halver, J., ed., Horvath, L., G. Tamas and I. Tolg. 1984. Special methods in pond fish husbandry. Academiai Kiado, Budapest, Hungary, and Halver Corporation, Seattle, Washington.

Huq, M.F. 1965. The effect of crowding on the growth of fry of channel catfish Ictalurus punctatus (Rafinesque). Scientific Researches (Dacca, Pakistan). 2:112- 117.

pond bottom was dry. Most ofthem hatch into females thatreproduce asexually until pondconditions become harsh. Thensexual reproduction occurs andresting eggs are again produced.

Copepod nauplii are the nextlargest zooplankton to appearafter ponds are filled and fertil-ized (Fig. 1). They hatch from rest-ing eggs that were dormant on thepond bottom. Resting eggs areproduced in the fall by sexualreproduction. At other timesreproduction may be sexual orasexual. After hatching, the youngcopepods grow by shedding theirexoskeletons up to 12 times instages called instars. They reachmaturity about 18 days afterponds are filled. As they growthey provide larger and largerfood for larval fish. The largestfreshwater copepods may reach 2to 3 mm. Only large fry, such asthose of channel catfish, havemouths big enough to eat adultcopepods initially.

Although copepods may be preyfor larger fish fry, sometimes theroles are reversed. Introducingsmall fish fry into a pond full oflarge copepods can be disastrous.One group of copepods, the

cyclopoids, is predaceous. Theyfeed on smaller zooplankton andeven on fish fry. When cyclopoidcopepods are prevalent, they mayeat all the fish fry stocked.

Cladocerans, often called waterfleas because of their shape and“hop-sink” type of locomotion,are the third major group of zoo-plankton found in freshwaterponds (Fig. 1). Larger fry andeven adults of some fish speciesoften selectively prey on thesecrustaceans. Cladocerans 2 to 3mm long are commonly found inculture ponds several weeks afterthe ponds are filled. Often onlyfemale cladocerans are found,except in early spring and late fall.Like some other zooplanktongroups, cladocerans hatch fromresting eggs when ponds arefilled. Later, eggs are held in thefemales before hatching.Cladocerans compete with rotifersand calanoid copepods for phyto-plankton.

Pond preparation Ponds that are drained, dried, andthen filled with well water aremuch safer for culturing fry thanare ponds filled with surfacewater. Starting with water that

does not contain zooplanktonmakes it much easier to predictwhen the right size zooplanktonfor the fry will appear in theponds. It also helps to ensure thatzooplankton, fish predators, para-sites, diseases, and a variety offry-eating insects are not abun-dant in the ponds when fish arestocked. If surface water must beused, it should be filtered througha filter fine enough to preventeven small copepods from passingthrough (125-micron mesh).

Ponds should be properly fertil-ized as they are being filled. If frywill depend on zooplankton forfood, a combination of organicand inorganic fertilizers is best.Organic fertilizers are the basis ofthe food chain that nourishes bac-teria, protozoans, zooplankton,and eventually the fish fry. (Someorganic fertilizers, such as ricebran, are fine enough to be direct-ly consumed by zooplankton). Asorganic fertilizers decompose,their nutrients are used by phyto-plankton, which is consumed bysome types of fry. Or, the phyto-plankton is eaten by protozoansor zooplankton before they areeaten by fry. Nutrients fromorganic fertilizers are releasedover time, so they produce lessdrastic changes in plankton popu-lations than do inorganic fertiliz-ers.

Inorganic fertilizers add nutrientsto the pond instantly. A phyto-plankton-based food chain candevelop very rapidly without theneed for bacterial action.However, the nutrients are oftenused up very rapidly by the tinyplants, and the risk of a bloom“crash” is greater than it is withorganic fertilizers.

Using a combination of organicand inorganic fertilizers results ina greater diversity of planktonthan if either fertilizer type is usedalone, and reduces the potentialfor a dangerous bloom crash.

Fertilizer nutrients are used quick-ly in the pond environment. Somenutrients are trapped in the bot-tom mud or otherwise lost fromwater. Therefore, nutrients shouldbe replenished often. Frequentapplications of small amounts aremore effective than a single large

Figure 1. Common zooplankton in fish culture ponds. (Drawings fromTaxonomic Keys to the Common Animals of the North Central States,1982. Burgess Publishing Co. Minneapolis, Minnesota, used with permis-sion from James C. Underhill.)

copepods and cladocerans preventa re-bloom of the smallest rotifers.However, modest populations oflarger rotifers may appear afterseveral weeks, particularly whenfish fry prey on the rotifers’ com-petitors and predators—cladocer-ans, copepods and insects.

Because of the ephemeral natureof high density rotifer popula-tions, timing is critical if the frybeing stocked are so small thatthey can eat only rotifer-sizedprey. Most fry 6 mm long or lessfall into that category. Fry must bestocked just before the rotifer pop-ulation begins its rapid growth(Fig. 2). If the fry are stockedwhen rotifer populations arerapidly rising there will be plentyof food and the fry should growrapidly and be large enough to eatcopepod nauplii and larger zoo-plankton when those organismsappear. The fry will also have amuch better chance of being largeenough to avoid being eaten bycyclopoid copepods.

Larger fry (more than 6 mm)should be stocked into ponds aspopulations of copepod nauplii,copepods and cladocerans beginto climb (Fig. 2). That usually hap-pens 2 to 3 weeks after ponds arefilled when water temperature is21 to 27o C (70 to 80o F). The frywill then have the right size food

application for maintaining a sup-ply of fry food organisms.Additional information can befound in SRAC Publication 469,“Fertilization of Fish Fry Ponds.”

Timing of fry stocking The proper timing of fry stocking,in relation to filling and fertilizingthe ponds, can make the differ-ence between having an abundantharvest or a complete crop loss.Proper timing is also importantfor optimum growth of the fry.Ponds must contain the appropri-ate type and size of food when fryare stocked. Large fry stocked intoponds with very tiny zooplanktonmay grow slowly because the frymust expend so much energy tocatch an adequate amount offood. Likewise, if the zooplanktonare mostly too large for the fry toeat they may starve, or becomeprey of cyclopoids or insects.

When ponds are filled and fertil-ized, the plant and animal popu-lations that invade or hatch fromwithin the bottom mud passthrough somewhat predictablechanges in sizes and species (Fig.2). This process is called succes-sion. At first there are usually afew small species in large concen-trations. Later there will be manyspecies in an array of sizes, buteach in moderate concentrations.The average size of organismsalso gets larger with time. Theearly community is unstable andgreat changes can occur quickly;later, the greater diversity oforganisms makes the communitymore stable.

Knowing how succession happensin fry culture ponds will help aproducer be more successful.Figure 2 illustrates the succession-al process in ponds in Arkansasduring the spring when tempera-tures are 21 to 24o C (70 to 75o F)(sunshine bass fry were also pre-sent in these ponds). When pondsare first filled with well water,there are few living organismsand few nutrients. The waterrapidly gains nutrients from thebottom, particularly when solubleinorganic fertilizers are added. Italso gains nutrients, but moreslowly, as organic fertilizers aredecomposed by bacteria. Phyto-

plankton and other bacteria rapid-ly use released nutrients. Within afew days, growing populations ofphytoplankton may provide agreen tinge or “bloom” to thewater. This indicates that there isa growing food base for single-celled protozoans and other zoo-plankton. In many ponds thewater first appears brownish. Thishappens when the bacterial foodlevels are large enough to causehuge protozoan or rotifer bloomswithout much phytoplanktonbeing present.

Although some protozoans maybe large enough for tiny fry to eat,it is the next stages in successionthat are of greatest importance tofry. Rotifers usually appear first.Rotifers feed on bacteria and phy-toplankton, and then reproduce toform huge populations. Whenwater temperature is 21 to 27o C(70 to 80o F), rotifers can go fromnearly nonexistent to concentra-tions in the thousands per liter bythe second week after a pond isfilled. As rotifers eat their ownfood supply the population dropsdrastically. Then copepod nauplii,adult copepods and cladoceransmake their appearance. Cyclopoidcopepods prey on small rotifers.Calanoid copepods and herbivo-rous cladocerans out-competethem for phytoplankton. Together,

Figure 2. Small fry, those less than about 6 mm long, should be stocked beforerotifers reach their initial peak density. Fry longer than 6 mm may be stockedslightly later but before predaceous insect populations are high. This graph illus -trates succession of the zooplankton community as it usually occurs in a fishculture pond that has been filled with well water and fertilized. This data wasobtained from ponds that contained sunshine bass fry that were preying on thezooplankton.

for rapid growth and can betterescape predation from aquaticinsects that soon begin to populatethe pond.

In general, fry must have zoo-plankton to survive, or at least tobe healthy and grow rapidly.Most fry are not particular aboutthe types of zooplankton they eat,but the organisms must be smallenough to fit into their mouths. Tomaximize survival, stock any fryjust as populations of zooplanktonsmall enough for the fry to eat arerapidly increasing and beforeinvading predators becomenumerous.

Stocking even large fry into apond that has been filled for morethan 3 to 4 weeks during warmweather can result in high mortali-ty. By that time, a variety of frypredators have invaded the pondand begun to reproduce. Theseinclude insects such as back-swim-mers, diving beetles and whirlygigbeetles. Later, even larger insectssuch as water scorpions, giantwater beetles and the larval stagesof dragonflies will appear.

Insects begin to colonize as soonas ponds are filled during warmweather. However, it usually takesseveral weeks for their popula-tions to reach levels threatening tosmall fish.

Predaceous cyclopoid copepodsare often a much greater threat tofry than insects. Many of thesetiny zooplankton will prey uponfry unless, or until, the fry arelarge enough to prey upon them.Cyclopoid copepods often areabundant in ponds after about 10days when water temperature is20 to 25o C (68 to 77o F). Becausethere are no legal means of con-trolling undesirable zooplanktonor insects, it is important that fry,particularly small fry, be stockedinto ponds as early as appropriateafter ponds are filled.

The effects of weather must alsobe considered when stocking fry.Temperature has a profound effecton the successional process. Figure3 shows that the colder the waterthe more time is required forrotifers to reach their initial peakpopulation.

Researchers are developing meth-ods of predicting when some zoo-plankton events will occur underdifferent temperatures. Figure 3shows that the time it takes toreach an initial rotifer peak isrelated to the mean morning watertemperture in the following way:

Days to rotifer peak = 29.7 – 0.95 (average morning

water temperature in oC)

= 46.57 – 0.53 (average morning water temperature in oF).

If you know the average watertemperature on a farm for selecteddates you can predict how long itwill take to reach a peak in therotifer population. If the fry beingrasied require rotifers, they should

be stocked several days before thepeak.

There is also a relationshipbetween the mean average dailyair temperature on days betweenfilling the pond and reaching thepeak and the time it takes rotifersto reach their initial peak density(Fig. 4). The relationship is

Days to reach rotifer peak = 27.4 – 0.89 (mean average

daily air temperature in oC)

= 43.22 – 0.49 (mean average daily air temperature in oF)

where average daily air tempera-ture is defined as high daily tem-perature plus low daily tempera-ture divided by 2.

Figure 3. The effect of water temperature on the time to peak rotifer populations ina fertilized culture pond. Cold water slows the development of zooplankton popu -lations.

Figure 4. The number of days it takes to reach a peak in the rotifer population inculture ponds at Stuttgart, Arkansas during the spring.

Fish farmers can use this relation-ship to approximate the timerequired for rotifer populations toreach an initial peak starting anyspring day for any location. To dothat, normal, daily average airtemperatures for the location aresubstituted into the equation anda curve is drawn. Figure 4 illus-trates this procedure for Stuttgart,Arkansas. Normal daily air tem-perature data can be obtainedfrom local airports or from theU.S. Department of Commerce,National Climatic Data Center,Federal Building, Asheville, NorthCarolina 28801. “LocalClimatological Data—Unedited”can also be obtained atwww.ncdc.noaa.gov.

Weaning fry to artificialfeed Some fry, such as channel catfish,can be grown on artificial feedsalone, but even these fry will growbetter and be healthier when zoo-plankton are present. However, anobjective in all fry culture is totrain the fry to eat artificial feeds,because fry ponds are stocked atdensities so high that naturalfoods alone cannot sustain goodgrowth of fingerlings. Unless fishare adapted to filter feeding (suchas paddlefish) they expend moreenergy catching zooplankton thanthey derive from eating them.

At some point, fingerlings will beconsuming zooplankton fasterthan it can be produced in thepond. By that time they should bewell on their way to learning howto consume artificial feeds. Thislearning process should be com-pleted before zooplankton stocksare exhausted.

Sometimes, however, small finger-lings are brought into tanks andtrained to eat manufactured feed.This abrupt change from naturalfeeds can cause high mortality. Itis better to wean fish to manufac-tured feeds by a process called“feeding the pond.”

Feeding the pond means spread-ing finely ground feed across thepond at a rate of about 10 to 50percent of the weight of fry in thepond. It is hard to determine theweight of the fry, so the typicalamount used is 1 to 2 pounds offeed per acre for small fry and 4 to8 pounds of feed per acre for cat-fish and other large species. Thefish should be fed twice a day for3 weeks. For small fry, use thefinest size feed available. For larg-er fry, a fine crumble may be used.After the initial 3-week period,increase the amount of feed to 3 to7 pounds per acre (12 to 24pounds for large fry) appliedtwice a day for another 3 weeks.At the same time, increase the sizeof the feed. After the second 3-week period, check to see if the fryare large enough to eat a crumbledfeed.

When the fish begin to take feed atthe surface, the amount of feedoffered can be adjusted to meettheir needs by watching the fishes’behavior. Feed just a small amountmore than they immediately con-sume and later check to see if theadditional feed is eaten. Adjustthe amount of feed offered accord-ingly.

Fry feeds have a higher proteincontent than feeds for larger fish,because fry are growing at a fasterrate and must consume largeamounts of protein and othernutrients. Fry of fish that are pis-civorous (fish eaters), such asstriped bass and their hybrids,require a protein content of about55 percent. Minnows and catfishrequire feeds with 40 to 50 percentprotein. The general rule is thatthe higher the adult fish are on thefood chain, the more protein thefry require.

Most fingerling producers removetheir fish from the ponds and sellthem, or restock them at lowerdensities, at the time when naturalfood in the pond can no longersupport the standing crop of fin-gerlings.

A fingerling operation that is care-fully planned and based on whatis happening in the pond has thebest chance of being profitable.

The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 fromthe United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.

NRAC Fact Sheet No. 222-1994

Northeastern RegionalAquacultureCenterUniversity of MassachusettsDartmouthNorth DartmouthMassachusetts 02747

Evaluation of Artificial Diets for Cultured FishMichael A. Rice, Fisheries, Animal & Veterinary Science, University of Rhode IslandDavid A. Bengtson, Zoology, University of Rhode IslandCarole Jaworski, Rhode Island Sea Grant, University of Rhode Island

IntroductionOne of the major expenses in any fish culture

operation is the cost of feeds for the fish, and theprofitability of many operations is frequently tied tothe cost of feed. Hatchery production of fish larvaemost often requires the expensive production of livefood (phytoplankton and zooplankton), becauseartificial diets are either not available, or are grosslyinadequate. Artificial diets are available for growout offingerlings and adults of most cultured fish species, butthey may be less than optimal because they had beenformulated for another species. For example, in theUnited States,commercially formulated diets areavailable for catfish and salmonids, but these diets havebeen used without modification to feed other species offish, including hybrid striped bass, tilapia, carps, andothers. Less than optimum diets for growout of finger-lings will result in lowered growth rates and excessivewaste, either by excessive fecal material, excessiveurinary nitrogen, or uneaten food. Thus, less thanoptimum diets are not only wasteful in terms of moneyspent on feed, but they can cause increased wastemanagement problems. The key challenge of producingproduction feeds is the maximization of fish growthwith a minimization of waste.

The challenges of formulating diets for larval fishare more formidable, as evidenced by a reliance on livefeeds. One major challenge of larviculture is the pro-duction of organisms that areas similar as possible tothose in nature. Marine fish larvae that hatch and growin nature typically feed on zooplankton in an environ-ment of low fish density and good water quality;mortality is often >90 percent, due primarily to prob-lems at first feeding and to predation. By contrast,those that hatch and grow in larval rearing systemstypically feed on rotifers, Artemia and prepared diets inan environment of high fish density and (at best)adequate water quality; mortality, due primarily to

problems at first feeding and disease, can often begreater than 50 precent. The development ofhigh-quality artificial diets could potentially amelioratewater quality and disease problems, as well as reducethe high cost of live feed.

Evaluation of Production DietsFor most practical applications, evaluation of

production diets (diets for fingerling and adult produc-tion) can be adequately done in feeding trials. Sincediets are available that have a well-defined composi-tion, growth performance of fish can be readily deter-mined after modifications of a control diet are made.Typically, the total feed utilization by fish, expressed asfood conversion ratio (FCR), or the protein utilization,expressed as protein efficiency ratio (PER), are calcu-lated. The highest quality production diets will haverelatively low FCRs and high PERs.

One of the simplest means for an aquacultureproducer to assess feed performance is to determine afood conversion ratio (FCR). The FCR is the weight offood supplied divided by the weight gain of the fishduring the feeding period. FCR can be expressed by theequation:

(1)

when, F

wo

W fand

FCR = F/(Wf-Wo)

is the weight of food supplied to fish during thestudy period,

is the live weight of fish atthe beginning of the studyperiod,

is the live weight of fish atthe end of the study period.

1

Example: A fish pond operator starts with 1,000fingerlings at an average weight of 200g each. The Now, if a feed were chosen with the reduced crudeaggregate W0 is 200 kg. The fish are fed 7g food/fish/ protein content of 35 percent and the fish growth is theday for 6 months when they are harvested at a final same, the PER would be:average weight of 900g each, but there has been 2percent mortality. The aggregate Wf would be: PER = (882kg - 200kg)/(1,274kg x 0.35) = 1.53

1,000- (1,000 X 0.02)= 980 fish x 900g = 882kg

The amount of food supplied would be: The PER values are reduced when protein levels inthe feed are either insuffient or are in excess. Opti-

7g/day/fish x 182 days x 1,000 fish= l,274kg mum protein content in fish feeds is species specificand occurs when PER is maximized.

Then, the FCR would be:

FCR = 1274kg/(882kg - 200kg) = 1.87

A very important factor to remember when FCRsare compared is that they are based on the wet weightof the feed. Different feeds may have very differentmoisture levels. For example, a dry catfish productiondiet may have a moisture content of around 10 percent,whereas a semi- moist diet for sea bass may have amoisture content of over 60 percent. Moisture does notcontribute to the growth of fish, but does add a bias tothe FCR values. Thus, if comparisons are made betweentwo or more diets, it is often useful to calculate the FCRon a dry weight basis. To make this easier, it is impor-tant to know the percent of moisture and dry weightin both your feeds and fish.

High protein ingredients are frequently the mostexpensive components of artificial diets. Consequently,feeding a diet too high in crude protein will not onlybe wasteful in terms of cost, but excess excretorynitrogen resulting from the breakdown of protein forenergy metabolism may be a stressor to the fish. Onemeans for determining the optimum level of protein ina selected feed is to compare the protein efficiencyratios (PER) of different feeds fed to fish. PER is theweight gain of fish divided by the dry weight of proteinin the feed. An equation describing PER would be:

(2) PER = (Wf-W0)/F x p

when, F is the weight of feedsupplied over the test period,

and p is the fraction of crudeprotein weight in the feed,

For example, if the percentage of crude protein inthe feed from the above example were 40 percent, thePER over the 6 month growth period would be:

PER = (882kg - 200kg)/(l,274kg x 0.4) = 1.34

Evaluation of Larval DietsDiets for larval fish are notoriously difficult to

evaluate because there are no completely defined artifi-cial diets that are adequate for fish growth. Larval fishproducers are currently reliant upon live feeds, so activework with artificial diets is largely confined to the re-search community. A comprehensive evaluation of anartificial diet only begins with a well-controlled experi-ment to compare survival, growth, and perhaps otherindicators (e.g., stress/activity tests) of the larvae tothose obtained with live food (either rotifers, Artemia ornatural zooplankton). If equivalency is not obtained,one then needs to investigate the causes of the defi-ciency, realizing that those causes may not even be inthe formulation of the diet. Two basic categories of re-search are required: 1) research on the physical andchemical state of the diet in the water column; 2) re -search on the physiological and biological requirementsof the larvae. Two special caveats should be noted here:1) all research should be conducted, and the resultsexpressed, relative to live food; and 2) if the artificialdiet happens to be microncapsulated, it is necessary toinvestigate deficiencies in the diet and themicrocapsule separately. One way to study themicrocapsule separately is to microencapsulate livefood, as Leibovitz (1991) has done with Artemia nauplii.

Diets in the Water ColumnAfter they are introduced into the water column,

diets should remain both available and palatable to thelarvae without leaching significant amounts of nutri-ents. Ideally, the diet should be neutrally buoyant; butin practice, this is very difficult to achieve. Manysinking diets can be kept in the water column withsufficient aeration, but the aeration levels required maybe detrimental to the larvae due to the excessive agita-tion. Estimation of the availability of the diet to thelarvae is possible even without the larvae in the systemsimply by measurement of the residence time of thediet: 1) at the surface; 2) in the water column; and 3) atthe bottom. For example, Leibovitz (1991) quantifiedthe percentages of diet particles (microencapsulatedArtemia nauplii) at each of the three locations over an8-hour period to demonstrate that they spent 2-4 hours

2

at the surface, 1-2 hours in the water column, and theremainder at the bottom.

Leaching of essential nutrients from larval dietshas long been considered a serious problem, with watersoluble vitamins being the most susceptible (Meyers1979). Microencapsulated and microbound diets canhelp to overcome leaching, but the diets should still beanalyzed to determine the extent of the problem in acomprehensive examination scheme. The degree ofleaching can be determined by chemical analyses forvarious substances conducted either on the diet par-ticles recovered from the water at various time inter-vals, or on the water itself, or on both. For example,Leibovitz (1991) showed that microencapsulatedArternia exhibited no significant change in proximatecomposition after 2 hours in seawater. The simplestdetermination of leaching includes simply the measure-ment of dry weight of particles recovered from thewater at various time intervals.

Palatability can be determined by the rate of therate of ingestion of feed particles by larvae. Simplevisual observations can be useful, provided that thelarvae and particles are large enough to be seen withthe naked eye. Alternatively, larvae can be videotapedto record the number of strikes at prey (or particles),number of successful ingestions, and number of rejec-tions. The recent use of image analysis to determinenumber of prey remaining in a bowl with predator(s) atfrequent time intervals (Letcher 1990) could be adaptedfor palatability determinations, but has so far beenattempted only with live Artemia as prey.

Determining Digestive Capabilities andNutritional Requirements of Larvae

The digestive capabilities and dietary requirementsof the larvae can best be determined through a combi-nation of biochemical, physiological, and morphologi-cal studies. Perhaps the greatest challenge to larval fishnutritionists is the integration of information fromthose studies in the formulation of adequate artificialdiets. Some approaches to estimation of the nutritionalrequirements of a given species have included bio-chemical analyses of: 1) yolk material in eggs of thatspecies; 2) zooplankton on which the species feeds, or3) Artemia (Leibovitz et al. 1987). Estimation of thephysiological capabilities of larvae have includedstudies of the development of digestive enzyme produc-tion (e.g., Baragi and Level 1986) and determinations ofthe pH of the digestive tract in which the enzymesmust function (e.g., Buddington 1985). Of particularvalue are those studies that locate the portions of thedigestive tract responsible for the addition of specificenzymes through histochemical means (e.g., Segner etal. 1989). The relative roles of exogenous and endog-

enous enzymes in the digestion of live vs. artificial foodin the larval fish digestive tract has been studied anddebated for years (Dabrowski and Glogowski 1977a,1977b); however, the addition of digestive enzymes toartificial diets has had varying degrees of success/failure(Dabrowski and Glogowski 1977c; Dabrowska et al.1979; Lauff and Hofer 1984; Tandler and Kolkovsky1991).

Morphological studies of development of larvaeinclude histological and histochemical methods withlight, scanning electron and transmission electronmicroscopy. In the context of determination of larvalcapabilities for utilization of artificial diets, the mostuseful studies include examination of sensory apparatus(e.g., taste buds) (Appelbaum et al. 1983), the alimen-tary canal (especially the mucosal epitheliums) (e.g.,Kjorsvik et al. 1991; Verreth et al. 1992), the liver andpancreas (Alami-Durante 1990). Larval fish are charac-terized by significant uptake of nutrients by the hind-gut epithelial cells and intracellular digestion in thesupranuclear vacuoles of those cells (Iwai and Tanaka1968; Watanabe 1984). Any morphological exam-ination of fish larvae by researchers should emphasizethe development of hindgut epitheliums.

Once the diet is ingested, the digestion, absorption, and assimilation of the food can be studied usingfluorescence, radiolabeling, and/or histological meth-ods. Walford et al. (1991) and Walsh et al. (1987) haveused fluorescence methods to follow particles passingthrough the larval fish digestive tract, particularlynoting time of passage and bottlenecks to passage.Determination of assimilation efficiency with radiola-beled carbon has long been practiced with larval fishfed live food (e.g., Boehlert and Yoklavich 1984), buthas recently been used also to compare uptake ofartificial and live diets (Tandler and Kolkovsky 1991).Assimilation effiency data, when combined with dataon rates of ingestion of live vs. artificial diets, canprovide valuable insight into artificial diet deficiencies(e.g., to what extent reduced growth is due to reducedingestion vs. reduced digestibility). Whereas fluores-cence and radiolabeling studies are most useful inestimating process rates for the whole organism,histological studies are most useful in identifyingspecific digestion and absorption sites within thedigestive tract. Bengtson (1993) and Bengtson et al.(1993) have studied uptake of live vs. artificial food byexamination of mucosal epitheliums in larval fish. Bysampling larvae at time intervals after a single feedingand examining histological sections, one can follow thepassage of particles through the digestive tract andanswer the question: Are there differences in thedigestion and absorption of live vs. artificial diets?Larval striped bass appear to absorb all of the nutrients

3

from live Artemia nauplii through the hindgut epithe-lial cells, but do not absorb nutrients from artificialdiets through those cells.

ConclusionThe evaluation of artificial diets for adult and

juvenile fish has been largely based upon feeding trialswith great success, because defined basal diets areavailable. Aquaculture producers can use simple feedingtrial techniques to evaluate the efficiency of feedutilization by their fish and the cost-effectiveness offeeds from different sources. Evaluating feeds for larvalfish is not as simple because of the lack of an adequateartificial basal diet. Research has evolved from theone-dimensional approach of formulating a variety ofdiets and simply obtaining survival and growth resultsfrom feeding trials with larval fish. The multi-disciplinary nature of a comprehensive evaluation of anartificial diet now requires that many groups commun-icate and cooperate with each other. These groupsinclude, but are not limited to, nutritional biochemists,food chemists and engineers, physiologists andmorphologists, and the Aquaculturists themselves.Continuation (and undoubtedly expansion) of such amultidisciplinary approach provides the best chance ofdefining an inert or artificial diet that can competewith live rotifers and Artemia as a nutritional source formarine fish larvae. The ideal artificial diet, however,will produce marine fish larvae that biochemically,physiologically, and behaviorally resemble wild larvaethat feed on natural zooplankton.

AcknowledgmentsThe authers gratefully acknowledge the construc-

tive criticisms of four anonymous NRAC reviewers whohelped to improve the manuscript. Preparation of thisfact sheet was supported by grants from the Northeast-ern Regional Aquaculture Center. This is ContributionNumber 2960 from Rhode Island Cooperative Exten-sion and RIU-G-94 -001 and P1363 from Rhode IslandSea Grant. Portions of this NRAC fact sheet have beenreprinted from the Journal of the World AquacultureSociety (vol. 24, pp. 285-293), with permission.

Literature Cited

Alami-Duruante, H. 1990. Growth of organs and tissues in carp (Cyprinus carpioL.) larvae. Growth, Development Aging 54:109-116.

Appelbaum, S., J. W. Adron, S.G.George, A. M. Mackie and B. J. S.Pirie. 1983. On the development of the olfactory and the gustatory organs of theDover Sole, Solea solea during metamorphosis. Journalof the Marine BiologicalAssociation of the United Kingdom 63:97-108.

Baragi, V. and R. T. Lovell. 1986. Digestive enzyme activities in striped bass fromfirst feeding through larval development. Transactions of the American FisheriesSociety 115:478-84.

on the structure and function of the digestive system of Menidia beryllina (Pices,Atherinidae).pp. 11%208. In: B. T.Walther and H. J. Fyhn, editors Physiology andbiochemistry of fish larval development. University of Bergen, Norway.

Boehlert, G. W. and M. M. Yoklavich. 1984. Carbon assimilation as a function ofingestion rate in larval Pacific herring, Clupea harengus pallasi Valenciennes.Journal 1 of Experimental Marine Biology and Ecology 79:251-262.

Buddington, R. K. 1985. Digestive secretions of lake sturgeon, Acipenserfulvescens, during early development. Journal of Fish Biology 26:715-723.

Dabrowska, H.C. Grudniewski and K. Dabrowski 1979. Artificial diets forcommon carp effect of addition of enzyme extracts. Progressive Fish-Culturist41:196-200

Dabrowski, K. and J. Glogowski. 1977a. Studies on the proteolytic enzymes ofinvertebrates constituting fish food Hydrobiologia 52:171-174.

Dabrowski, K. and J. Glogowski. 1977b. The role of exogenic proteolytic enzymesin digestion processes in fish Hydrobiologia 54:129-134.

Dabrowski, K and J. Glogowski. 1977c. A study of the application of proteolyticenzymes to fish food. Aquaculture 12:349-360.

Iwai, T. and M. Tanaka. 1%8. The comparative study of the digestive tract ofteleost larvae-III. Epithelial cells in the posterior gut of halfbeak larvae. Bulletin of theJapanese Society of Scientific Fisheries 34:44-48.

Kjersvik, E., T. van der Meeren, H. Kryvi, J. Amfinnson and P. G. Kvenseth1991. Early development of the digestive tract of cod larvae, Gadus morhau L.,during start- feeding and starvation. Journal of Fish Biology 38:1-1S.

Lauff, M. and R. Hofer. 1984. Proteolytic enzymes in fish development and theimportance of dietary enzymes, Aquaculture 37:335-346.

Leibovitz, H.E. 1991. Albumen-alginate microcapsules for delivering food to landinland silversides, Menidia beryllina. Doctoral dissertation. University of RhodeIsland, Kingston, Rhode Island, USA.

Leibovitz, H. E., D.A. Bengtson, P.D. Maugle and K. L. Simpson.1987. Effectsof dietary Artemia lipid fractions on growth and survival of larval inland silversides,Menidia beryllina. pp. 469478 in P. Sorgeloos, D. A. Bengtson, W. Decleir and E.Jaspers, editors, Artemia research and its applications, Volume 3. Universa Press,Wetteren, Belgium.

Letcher, B. H. 1990. Laboratory growth rate, daily food consumption, andwithin-&y patterns of food consumption by larval inland silvrsides, Menidiaberyllina; an investigation using image analysis. Masters thesis. University of RhodeIsland, Kingston, Rhode Island, USA.

Meyers, S. P. 1979, Formulation of water-stable diets for larval fishes. pp. 13-20 In:J. E. Halver and K. Tiews, editors. Finfish nutrition and fishfeed technology, Volume2. Heenemann, Berlin, Germany.

Segner, H., R. Rosch, H. Schmidt and K. J. von Poeppinghausen. 1989.Digestive enzymes in larval Coregonus lavaretus L. Journal of Fish Biology 35:249-263.

Tandler, A. and S. Kolkovsky. 1991, Rates of ingestion and digestibility as limitingfactors in the success/id use of microdiets in Sparus aurata larval mating. EuropeanAquaculture Society 15:169-171.

Verreth J. A. J., E. Torrele, E. Spazier, A. van der Sluiszen, J. H. W. M.Rombout, R. Booms and H. Segner. 1992. The development of a functionaldigestive system in the Afican catfish Clarias gariepinus (Burchell). Journal of theWorld Aquaculture Society 23:286-298.

Walford, J., T. M. Lim and T. J. Lam. 1991. Replacing live finds withmicroencapsulated diets in the rearing of seabass (Lates calcarifer) larvae: do thelarae ingest and digest protein-membrane microcapsules? Aquaculture 92:225-235,

Walsh, M., J. E. Huguenin and K. T. Ayers. 1987. A fish feed consumptionmonitor. Progressive Fish-Culturist 49:133-136.

Watanabe, Y. 1984. An ultrastructural study of intracellular digestion ofhorseradish peroxidase by the rectal epitheliun cells in larvae of a freshwater cottidfish Cottus nozawae. Bulletin of the Japanese Society Scientific Fisheries50:409-416.

Bengston, D.A 1993. A comprehensive program for the evaluation of artificial diets.Journal of the World Aquaculture Society 24:285-293.

Bengtson, D. A., D. N. Borrus, H. E. Leibovitz and K. L. Simpon1993. Studies

4

May 1993

Dietary Effects on

Catfish diets must provide enoughenergy, protein, vitamins and min-erals in the proper proportions forfast, efficient growth and healthmaintenance. Choosing the rightfeed plays an important role in de-termining the productivity andprofitability of aquaculture opera-tions. But, producers aren’t theonly people who are interested indiet quality. Certain characteristicsof the diet influence the quality ofcatfish products during processingand storage. As a result, catfishprocessors, wholesale marketersand retailers also depend onproper feed quality to yield desir-able results.

Effects of dietary protein

Because excess fat decreases thedress-out yield and potential shelf-life of processed catfish, questionsregarding the impact of diet com-position on product quality havearisen. Research projects evaluatedthe effects of dietary protein con-tent on body composition of vari-ous-sized catfish in differentproduction systems.

Catfish grow-out in ponds

At Kentucky State University, fin-gerlings stocked in intensivelymanaged ponds were fed commer-cial-type diets containing 34 or 38percent protein to satiation once ortwice daily. At the end of 170 days

SRAC Publication No. 186

SouthernRegionalAquacultureCenter

Channel Catfish

Body Composition and Storage Quality

The Southern Regional AquacultureCenter has supported research to deter-mine how diet affects the compositionand quality of processed channel catfishproducts. The results of-these researchprojects should help producers and pro-cessors modify management practices toimprove the value and marketability oftheir products. This publication wascompiled by James T. Davis, D.M. Gatlin, III and Max R. Alleger, based on re-search conducted at Auburn University,University of Georgia, Kentucky StateUniversity and Texas A&M University,Details are available from the applicablepublications listed.

the fish weighed an average of 1pound. Results of this experimentindicated that neither feeding fre-quency nor protein content of the

diet within this range affectedgrowth or important body compo-sition characteristics such as fatcontent or fillet yield.

In an Auburn University study,fingerlings were fed commercial-type diets containing 26,32 or 36percent protein for 125 days oneither a restricted basis or to satia-tion. Dressing percentage in-creased as dietary protein wasincreased from 26 to 32 percent,but then decreased as protein con-centration was increased to 36 per-cent. Whether fed on a restrictedbasis or to satiation, body fat de-creased as the diet’s protein levelincreased. Body fat content of fishfed to satiation was higher thanthose fed on a restricted basis,

Choosing the right feed plays an important part in profitability of aquacultureoperations.

suggesting that feeding rate doesinfluence important body composi-tion variables.

Another study conducted atAuburn University measured theeffects of feeding commercial-typediets containing 24,28,32,36 or 40percent protein to fingerlings inponds. Fingerlings were fed to anaverage of 1 pound in 151 days.They were fed to satiation oncedaily during the growing season.Dressing percentage increased asdietary protein was increasedfrom 24 to 36 percent, but de-creased when increased to 40 per-cent. Fat content in filletsdecreased, while protein and mois-ture increased when dietary pro-tein was increased.

Conclusions

These studies suggest that filletyield may improve as dietary pro-tein is increased up to 36 percent,and that feeding to satiation mayincrease body fat concentrations.However, the same studies sug-gest that producers can savemoney without sacrificing weightgain by feeding diets that containmuch less than 36 percent protein.These trade-offs betweeneconomic savings and potentialchanges in product quality de-serve further attention, especiallyif fat content and other body com-position characteristics are provento reduce the quality and con-sumer acceptance of catfish pro-ducts.

Catfish grow-out in cages

Cage culture offers an opportunityto produce fish in ponds that maybe poorly suited for conventionalpond culture because of their size,depth or the presence of other fish.However, successful cage culturealso provides unique managementchallenges to the producer.

A study conducted at KentuckyState University focused on the nu-tritional needs of fingerling chan-nel catfish stocked in cages. Fish incages were fed to satiation once ortwice daily for 105 days with com-plete, commercial-type feeds con-

taining either 34 or 38 percent pro-tein.

Body composition of fingerlings inthis study was not affected by feed-ing frequency or dietary proteinlevel. Fish grew faster on thehigher protein diet, and fish fedtwice daily had a higher dressingpercentage than those fed onlyonce per day.

Grow-out of third-year fish inponds

Little information is available re-garding how diet affects thegrowth or body composition ofthird-year fish. Studies conductedat Auburn University and Ken-tucky State University measuredthe effects of feeding commercial-type diets containing various con-centrations of protein to third-yearcatfish in ponds. Results of thesestudies indicate that, althoughbody fat decreases when dietaryprotein is increased, fish growthand dressing percentage were un-affected. It remains unclearwhether diets containing morethan 32 percent protein improvethe quality of fillets from third-year fish enough to justify the ac-companying higher feed costs.

Supplementing aminoacids in catfish diets

Amino acids are the buildingblocks of protein; they are essen-tial for good fish growth andweight gain. Several research pro-jects have focused on the effects ofspecific amino acid supplementsin catfish diets. These studies wereconducted with catfish fingerlingsin aquaria maintained under opti-mum conditions.

Lysine is one of ten amino acidsthat must be provided by the diet;it is also the least abundant aminoacid in many feedstuffs. As a re-sult, extra care must be taken toprovide enough lysine when for-mulating catfish diets containing alarge percentage of protein fromplant sources. Also, lysine supple-mentation above requirement lev-els has been shown to reduce body

fat of some terrestrial animals. Re-searchers at Texas A&M Univer-sity compared diets that containedeither 25 or 30 percent proteinfrom soy isolate or casein and gela-tin, and either 0 or 0.5 percent sup-plemental lysine. Fingerlings feddiets with protein from casein andgelatin gained more weight thanthose fed diets containing soy-based protein. Also, fish fed soy-based diets contained more lipidand less protein than those fed thecasein-based diet.

Supplemental lysine improvedprotein conversion efficiency andfeed efficiency of catfish fed soy-based diets, but not of those fed ca-sein-based diets. Fish fed a 30percent protein soy-based dietwithout added lysine performedbetter than those fed a 25 percentprotein soy-based diet with extralysine. However, supplementally-sine did not influence body compo-sition characteristics at any proteinlevel.

Results suggest that both thesource and concentration of die-tary protein impact catfish per-formance, and that supplementallysine does not influence bodycomposition.

Carnitine is a naturally-occurringcompound that animals typicallyproduce from lysine. Some re-search suggests that providingsupplemental carnitine in the dietincreases the quality of processedanimal products by reducing fatcontent. A study conducted at theUniversity of Georgia comparedthe benefits of feeding diets thatcontained 0.1 percent carnitineand 1.1, 1.4 or 1.7 percent lysine.Feeding diets that included bothsupplemental carnitine and lysineproved most beneficial. When car-nitine was added to diets contain-ing lysine close to or above therequired dietary level (1.4 and 1.7percent, respectively), fat contentin the viscera and dark muscle tis-sue decreased and whole-fish pro-tein levels increased.

Results indicate that feeding high-quality diets supplemented with

carnitine may reduce body fat con-tent.

Effects of vitaminfortification

Improved storage quality dependson management practices duringgrow-out, as well as procedurescarried out during and afterprocessing. Storage quality of poul-try and some kinds of fish hasbeen improved by feeding dietsthat increased concentrations of vi-tamin E in muscle tissues prior toprocessing. Vitamin E, and similarsynthetic products, are called anti-oxidant because they help reducelipid oxidation and maintain thefreshness of products during stor-age.

Researchers at Texas A&M Univer-sity evaluated the benefits of ad-ding synthetic and naturalantioxidants to channel catfishdiets. Fingerling catfish were fedexperimental-type diets that satis-fied all known requirements andcontained one of two concentra-tions of vitamin E (60 or 240mg/kg), either alone, or in combi-nation with one of four syntheticantioxidants. None of the syntheticantioxidants affected weight gain,feed efficiency, survival or tissuecomposition. Fillet samples fromfish receiving each diet were fro-zen at -10°F for six months. TheTBA number, a measure of rancid-ity caused by oxidation, was deter-mined for fillets to assess howstability during frozen storage wasaffected by diet composition. Fishfed the higher level of vitamin Ehad reduced TBA numbers, butsynthetic antioxidants did not af-fect this measure of storage quality.

Fortification of catfish diets withhigh levels of vitamin E probablyoffers an effective means of main-taining fillet stability during fro-zen storage. Results from anotherstudy indicate that maximumbenefits from vitamin E supple-mentation are achieved within 2weeks of feeding a diet fortifiedwith 1,000 mg vitamin E/kg. Feed-ing diets fortified with this highlevel of vitamin E throughout the

Dietary protein concentration does not appear to affect storage quality of catfishfillets.

grow-out period may be unneces-sary.

Vitamin C is essential for normalfish growth and has some proper-ties that allow food products to re-sist oxidation. Rutin, a compoundclassified as a bioflavonoid, mayproduce beneficial responses simi-lar to vitamin C when available inthe diet. A Texas A&M Universitystudy compared diets containingvarious concentrations of vitaminC and rutin. By the end of eightweeks, no differences in weightgain, feed efficiency or survivaldue to diet were observed. But,within 10 weeks, fish fed dietswithout supplemental vitamin Cor rutin had developed deformedspinal columns, external hemor-rhages and eroded fins. By week12, fish receiving supplementalrutin but no vitamin C showed thesame symptoms. By the end of 16weeks all fish that had not re-ceived supplemental vitamin Chad reduced weight gain and feedefficiency and decreased survival.Rutin had no significant effect onweight gain, feed efficiency orother variables, either alone orwhen fed with added vitamin C.

According to this study, high con-centrations of supplemental vita-min C (1,500 and 3,000 mg/kg)improved the oxidative stability ofchannel catfish fillets, but supple-mental rutin was not beneficial.

Dietary impacts onstorage quality

Studies at the University of Geor-gia focused on how dietary pro-tein concentration and packagingmethod may affect the quality offrozen catfish fillets. Year-2 andyear-3 catfish stocked in researchponds were fed diets containing24,28,32,36 or 40 percent proteinto an average harvest weight of 3.3pounds. Upon processing, filletswere packaged using PVDC filmoverwrapping, vacuum packagingwith Eva bags or vacuum skinpackaging and stored at -10°F. Fil-lets were removed from frozenstorage after O, 30 and 90 days forchemical analysis and sensoryevaluation. Chemical analyses in-cluded pH, TBA number, ammo-nia and free fatty acid content. Aconsumer panel evaluated broiledsamples for off-flavor, greasinessand texture. Although lower die-tary protein increased fillet fat con-tent, it did not directly affect TBAnumber, pH or sensory attributes.Sensory panelists reported that allfillets became tougher, but greasi-ness decreased as storage time in-creased. Packaging treatment didnot impact the free fatty acid char-acteristics of fillets.

Results indicate that lower proteindiets may increase the fat contentof catfish fillets, but not to a de-gree that reduces consumer satis-faction. Also, current processingand packaging methods for catfishprovide adequate quality protec-tion for up to 3 months of frozenstorage.

Another University of Georgiastudy evaluated the impact of die-tary protein on channel catfishstored on ice. Fish were fed to anaverage size of 3.3 pounds on com-mercial-type diets containingeither 24, 28, 32, 36 or 40 percentprotein in production ponds. Fishwere processed upon harvest, andfillets were placed on polystyrenetrays overwrapped with plasticwrap and stored in drained icechests.

Sensory and chemical evaluationsof iced fillets were conducted after1,7,14 and 21 days. Catfish fedthe lowest-protein diet had morebody fat than those fed higher-pro-tein diets, but fillet fat content hadno direct effects on free fatty acidcontent during storage. Also, die-tary protein did not affect ammo-nia concentration, pH, TBAnumber or bacterial counts of fil-lets stored on ice for up to 2weeks. Sensory panelists reportedthat the texture of fish fed the 32percent protein diet was superiorto those fed 36 percent proteinafter 1 day of storage. After 1week, fish fed 28 percent proteinwere more greasy than those fedthe other diets, but other differ-ences were not detected.

Results suggest that dietary pro-tein does not influence importantquality attributes of channel cat-fish fillets, and that fillets can bestored on ice for up to 2 weekswithout compromising quality.

References

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Bai, S. C. and D. M. Gatlin III.1992. Dietary rutin has limitedsynergistic effects on vitamin Cnutrition of fingerling channelcatfish. Fish Physiology and Bio-chemistry 10:183-188.

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The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 89-38500-4516 from the UnitedStates Department of Agriculture.