Fish Ecology

Species Diversity and Distribution

Growth and Spawning

Population Dynamics

Stocking

Species Diversity & Global Distribution

• Marine biodiversity generally higher than freshwater.

• ~70% of the earth covered by salt water, 1% freshwater

• 97% of all water in the world is ocean (0.009% freshwater)

• Are there more marine or freshwater species of fish in the world?

58% of all species marine

41% freshwater

1% both (diadromous, euryhaline)

• More species on large than small continents.

• Higher species richness in tropics. Why?

Lake Species Diversity

• Seasonal stability / variability of lake habitats influences species presence.

• Abiotic factors that most influence fish species distribution in lakes:– Temperature– Oxygen availability

• Main biotic factor: primary productivity (i.e., food availability)

Four groupings of fish communities based on lake temperature regime and trophic status.

Stream Species Diversity• Number of species increases as stream order increases.

– Different species / predator types become more predominant due to changes in abiotic/biotic features of river habitat.

– Relates to changes in flood frequency, temperature regime, substrate, riparian canopy, etc.

• Functional groups (feeding strategies) change with increased stream order. What are these strategies?– Carnivores:

• Piscivores• Benthophages• Zooplanktivores• Epifauna eaters• Parasites

– Omnivores– Herbivores– Detritivores

Spawning, Hatching & Dispersal• Broadcast spawning

– Meroplanktonic (not seen in exclusively lotic species)

• Benthic spawning– Brood hiding (trout)– Brood guarding (cichlids)– Nest spawning (bass, sticklebacks)

• Internal bearers: internal fertilization.– Oviparity: egg-laying fishes. Very little embryonic development

before eggs are laid.

– Ovoviviparity: embryos develop internally, but only receive nutrition from yolk-sac.

– Viviparity: embryos develop internally, mother provides additional nutrition after yolk sac used up. Also some instances of in-utero cannibalism.

Juvenile Development and Growth

• Transition from larvae to juvenile stage can involve a dramatic metamorphosis, but changes generally more subtle:– Appearance of fully formed fins– Organ systems fully formed (or nearly so)– Juvenile period lasts until first onset of sexual maturity (initial

development of gonads)

• Growth: G = C – (R + E)– Somatic (body) growth is a function of the balance between

consumption/assimilation, respiration rate, and waste excretion

Factors Affecting Growth

• Temperature: dependent on tolerance range of species. Growth increases with temperature to a point, then falls off as temperature increases further

• DO: decrease in DO associated with decrease in growth rate

• Salinity: Important in euryhaline / diadromous fishes

• Food abundance, competition (inter- and intraspecific),

Growth for Reproduction

• Gonadal development occurs prior to spawning, claims a significant percentage of ingested nutrition– Testes: up to ~ 12% of male body weight– Ovaries: up to ~70% of female body weight

• Sexual dimorphism:– Females often larger (to produce more/larger eggs)– Males sometimes larger when territorial during spawn (e.g.

salmon)– Sometimes accompanied by changes in color (dichromatism)

and body structure during spawning season

Timing of Spawning

• Semelparity: Fish reproduce once, then die.

• Iteroparity: Fish reproduce repeatedly during adult lifespan.

• Seasonal cycles:– In temperate areas where seasonal fluctuations in climate are

significant, spawning usually happens as a discrete event (once per year)

– In tropics, many fish species spawn year-round in either distinct peaks or at a constant rate.

– Timing of spawning usually linked to corresponding conditions for larval development and early growth.

Spawning Migrations

• Some fishes undergo directed movements to a specific location for spawning– In less dramatic instances, spawning may merely

involve a shift in habitat preference.

• Anadromy: Most growth takes place in salt water, adult migrates into freshwater to spawn (e.g. coastal salmonids) – Lentic salmonids often migrate into feeder streams to spawn.

• Catadromy: Most growth in freshwater, migrate to saltwater to spawn (e.g. anguillid eels)

Note more catodromous species in the tropics and the reverse for temperate latitudes; why?

• Tropical oceans are extremely oligotrophic; yet rainforest streams are rich.

• Temperate coastal oceans and higher latitude open ocean are very productive relative to some streams.

Atlantic salmon (Salmo salar)

mountain mullet (Agonostomus monticola)

Overview of Tolerance Range versus Survival, Growth, & Reproduction

Population Dynamics & Management

• Stock: total numbers of a population; sum of year classes.

• Production: biomass * growth rate of population.

• Age (size) classes: sub-groupings or cohorts within a population.

• Recruitment: Numbers entering a new year class; can be defined for each year class. Can have inter-annual variability.

• Mortality: Loss between years.

• Fecundity: Offspring numbers per female; may differ between mature classes.

Age and Growth• The patterns of abundance at age gives an indication of

the annual mortality of the population; the number of individuals in each age class will decline at a rate dictated by the total annual mortality.– Therefore, characterizing the age structure of the population is a

central component in studying fish population dynamics.– Age structure can be monitored to determine impacts of

exploitation / environmental perturbation on population

Determining Age of Fishes:

1. Length frequency analysis

2. Estimation from analysis of “hard parts” (scales, otoliths, etc).

Length Frequency Analysis• A comprehensive sampling of fish in the system is

required for complete age analysis; bias in the sample leads to poor characterization of ages and lifespan.

• Lengths of all fish in sample measured, frequencies of each length (i.e. number of fish present of a given length) plotted, and distinct “groups” of length distributions are identified:

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Analysis of Hard Structures

• Seasonal variation in growth rates creates distinct “marks” in radial expansion of hard structures such as scales, otoliths (ear bones), spines, etc.

• Similar to rings on trees, these marks can be used to count the number of days or annual cycles the fish has experienced.

• Distinct changes in growth rate associated with seasonal variation in temperature, seasonal variation in resource availability, energetic losses due to reproduction / spawning, etc.

• Growth can be verified by chemical “marking” of hard parts (OxyTertraCycline, etc.) and rearing / recapture of fish.

Recruitment• Recruitment is a function of spawning stock size, density-

dependent forces, and physical influences on mortality and survival. – Increasing numbers of mature classes produces large number of

offspring up to a limit.– Too many mature individuals leads to competition for limited

resources, meaning more energy goes toward competing and growth; less toward reproduction -> lower number of offspring.

– Large number of offspring compete for limited food supply, leading to slower growth and increased mortality under crowded conditions.

– Physical and disease conditions play an unpredictable role.

• Environmental influences on spawning and juvenile recruitment:– Spawning habitat; including physical and chemical conditions.– Timing of hatching and larval development with prey resources.

Fisheries

• Commercial fisheries: Large numbers of fish harvested for sale, generally on significant scales of exploitation. Fisheries hold high importance both as food source and as source of income.

• Sport / Recreational Fisheries: Fewer numbers of fish harvested per individual; however, much greater numbers of individuals participating in fishery. Very difficult to assess impact of these fisheries due to cryptic catches and mortality.

• Sustenance Fisheries: Can be small-scale commercial fishing operations limited to local sale, or consistent harvest from a fishery for purposes of nutrition / sustenance.

Fishery Data

• Historical catch data from fisheries provide indications on stock status compared to previous (especially unexploited) levels.

• Trends in historic data can be difficult to interpret due to changing abilities of fisheries (technological advancements, improved techniques, etc.)

• Number / biomass of fish caught (catch) is not an unbiased means of estimating of stock size; instead, must consider catch per unit effort (hooks deployed, hours fished, # net sets, etc.) or yield.

• This requires understanding efficiency of different fishing approaches and standardizing them against to a single unit of effort.

Response to Exploitation

• Under the simplest assumptions, exploitation (harvest) of a stock functions as an increase in mortality. We would expect a compensatory response of increased production due to decreased density and competition.

• (see Maximum Sustainable Yield approach).

Managing Fisheries: MSY• The concept of Maximum Sustainable Yield is based

upon the principles of logistic growth and density dependent net production.

At an intermediate stock size, the rate of net production (growth not offset by mortality) is highest.

If this net production is harvested, the stock will continue to produce this highest level of net “surplus”

MSY is catching fish in excess of the number needed to maximize production of reproductive adults.

MSY and MEY• Maximum economic yield (MEY) is different from MSY.• Assuming a proportional relationship between effort and cost, we

can plot cost of effort versus yield on the same graph:

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0 50 100 150 200

Fishing effort (f)

Yie

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MSY

MEY Break-even point

But it’s NOT that simple:

– Stock is made up of fish of different ages, sizes, maturity levels, reproductive capacity (i.e. fecundity).

– Fishing often targets largest & easiest-to-catch fish foremost, leaving smaller, less fecund and sexually immature fish.

– This can at best reduce the reproductive capacity of the stock (spawning stock biomass as indicator of stock health).

– At worst, this can in time change the biological characteristics of the stock (size at maturity, growth rates, etc.).

• Controlling effort:– By limiting or allocating effort, stock can be kept at sustainable

levels.– Difficulties in enforcement, sources of cryptic mortality (hooking

mortality, etc.).– Example: bag limits, gear restrictions, etc.

• Managing Production:– Size restrictions are designed to increase reproductive potential

of unexploited stock.– Minimum size of capture set above size (age) of maturity.– Slot limits allow removal of medium sized fish, keep large (highly

fecund) fish in stock.

Methods for Managing Fisheries

• Protecting vulnerable stages:– Spawning aggregations– Easily exploitable stages– Example: closed seasons, closed areas

• Providing refuge: Protected Areas– Allocates an area closed to harvest, provides spatial

refuge for stock.– Can provide a consistent source of spawning stock,

dispersal extends recruitment to surrounding / outlying areas.

– Currently at the forefront of fishery management.

Stock Enhancement

• Fish are artificially reared for release into natural systems to augment natural reproduction.– Upside: consistent recruitment managed to

offset harvest.– Downside: dangers to genetic makeup of wild

stock, deficiencies of hatchery fish.– Stock enhancement vs. “Put-and-Take”

Rainbow Trout Oncorhynchus mykiss

Brown Trout Salmo trutta

Brook Trout Salvelinus fontinalis

Trout stocked in PA!

Aquaculture:• Has potential to relieve pressure on wild stocks if net profit can

exceed that of commercial fishing.

• Becoming more widespread and including more prominent fish species (salmon, shrimp, seabass) due to improving technology.