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Rapp. P.-v. Réun. Cons. int. Explor. Mer, 178: 352-354. 1981.

FEEDING NICHE DIMENSIONS IN LARVAL RAINBOW SMELT (O SM E RU S M O R D A X )

R u s s e l l . D. M c C u l l o u g h and J o n G. S t a n l e y

Maine Cooperative Fishery Research Unit1,

and

Migratory Fish Research Institute,University of Maine,

Orono, Maine 04469, USA

INTRODUCTION

The rainbow smelt is anadromous, but landlocked populations in many lakes are important as forage, game, and bait (Kendall, 1935; Havey and Warner, 1970). Its usefulness is limited because of fluctuations in populations, for which the cause is not known. In many species, year-class strength is determined during critical periods of early development at the time of initial exogenous feeding. The decisive factor is often the density of suitable prey. Freshwater species that apparently have critical feeding periods include Coregonus wartmanni (Braum, 1967) and C. artedii (Anderson and Smith, 1971). Prey availability for fry of the walleye, Stizostedion vitreum, depended more on size of prey than on its taxonomic identity (Merna, 1977).

The purpose of this study was to define the taxonomic and size compositor of the food niche of larval rainbow smelt and to trace food changes that occur during early development, with the aim of better understanding critical factors that influence year-class strength.

M ATERIALS AND METHODS

Fish larvae and plankton were collected in Echo Lake, an oligotrophic lake of glacial origin on Mount Desert Island, Maine, 95 ha in surface area and 20 m deep. Specimens were gathered by Nitex plankton nets towed horizontally. Eight tows, alternating between

1 Jointly supported by the U. S. Fish and Wildlife Service, University of Maine, and Maine Department of Inland Fisheries

and Wildlife.

plankton and larvae, were made at the surface, 2, 4, and 10 m during each of 25 sampling periods, between early May and mid August 1977. Collections were made in the evening between 1800 and 2200 h. Samples were preserved immediately in 10% Formalin. Larval length was measured to the nearest 0.1 mm, and mouth width to the nearest ocular micrometer unit (0.011 mm). The gut was opened with micro-dissecting needles and the contents identified, counted, and measured. Plankters were identified to the same taxonomic level as to gut contents, counted, and length and width (exclusive of appendages) measured.

RESULTS

One of the more striking findings was that many larvae had no food in their guts. The percentage of larvae with food in their guts increased from 4.4% in the smallest to 77.8% in the largest.

The food of larval rainbow smelt consisted of microplankton — largely six kinds of rotifers. Floscularids and M ono sty la were eaten frequently and consum ption increased with larval grow th, consumption of Keratella decreased with larval growth; and Trichocerca, Dicranophorinae, and collothecids were eaten in lesser amounts. Kellicottia was common in the plankton but never seen in guts of larvae.

Cyclopoid copepods were frequent in the larger larvae. In contrast, consumption of calanoid copepods and copepod nauplii apparently decreased as the length and mouth width of larvae increased. Bosmina was the only cladoceran frequently eaten, even though other cladocerans were abundant in the plankton.

Feeding Niche Dimensions 353

We predicted that larger larvae could consume larger size prey, and this appeared to be true for cladocera and copepods, but not for rotifers, the principal food item. In fact, the shortest rotifers were found in the longest smelt. Prey width for all prey combined increased slightly as mouth width increased. In a regression analysis of the various dimensions of prey and larvae only the logarithm of prey width vs. mouth width yielded a significant correlation coefficient (r = 0.42). For the other regressions there were no significant correlations, probably because the larger larvae continued to eat small prey. A particularly important observation was that although maximum prey dimensions increased with larval growth, minimum prey dimensions fluctuated about a constant value. Thus a wider range of prey sizes was utilized by larger than smaller larvae; i.e., larger larvae occupied a wider niche.

DISCUSSION

The absence of food in most larval rainbow smelt suggested that initiation of feeding may pose a serious problem for larval smelt. The low incidence of feeding in very young fish appears to be characteristic of species with strong fluctuations in year-class strength. DeMendiola (1974), for instance, found only 22% incidence of feeding in larval anchoveta, Engraulis ringens. Since the ability of larval fish to survive periods of starvation is limited in comparison with that of adult fish, short periods of deprivation can cause severe larval mortality. The critical period for starvation mortality was passed when the smelt reached 12 mm. Leslie and Kelso (1977) found that only 7% of larval smelt had empty stomachs (perhaps the larvae were older). The increased incidence of feeding as larvae grew seems typical of larvae of some other fish. The incidence of feeding soon after yolk sac absorption was 10% for northern anchovy, Engraulis mordax (Hunter, 1972), 2-4% for Atlantic herring larvae (Rosenthal and Hempel, 1970; Blaxter and Staines, 1971), and 3-5% for coregonids (Braum, 1967). The incidence increased to 90% after 3 weeks for the anchovy (Hunter, 1972) and after 7 weeks for the herring (Blaxter and Staines, 1971). In contrast, certain other larvae often have much higher initial feeding success, possibly because they have better maneuverability. Incidence of feeding was 32-62% in early larvae of plaice, Pleuronectes platessa (Blaxter and Staines, 1971) and 60-100% in larval garfish, Belone belone (Rosenthal and Fonds, 1973).

Larval rainbow smelt had definite prey selectivity that appeared to be independent of size alone, and this pattern was found in rainbow smelt from Nipigon Bay

(Leslie and Kelso, 1977) and from St. Louis Bay (Siefert, 1972), both in Lake Superior. On the basis of the literature, it seems that the larvae of marine fish depend primarily on copepods, whereas larvae of freshwater fish feed heavily on rotifers. Freshwater species that apparently select for rotifers include the white sucker, Caloslomus commersonr, bluegill Lepomis macrochirus; and emerald shiner, Notropis atherinoides (Siefert, 1972). Cyprinid larvae in general depend on rotifers as their first food (Rozmanova, 1966; Korniyenko, 1971).

Prey size is an important factor in selectivity, seen in clupeiform larvae of scaled sardine, Harengula pensacolae, and bay anchovy, Anchoa mit chilli (Detwyler and Houde, 1970), northern anchovy (Lasker et al., 1970); and rainbow smelt (present study). The critical dimension is probably prey width, excluding appendages (Blaxter, 1965; Arthur, 1976). The utility of prey, in terms of energy gain, increased greatly with increasing prey size (Werner, 1974). However, small prey of larval fish have the advantage of low time and energy costs for search and capture if in high densities or in dense patches. The relatively wide feeding niche of large larvae allows them to eat large, high energy content prey when such prey are present, thus promoting high growth rates and enhancing survival, but they can continue to eat small, low-cost prey if necessary. Small larvae, on the other hand, are dependent on small prey. The relatively narrow feeding niche of small larvae might make them more vulnerable to starvation when prey densities are low or competition from other larvae is intense.

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