7405 - Laserfiche WebLink

Home Browse Search

20 BIOLOGICAL REPORT 90(5) Fish Larvae-Ecologically Distinct Organisms1 Darrel E. Snyder by Larval Fish Laboratory Colorado State University Fort Collins, Colorado 80523 The behaviors and habitat requirements of larvae of most warm water stream fishes are distinctly different from those of their juvenile and adult counterparts (Braum 1978; Marcy et al. 1980; Sny- der 1983, 1985; Floyd et al. 1984; Faber 1985). The larval period-the interval of fish development from hatching or birth to loss of finfolds and development of adult complement of fin spines and rays-entails many and often dramatic changes in morphology and physiology. Also, it usually covers a number of short- term shifts in lifestyle and habitat. Indeed, a fish larva is often ecologically distinct not only from its juvenile and adult counterparts but from itself dur- ing certain intervals within the larval period. The initial habitat of most fish larvae is the spawn- ing ground. For warmwater stream fishes and anadromous species, there is much diversity in the earliest habitat requirements and behavior of larvae. Initial larval habitats, associated environmental re- quirements, and behaviors are considered in a classification of reproductive guilds by Balon (1981, 1984). The 22 families and over 260 species (Swift et al. 1986) of freshwater and anadromous fishes in the southeastern United States represent at least 17 of Balon's reproductive guilds (Table). Some fishes remain on or near the spawning grounds throughout much or all of their early development, and some may use the same habitat throughout their life cycle. However, for most stream fishes, the initial habitat rapidly becomes in- adequate, and the larvae either drift or actively migrate to more suitable nursery grounds, some- times hundreds of kilometers downstream. The larvae and early juveniles of most stream fishes tend to use nearshore areas with relatively slow- moving water near cover, vegetation, or sharp ver- tical relief. 1Contribution 42 of the Larval Fish Laboratory, Colorado State University. Larval fish size largely dictates the characteristics of suitable nursery habitat and the role larvae play in the aquatic system. In southeastern fresh waters, fish larvae may be as small as 2 mm in total length (TL) at hatching (e.g., white crappie [Pomoxis an- nularis]) and early larvae of some (e.g., striped bass [Morone saxatilis], freshwater drum [Aplodinotus grunniens], and emerald shiner [Notropis atherinoi- des]) inhabit pelagic waters and constitute a part of the planktonic community. These early pelagic lar- vae may be food for predatory copepods, whereas later larvae of these fishes may reverse the roles and prey on the copepods. Most fish larvae feed on rotifers and other small zooplankton; however, some are piscivorous and may consume other fish larvae nearly as long as themselves. Clark and Pearson (1979) observed fish larvae in the stomachs of over 25% of small (4-5 mm standard length [SL]) fresh- water drum larvae. For this species, piscivory decreased with size and was not observed in larvae or early juveniles over 9 mm SL. For most stream fishes, extremely high mortality during the embryonic and larval period is normal and accommodated by their reproductive strategy. En- vironmental effects that substantially add to, or reduce, natural larval fish mortality can have a cor- responding effect on the eventual size of the. adulV population. The effects of changes in habitat and community structure on fish larvae can be quite~ ferent from those on juven~~n4 ~'.:~~::~ most common effect is loss 6r alteration of lar 'al fish habitat. Fish larvae C~m be particularly semlttive to physical and chemical water pollution. Biotic altera- tions that might not be expected to have an adverse effect on native fishes, such as the introduction of exotic forage species, might indeed affect native populations by excess predation on, or competition with, their larvae. Fish larvae of many species are especially vulnerable to entrainment in water with- drawal systems for irrigation, domestic and indus- trial water supplies, and power plant cooling. , ~