Larval Fish Distribution and Microhabitat Use in Free-Flowing and Regulated Rivers

Abstract

The ecology of larval fish is poorly documented in natural and altered rivers even though larval fish distributions and habitat requirements are often distinctly different from larger and older fish. Larval fish were collected every two weeks from late winter to late summer in 1988 and 1989 in the Tallapoosa River, a highly flow-regulated river, and the Cahaba River, an unregulated river, in Alabama. Larval fish were collected in offshore waters using a pushnet and in potential nursery habitat along the river margins using 1-im, 500-jum mesh seine. Nursery habitat was defined as shallow water (< 1.3 m) with currents less than the maximum sustained swimming speed of larvae (estimated as 8.4 cm/sec). A total of 17,675 larvae were collected in 1529 samples. Most (77%) larvae were collected in the unregulated river, and a majority of larvae (83%) were captured by seining in nursery habitat. Identifications and analysis were done at the family level, and taxonomic diversity was similar between rivers and between nearshore and offshore collections. The abundance and taxonomic composition of the larval fish assemblages varied by river, year, and Tallapoosa River study site. The most common families were catostomidae, cyprinidae, percidae, and centrarchidae. Nursery habitat at the most flow-regulated site had very low abundances of larvae and a distinctly different family composition from the other sites. All taxa commonly recorded in the Cahaba River and the least regulated Tallapoosa River study site used a statistically distinct subset of available microhabitats in nursery waters. There was little evidence for microhabitat specialization by larval fish at the most regulated Tallapoosa River site. Flow regulation appeared to reduce the abundance of larval fish in nursery habitat, alter taxonomic composition, and disrupt microhabitat relations. Flow regulation and the associated degradation of nursery habitat appears to be a potential threat to the conservation of natural and diverse riverine fish faunas.