The Role of Predation in Age‐ and Size‐Related Habitat Use by Stream Fishes

I examined the effects of age and body size on intra- and interspecific habitat relationships for 15 species of fishes in a natural second-order warm water stream. Juveniles (age 0) of all taxa and adults (>age 0) of taxa with small maximum body size (darters and cyprinids) were at high densities predominantly in shallow riffle and raceway habitats. Adults of taxa with large maximum body size (catastomids and centrarchids) were at low densities predominantly in deep pool habitats. Based on the complementarity in depth distribution of large and small fishes, controlled experiments were conducted in a semi- natural experimental stream to assess (1) the effect of large centrarchid piscivores in pools on the habitat use of small fishes, (2) the influence of structural complexity of pool habitats on the interaction between centrarchid predators and their prey, and (3) the effectiveness of centrarchid predators at capturing small fishes when shallow refugia exist. In the ex? periments, juvenile hornyhead chub, white sucker, and smallmouth bass were allowed to select one of four physical habitats in the presence and absence of adult smallmouth bass: riffle, raceway, structurally simple pool, or structurally complex pool. Predators were re? stricted to pool habitats so that riffles and raceways were effective refugia. Predation by bass occurred even though effective refugia were present. However, the taxa exhibited considerable variation in susceptibility to predation; white suckers were most vulnerable and smallmouth bass least vulnerable. In the absence of bass, juveniles of all taxa preferred structurally complex or structurally simple pools, even though benthic insect densities were higher in riffles than pools. In the presence of bass, juveniles of all taxa were largely restricted to shallow riffle or raceway refugia, with the extent of the shift in distribution to shallow habitats related to the vulnerability ofthe taxa to predation. If juvenile fishes occurred in pools with centrarchid predators, juveniles were at low densities and only in pools with high structural complexity. These results suggest that the high density and extensive overlap in habitat use of small fishes in shallow habitats of small warm water streams is related to the increased risk of predation by centrarchids in pools.