|Abstract or Summary
- Shallow-water habitats (< 0.5 m depth) of the upper littoral zone of John Day Pool of the Columbia River were heavily used by young fishes in comparison with lower littoral and limnetic waters. Through the season, 96.9% of the 64,700 larvae and juveniles collected (corrected for effort) were from upper littoral habitats. The sequential appearance of species and their change in life stage was the dominant ecological process identified in the ordination of sites and species. Spatially distinct species associations were identified in the cluster analysis, but they were temporally restricted because the composition of the community of young fishes changed continually. Three general patterns of habitat use by larval and juvenile fishes emerged: 1) a strongly littoral assemblage of native species of cyprinidae and catostomidae that was dominant in the tailrace and transition zones of the reservoir; 2) as association of introduced fishes with more lentic habitats (primarily centrarchidae), most abundant in the forebay and backwaters, and 3) larvae of two species, more limnetic in dispersal than others, rearing throughout the
reservoir. General patterns of habitat use by larvae were consistently reflected in correlations of their abundance with physical variables. Larvae of three littoral native species, northern
squawfish (Ptychocheilus oregonensis), largescale sucker (Catostomus macrocheilus), and peamouth (Mylocheilus caurinus), dominated the catch numerically (> 75%) , and were in great abundance during the day in the margin microhabitats (< 0.15 a depth). Young fish may inhabit the margins in response to reduced predation, higher water temperatures, or possible differential in availability of food. Structurally complex areas within the upper littoral and margins, which were provided by aquatic macrophytes and submerged terrestrial vegetation, were used by larvae in greater abundance than unvegetated sites. Competition among young fishes in the upper littoral habitats may be reduced, even though they were present in great abundance, because sizes of fishes at developmental stages and their seasonal occurrence differed among species. Larvae also exhibited ontogenetic changes in habits as they became more specialized in resource use. If
water-level fluctuations in the reservoir affect larval survival, the strongly-littoral native species would seem to be most adversely influenced.