DIEL AND DISTRIBUTIONAL ABUNDANCE PATTERNS OF FISH EMBRYOS AND LARVAEIN THE LOWER COLUMBIA AND DESCHUTES RIVERS

Diel and distributional abundance patterns of free embryos and larvae of fishes in the lower Columbia River Basin were investigated. Ichthyo plankton samples were collected in 1993 during day and night in the ma in-channel and a backwater of the lower Columbia River, and in a tribu tary, the Deschutes River. Fish embryos and larvae collected in the ma in-channel Columbia River were primarily (85.6%) of native taxa (peamo uth Mylocheilus caurinus, northern squawfish Ptychocheilus oregonensis , suckers Catostomus spp., and sculpins Cottus spp.), with two introdu ced species (American shad Alosa sapidissima and common carp Cyprinus carpio) comprising a smaller percentage of the catch (13.3%). Similarl y, in the Deschutes River native taxa [lampreys (Petromyzontidae), min nows (Cyprinidae), and suckers Catostomus spp.] dominated collections (99.5% of the catch). In contrast, 83.5% of embryos and larvae in the Columbia River backwater were of introduced taxa [American shad, commo n carp, and sunfishes (Centarchidae)]. In all locations, all dominant taxa except sculpins were collected in significantly greater proportio ns at night. Taxon-specific differences in proportions of embryos and larvae collected at night can in some instances be related to life his tory styles. In the main-channel Columbia River, northern squawfish an d peamouth were strongly nocturnal and high proportions still had yolk sacs, suggesting that they had recently hatched and were drifting down river to rearing areas. In contrast, sculpin abundances were similar d uring day and night, and sculpins mostly had depleted yolksacs, indica ting sculpins were feeding and rearing in offshore limnetic habitats. Taxon-specific diel abundance patterns and their causes must be consid ered when designing effective sampling programs for fish embryos and l arvae.