Environmental Biology of Fishes 51: 353–368, 1998.  1998 Kluwer A cademic Publishers. Printed in the Netherlands. D iel and distributional abundance patterns of fish embryos and larvae in the lower Columbia and D eschutes rivers Dena M. Gadomski & Craig A. Barfoot U.S. Geological Survey, Biological Resources Division, Columbia River Research L aboratory, 5501 A Cook- Underwood Road, Cook, WA 98605, U.S.A . (e-mail: dena_gadomski@nbs.gov) R eceived 11.2.1997 A ccepted 6.10.1997 Key words: ichthyoplankton, early life history, drift, reservoir, backwater, vertical distribution, nocturnal, introduced species Synopsis Diel and distributional abundance patterns of free embryos and larvae of fishes in the lower Columbia River Basin were investigated. Ichthyoplankton samples were collected in 1993 during day and night in the main- channel and a backwater of the lower Columbia River, and in a tributary, the Deschutes River. Fish embryos and larvae collected in the main-channel Columbia River were primarily (85.6%) of native taxa (peamouth Mylocheilus caurinus, northern squawfish Ptychocheilus oregonensis, suckers Catostomus spp., and sculpins Cottus spp.), with two introduced species (American shad A losa sapidissim a and common carp Cyprinus car- pio ) comprising a smaller percentage of the catch (13.3%). Similarly, in the Deschutes River native taxa [lam- preys (Petromyzontidae), minnows (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, common carp, and sunfishes (Centrarchidae)]. In all locations, all dominant taxa except sculpins were collected in significantly greater proportions at night. Taxon-specific differences in proportions of embryos and larvae collected at night can in some instances be related to life history styles. In the main- channel Columbia River, northern squawfish and peamouth were strongly nocturnal and high proportions still had yolksacs, suggesting that they had recently hatched and were drifting downriver to rearing areas. In con- trast, sculpin abundances were similar during day and night, and sculpins mostly had depleted yolksacs, in- dicating sculpins were feeding and rearing in offshore limnetic habitats. Taxon-specific diel abundance patterns and their causes must be considered when designing effective sampling programs for fish embryos and larvae. Introduction Despite the biological and socioeconomic impor- tance of the Columbia River to the Pacific North- west, comparatively few studies have addressed the ecology of nonsalmonid fishes in the Columbia Riv- er Basin. Knowledge of the ecology of fish larvae in this area is particularly scarce, although this infor- mation is important for effective fisheries manage- ment since survival during early life may strongly affect recruitment success (Houde 1987). Those studies that have been conducted on the ecology of fish larvae have examined littoral zones of the lower Columbia River (LaBolle et al. 1985), food habits of fish larvae in the upper Columbia River (Faurot & White 1994), marine ichthyoplankton of the Co- lumbia River estuary (Misitano 1977), and rearing habitats of white sturgeon, A cipenser transmonta- nus, larvae (Parsley et al. 1993). Because general early life history information on