Barriers to upstream passage by two migratory sculpins, prickly sculpin (Cottus asper) and coastrange sculpin (Cottus aleuticus), in northern Puget Sound lowland streams

Abstract

We identified and characterized barriers to the upstream distribution by prickly sculpin (Cottus asper) and coastrange sculpin (Cottus aleuticus), based on their longitudinal distribution in 11 streams in the Puget Sound lowlands of Washington State. Eight streams had upstream limits associated with particular instream structures, and these were compared to distinguish features of barriers and non-barriers. In seven streams perch height greater than 15 cm resulted in a complete barrier. In one stream water velocity >4 m·s−1 created a partial barrier. Five of the eight barriers were one- or two-step fish ladders installed to facilitate upstream passage of Pacific salmon and trout. Water quality, physical habitat, and the presence of other fishes were not related to the presence or absence of sculpin species. Fish passage criteria in western North American streams are based on the swimming abilities of Pacific salmon and trout, without consideration for other, less motile native fishes. Structures installed to surmount previously identified impediments to upstream migration continue to inhibit sculpin movement, resulting in loss of upstream habitats for adults, with potential consequences to population size and trophic dynamics.