Ontogenetic Diet Shifts of Prickly Sculpin in the Lake Washington Basin, Washington

Abstract

We studied the ontogenetic diet shifts of prickly sculpin Cottus asper (the largest North American freshwater cottid) in the Lake Washington basin from a variety of habitat types, including fluvial and lacustrine. In all habitats, prickly sculpin progressively shifted to larger prey, such as fish and crayfish (Decapoda), as they increased in size. In offshore areas of Lake Washington, amphipods were the dominant prey by weight consumed by prickly sculpin of 75-124 mm total length (TL). Although generally uncommon numerically in the diet, fish (primarily small cottids) made up a large percentage of the diet by weight for prickly sculpin larger than 125 mm TL. In the lower end of the Cedar River, juvenile sockeye salmon Oncorhynchus nerka (23-30 mm TL) were most commonly observed in the diets of 50-99-mm prickly sculpin, while larger prickly sculpin tended to consume larger fish, including adult longfin smelt Spirinchus thaleichthys, lampreys Lampetra spp. (ammocoetes and adults), and small cottids. For each habitat type, diet overlap tended to decrease as size-classes became more dissimilar. Overall, the size of fish eaten was strongly related to prickly sculpin size, but the type of fish eaten influenced the relationship between prey size and predator size. The higher percentage contribution of fish to prickly sculpin diets seen here relative to other studies can be explained by the fact that (1) we collected many large prickly sculpin and sampled a wide variety of habitat types and (2) potential prey fish in the Lake Washington system are abundant and diverse. Our results, in combination with other research on the Lake Washington ecosystem, suggest that because of their size, abundance, wide range in habitat use, and breadth of diet, prickly sculpin are an especially important species in the food web of this system. Prickly sculpin are directly linked to the pelagic food web as both predators and prey, and they play a key role as benthic predators.