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Diet of the Torrent Sculpin, Cottus rhotheus, in the Cedar River, Washington: Effect of Season, Habitat Type, and Predator Size with Emphasis on Piscivory

Abstract Torrent Sculpin (Cottus rhotheus) can play an important role in riverine ecosystems in the Pacific Northwest, but factors that influence their feeding ecology are not well understood. To better understand how environmental conditions may influence the diet of Torrent Sculpin in the Cedar River, Washington, we collected diet information from fish across seasons and habitats. Numerically, aquatic insects were usually the dominant prey type for each size class in each sampling effort. Overall, Torrent Sculpin prey on a wide variety of aquatic invertebrates but can also take advantage of other prey types, such as fry of Sockeye Salmon (Oncorhynchus nerka), juvenile Chinook Salmon (O. tshawytscha), and eggs of Largescale Suckers (Catostomus macrocheilus), when these prey become abundant. In February and March, predation on Sockeye Salmon fry was substantially higher in large, deep pools (>2.5-m depth) than in either riffles or slow-water secondary habitats. Besides fry of Sockeye Salmon, the only salmonid observed in the diet of Torrent Sculpin was juvenile Chinook Salmon. Sculpin (Cottus spp.) were an important prey type of Torrent Sculpin ≥100 mm total length (TL) in each sampling effort. This was especially evident during the summer when sculpin represented over 90% of the diet by weight of Torrent Sculpin ≥100 mm TL. Feeding strategy plots demonstrated that Torrent Sculpin exhibit an opportunistic feeding strategy for large-bodied prey (for example, fish, oligochaetes, and large plecopterans) and a more generalized feeding strategy for small-bodied prey (chironomids and small-bodied ephemeropterans). Prey size, abundance, and activity are likely important factors influencing prey selection in Torrent Sculpin. Because of their size, abundance, wide range in habitat use, and breadth of diet, Torrent Sculpin are an important species in the Cedar River food web.