Factors influencing diet and prey selection of the sculpin Cottus cognatus and Myoxocephalus thompsonii

Selective feeding by fish can have profound impacts on prey community structure and food web dynamics in aquatic ecosystems, but few studies have examined prey selection in benthic fishes. In this study, diet and prey selection dynamics were analyzed for two benthic fish from Lake Michigan, slimy sculpin Cottus cognatus and deepwater sculpin Myoxocephalus thompsonii (Family Cottidae). The benthic amphipod Diporeia was an important prey for both sculpin, but relative frequencies of other prey in sculpin diets differed between species. For example, Mysis relicta was significantly more common in the diet of deepwater sculpin, whereas chironomids (Order: Diptera; Family: Chironomidae) were more important prey for slimy sculpin. These patterns suggested differences in prey selection between the two sculpin species. This was confirmed by a field-based study of sculpin prey selection, which showed that chironomids and Diporeia were preferentially consumed by slimy sculpin, whereas Mysis was preferentially consumed only by deepwater sculpin. Variation in prey-species selection within and between sculpin species was associated with differential prey capture and encounter rates. For example, due to differences in prey evasiveness, prey capture success by sculpin in laboratory feeding trials was greatest for chironomids, lowest for Mysis, and intermediate for Diporeia. This pattern was consistent with trends in slimy sculpin prey-species selection observed in the field. Similarly, greater occurrence of Mysis in deepwater sculpin diets was partly attributable to higher capture success with mysid prey. Encounter rates with Mysis also may be higher for deepwater sculpin than for slimy sculpin due to the former's larger reactive volume. The larger reactive volume of deepwater sculpin may give it a competitive advantage over slimy sculpin in areas of sympatry, a hypothesis supported by the relative scarcity of slimy sculpin at depths where deepwater sculpin are abundant. This study also showed that sculpin are size-selective predators. Results suggested that partitioning of prey size may be one way to alleviate interspecific competition for prey such as Diporeia that were common in the diets of both sculpin.