Comparative feeding ecology of two sympatric rockfish congeners, Sebastes caurinus (copper rockfish) and S. maliger (quillback rockfish)

Abstract

Feeding ecology was compared between sympatrie populations of Sebastes congeners, S. caurinus (copper rockfish) and S. maliger (quillback rockfish), to determine the potential for interspecific competition for food resources. A total of 602 copper rockfish and 285 quillback rockfish were collected from rocky reefs in Saanich Inlet, British Columbia, Canada, from October 1986 to August 1990. All fish were collected in 15 to 40 m depth, where the species are sympatric. Seasonal and size-related differences in diet composition, niche breaidth and overlap of diets, diel variation in feeding, and quantity of food consumed were analysed to construct ecological profiles of the species. Overall, these two species had similar diets composed of pelagic and demersal fishes and crustaceans; both species consumed primarily demersal crustaceans throughout the year. Copper rockfish, however, consumed a greater proportion of pelagic fishes than quillback rockfish. Quillback rockfish had a greater proportion of pelagic crustaceans in their diet, especially during the spring and summer. The importance of fish in the diets of both species also increased with size. Copper and quillback rock fish consumed the greatest mass of food during the winter when feeding on juvenile herring (>90% of the mass of their diets). Copper rockfish also consumed a greater quantity of food than quillback rockfish during the winter. This winter feeding may be significant in the timing of the reproductive cycle in these rockfish. Values for niche overlap in food habits based on the mass of food resources consumed by copper and quillback rockfish were relatively high (>0.55) throughout the year, and in particular during the winter (0.99). Extensive niche overlap in the winter, however, occurred when niche breadths based on mass contribution were narrow. This was coincident with the presence of large schools of juvenile herring, and it was therefore assumed that herring were not a limited resource. Maximum niche overlap was therefore correlated with an abundance of a shared resource and hence did not indicate the presence of interspecific competition between copper and quillback rockfish.