The Role of Size in Trophic Niche Separation between Two Groundfish Predators in Alaskan Waters

Abstract

AbstractTo minimize negative effects of competition, predators must divide resources along one or more niche dimensions. Trophic niche separation is a principal method of resource partitioning in marine environments and is often assessed by quantifying diets of fishes with similar lengths. Taxonomic differences in allometric growth, however, suggest that the degree of gape limitation at a given body size varies among species. We studied resource overlap between two potential competitors in Southeast Alaska: Pacific Halibut Hippoglossus stenolepis and Arrowtooth Flounder Atheresthes stomias. We quantified length–gape relationships and assessed effects of fork length (FL; cm), gape height (GH; mm), and gape width (GW; mm) on interpretations of trophic niche separation. Gape sizes for Arrowtooth Flounder were nearly twice those of Pacific Halibut at similar FLs. Predators with smaller gapes exhibited moderate to high dietary overlap ( = 0.554; = 0.731), whereas those with larger gapes had lower dietary overlap and distinct diet compositions ( = 0.421; = 0.265). These findings suggest that the greatest potential for food competition is among small‐bodied Arrowtooth Flounder and relatively large‐bodied Pacific Halibut, with trophic niche separation increasing as these predators progress throughout their ontogeny. When comparing resource use among predators with similar FLs, we found high dietary overlap but significantly different diet compositions ( = 0.657). Thus, we postulate that GW is more appropriate than FL when quantifying trophic niche separation among sympatric fishes, especially when disparate length–gape relationships are detected. Studies of resource partitioning would benefit from examining multiple size metrics and explicitly considering differences in allometric growth, thereby addressing potential mechanisms for resource overlap among hypothesized competitors.