Stable Isotope Analyses Of Food Web Structure And Diet Of Lake Superior Nearshore Native And Introduced Fishes

Abstract

The objective of this research was to delineate the trophic relationships among the nearshore salmonid complex and other nearshore predators within Lake Superior’s Western Arm to provide new insight into interactions among these potentially competing fishes. The focus of our study was to measure both trophic niche overlap and trophic position based on carbon and nitrogen stable isotope ratios. For the analysis, we used two main collections: angler-caught fish and MN DNR survey-captured fish. Among species, the nitrogen stable isotope ratio (15N) range was large, >5‰, indicating that our study species occupy ~2 trophic positions: omnivores (e.g., Steelhead Oncorhynchus mykiss, Coho Oncorhynchus kisutch) and piscivores (e.g., Siscowet Lake Trout Salvelinus namaycush). The carbon stable isotope ratio (13C) range among species was consistent with a mix of pelagic and benthic diet habits. Combined, the 13C and 15N values revealed some species have a relatively distinct trophic niche (e.g., Siscowet Lake Trout) whereas other species form an overlapping complex of species that occupy similar trophic niches (e.g., Walleye Sander vitreus and Lean Lake Trout or Steelhead and Chinook Salmon Oncorhynchus tshawytscha). A dual isotope mixing model revealed Siscowet Lake Trout, Lean Lake Trout, and Walleye all have the potential for diet competition since they all have a high energetic contribution from Rainbow Smelt, though Siscowet Lake Trout has a diverse diet whereas Lean Lake Trout and Walleye both heavily rely on Rainbow Smelt. Further, Chinook Salmon, Coho Salmon, and Pink Salmon exhibited high amounts of overlap with one another, largely relying on Diporeia spp. and other invertebrates. The management implications of this study are two-fold. First, introduced salmonids are in apparent existing resource competition for benthic invertebrates, which represents a long-term shift in trophic position. Second, given high niche overlap among introduced salmonids, routine monitoring of benthic invertebrates and supporting bioenergetic modeling to determine carrying capacity could help to further population targets for management actions.