Stable isotopes reveal niche separation among three piscivores in a northern mesotrophic lake

Background/Question/Methods

The piscivorous Muskellunge (Esox masquinongy), Northern Pike (Esox lucius), and Walleye (Sander vitreus) have strong influences on trophic structure and energy flow in North American lakes.  All three species are popular gamefish with both recreational and economic value, often coexisting in individual lakes either naturally or through stocking efforts.  Given the high degree of piscivory in all three species, there is high potential for interspecific competition and niche overlap.  However, despite their ecological and economic importance, the degree of niche overlap among these three species is poorly known.  We assessed niche overlap by measuring the stable isotopes ¹³C and ¹⁵N in Walleye, Muskellunge, Northern Pike, and their potential prey fish in Elk Lake, Minnesota.  

Results/Conclusions

Results indicated three general groups of prey fish based on δ¹⁵N and δ¹³C levels.  Cisco (Coregonus artedi) had high δ¹⁵N and low δ¹³C due to its deep-water, pelagic niche. Yellow Perch (Perca flavescens) had high δ¹⁵N due to higher levels of piscivory and also high δ¹³C due to its more littoral niche. Several species of minnows and sunfish had low δ¹⁵N from feeding on aquatic invertebrates and high δ¹³C due to their littoral niche.  The three piscivore species we evaluated appeared to have significantly different diets and niches, each focusing to a greater extent on one of the three groups of prey fish.  Overall, Muskellunge had significantly lower δ¹³C than both Northern Pike and Walleye, while Northern Pike had significantly lower δ¹⁵N compared to Muskellunge and Walleye.  This indicates Muskellunge prey more heavily on pelagic Cisco and Walleye feed more heavily on piscivorous Yellow Perch, while Northern Pike feed on a mix of Yellow Perch, as well as the sunfish and minnow assemblage.  Our results indicate the response of Walleye, Northern Pike, and Muskellunge to changes in prey fish abundance will depend on the specific species of prey fish in lakes.  Of particular concern is the reliance of Muskellunge on Cisco, as Cisco populations are threatened by eutrophication and climate change.  Our results indicate Muskellunge would be impacted most by reduced abundance of Cisco, potentially forcing Muskellunge to alter their niche and compete more directly with either Walleye or Northern Pike.