Characterizing Shifts in Fish Community Diversity of Isle Royale National Park’s Inland Lakes

Gallagher, David; Hrabik, Thomas; Glase, Jay; Moore, Seth; Romanski, Mark

doi:10.36967/2316629

The inland lakes of Isle Royale National Park have remained relatively isolated from human impact, providing an opportunity to explore how fish diversity responds to climate change with minimal confounding effects. We analyzed fish assemblages of 23 lakes sampled in the 1920s, 1990s, and 2020s to 1) assess shifts in multiple diversity metrics, 2) identify lakes and species experiencing the most change, 3) explore environmental factors driving community structure, and 4) determine changes in length and abundance of common species. We used species-area curves, Permutational Multivariate Analysis of Variance (PERMANOVA), and multivariate dispersion tests to quantify differences in diversity between surveys, temporal beta diversity indices to identify lakes and species changing most, and Non-metric Multidimensional Scaling (NMDS) to examine lake factors influencing composition. We assessed changes in length and abundance using linear mixed-effect models and Wilcoxon signed-rank tests. Mean richness and species-area curves did not significantly differ between survey dates, although both declined slightly. PERMANOVA and dispersion tests indicated overall community composition and spatial variability remained mostly stable, but larger lakes became more dominated by cool-water species. Species gains and losses were associated with isostatic rebound rate, where lakes with fast and slow rebound rates tended to gain and lose species, respectively, compared to prior studies. Sculpin and stickleback species were most commonly lost, while blackchin shiner and pumpkinseed were most commonly gained. Larger lakes hosted more diverse communities, and lakes with higher elevation lacked northern pike and yellow perch. Abundance increased for northern pike but declined for yellow perch and walleye, while average size increased for northern pike and walleye. Overall, these results suggest the inland lake fish communities of Isle Royale have not homogenized over time, but assemblages have gained cool-water and lost several cold-water species. More consistent surveys will be required to determine which species will persist in the future and whether undetected species have been extirpated.

Characterizing Shifts in Fish Community Diversity of Isle Royale National Park’s Inland Lakes

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