Date of Award

2022

Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Biological Sciences (MS)

Administrative Home Department

Department of Biological Sciences

Advisor 1

Gordon Paterson

Committee Member 1

Jill Olin

Committee Member 2

Casey Huckins

Abstract

The Lake Superior lake trout population consists of siscowet, lean, humper and redfin ecological morphotypes or ‘ecotypes’. In addition to morphological differences, these ecotypes occupy different habitats in Lake Superior and partition various invertebrate and fish prey resources. In this study, I measured the stable isotopes of carbon (δ13C) and nitrogen (δ15N) and mercury concentrations among these ecotypes collected from Lake Superior waters surrounding Isle Royale. Average mercury concentrations were highest for redfins (1389 ng/g dry wt) followed by siscowets (823 ng/g), humpers (799 ng/g) and leans (519 ng/g) and differed significantly among the ecotypes (p < 0.05). This pattern in Hg contamination among the ecotypes was consistent even when data were corrected for differences in fish ages among the ecotypes. Diet predictions indicated that the predominance of diet energy (kJ/g) for redfins (52.2 %) and humpers (64.8 %) was gained from invertebrate prey for the size ranges of these ecotypes included here. In contrast, fish prey were predicted to be the primary source of dietary energy for the sizes of siscowets (63.0 %) and leans (72.6 %) sampled. Relationships between Hg concentrations and fish lengths also differed significantly (p < 0.05) among ecotypes and were concluded to reflect the extent to which insect or fish prey dominate ecotype diets. This difference was concluded to represent the increased time and energy required by redfin and humper ecotypes to forage on small bodied and low energy invertebrate prey. Stable isotope niche modeling demonstrated that siscowets occupy the largest niche in Isle Royale waters with humpers occupying the smallest. However, the δ13C and nitrogen δ15N results also demonstrated a high degree of overlap suggesting that none of these ecotypes occupy a unique niche in the waters surrounding Isle Royale. This study emphasizes the role of ecology in regulating mercury bioaccumulation among these sympatric lake trout.

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