**ABSTRACT NOT FOR CITATION WITHOUT AUTHOR PERMISSION. The title, authors, and abstract for this completion report are provided below.  For a copy of the full completion report, please contact the author via e-mail at hswanson@uwaterloo.ca. Questions? Contact the GLFC via email at frp@glfc.org or via telephone at 734-662-3209.**

trophic ecology and isotopic niche of humper lake trout (salvelinus namaycush) in lake superior” comparison with other morphotypes

Heidi Swanson², Justin Hoffman², Andrew Muir³, Mark Vinson⁴, Chuck Krueger⁵, Chuck Bronte⁶, Lisa Loseto⁷, Michael Hansen⁸, and Shawn Sitar⁹

² Department of Biology, University of Waterloo, Waterloo ON

³ Great Lakes Fishery Commission, Ann Arbor, MI

⁴ USGS Great Lakes Science Center, Ashland, MI

⁵ Michigan State University, East Lansing, MI

⁶ US Fish and Wildlife Service, New Franken, WI

⁷ Fisheries and Oceans Canada, Winnipeg, MB

⁸ USGS Great Lakes Science Center, Millersburg, MI

⁹ Michigan Department of Natural Resources, Marquette, MI

October 2017

ABSTRACT:

Four principal morphs of lake trout (Salvelinus namaycush, Walbaum 1792) persist in Lake Superior: leans, siscowets, humpers, and redfins. The trophic ecology and resource partitioning of lean and siscowet lake trout are well documented; however, comparatively little is known about the trophic ecology of the humper morph or the recently re-described redfin morph. Using samples collected from Superior Shoal and Stannard Rock, Lake Superior, during summers 2013 to 2015, the trophic ecology of all four lake trout morphs was characterized using stomach contents, fatty acid methyl esters, and stable isotope ratios (δ¹³C, δ¹⁵N, and δ³⁴S). Based on stomach contents, leans, siscowets, and redfin diets were mostly piscivorous (55-90% fish by mass); whereas, humper diets were more planktivorous (43-55% fish and 40-46% Mysis by mass). Fatty acid profiles showed that morph diets differed between sites. Stable isotope analysis revealed among-morph differences along a pelagic-profundal gradient (d³⁴S), but trophic position (d¹⁵N) and basal carbon sources (d¹³C) did not differ among morphs. Using the nicheROVER software package, probabilistic 4-dimensional trophic niches for each morph were modelled using stable isotope ratios (δ¹³C, δ¹⁵N, and δ³⁴S) and a multivariate axis of the variation in 30 dietary fatty acids. Contrary to our predictions, humpers had the widest dietary 95% niche region, and redfins the narrowest. Although stomach contents and stable isotopes indicated generalist feeding for all morphs, pairwise probability of overlap in trophic niche was determined to be < 50% between most morph pairs, and differentiation among niches was driven primarily by dietary fatty acids. Our results suggest that humpers may be good potential candidates for lake trout re-establishment programs due to their wide dietary niche region and low niche overlap with other morphs.