**ABSTRACT NOT FOR CITATION WITHOUT AUTHOR PERMISSION.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 rick@uwm.edu or rick.goetz@noaa.gov . Questions? Contact the GLFC via email at frp@glfc.org or via telephone at 734-662-3020.**





Reproduction in lake trout morphotypes surrounding Isle Royale


Frederick Goetz2, Shawn Sitar3, Charles Bronte4, Michael Seider5, Jay Glase6, Emily Latch7


2 School of Freshwater Sciences, U. Wisconsin-Milwaukee, 600 E. Greenfield Ave., Milwaukee, WI 53204, and NOAA NWFSC, Seattle, WA

3 Marquette Fisheries Research Station, Michigan Department of Natural Resources 484 Cherry Creek Road, Marquette, MI 49855

4 USFWS (Green Bay Fish and Wildlife Conservation Office), 2661 Scott Tower Drive, New Franken, WI 54229

5 USFWS, 2800 Lake Shore Drive, Ashland, Wisconsin 54806

6 National Park Service, 2800 Lake Shore Drive, Ashland, Wisconsin 54806

7 U. Wisconsin-Milwaukee, Biological Sciences Department, 600 E. Greenfield Ave., Milwaukee, WI 53204


February 2019




The reproductive biology of male and female lean, siscowet, redfin and humper lake trout was assessed by measuring the gonadosomatic index (GSI) and by histological analysis of the gonads over two years from spring to fall at sites surrounding Isle Royale National Park, Lake Superior. In addition, muscle lipid levels were measured in all fish using a Distell fatmeter and the gut contents of all fish were assessed. In general, GSIs increased from spring to summer in male and female siscowets, leans, redfins and humpers, and ovarian GSIs peaked by September. Postspawned ovaries and regressing testes were predominant in all morphotypes by October but were also present in September indicating that spawning occurred at least by September. Further, preovulatory siscowet females (GSIs >10) and siscowet males with regressing testes were observed in the spring and summer, indicating that spawning occurs at least in the spring and fall in siscowets at Isle Royale. Fecundity was positively related to size and age for all morphotypes. Humpers had the lowest fecundity as a function of length and age. Leans had higher fecundity as a function of weight than redfins and siscowets. The results demonstrate that deepwater siscowet lake trout reproduce during the spring and fall at Isle Royale and that leans, humpers and redfin reproduce during the fall. In lakes that exhibit sympatric populations of deep and shallow water arctic char ecotypes, the deepwater form also spawns in both spring and fall. Our data and those on arctic char suggest that there is an adaptive strategy for maintaining alternative spawning times in the deepwater char ecotypes in these populations. Generally, we found little relationship between lipid level and total length or age of fish. Lipid levels were highest in siscowets averaging 47.3% and lowest in redfins which averaged 15.2%. Mean lipid levels were 25% in leans and 18.3% in humpers. Lipid levels differed between female and male siscowets but not for other ecotypes. Higher lipid levels were measured in deeper water in leans, redfins, and siscowets. All lake trout ecotypes had similar diets with small fish mainly consuming invertebrates and shifting to consumption of prey fish at larger sizes. Schoener's diet overlap index indicated that all ecotypes had ecologically similar diets with the exception of the largest fish. Therefore, diet did not explain the differences in lipid levels measured among ecotypes. It is likely that lipid levels in lake trout ecotypes have a genetic basis that relates to the scope of depth they inhabit. Siscowets have the greatest depth distribution and vertical movement patterns among the ecotypes in Lake Superior and this may be facilitated by the high levels of intramuscular lipid.