**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NOT FOR CITATION WITHOUT AUTHOR PERMISSION.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.**
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
ABSTRACT:
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.