The three principal types of lake trout found in Lake Superior -- the lean, siscowet and humper. (From Moore and Bronte 2001, Transactions of the American Fisheries Society)

Siscowet, a deep water form of lake trout, is the top predator in Lake Superior and currently makes up most of its lake trout biomass. Anecdotal accounts indicate that siscowet made up some portion of the historical lake trout commercial fishery but estimates of harvest and relative abundance were lacking.

Charles Bronte of the Green Bay Fishery Resources Office and Shawn Sitar of the Michigan Department of Natural Resources examined historical commercial fishing catch reports from the lake trout fishery.  Using the location information provided by the anglers and past and contemporary knowledge of the depth distribution of the siscowet, they provide the first estimates of historical siscowet commercial harvest, fishing effort, and changes in relative abundance for Michigan waters of Lake Superior from 1929-1961.

Results from the study indicate that siscowet lake trout made up about 27 percent of the historical yield of lake trout in Michigan waters during this time period, but this composition varied greatly among areas.  The relative abundance of siscowet in their principal habitat (waters deeper than 250 feet) generally declined in most areas prior to an increase in fishing effort in the mid to late 1940s and the invasion of sea lamprey during the 1950s--these factors led to the collapse of near shore lean lake trout populations by the late 1950s. 

Modest levels of fishing effort prior to sea lamprey invasion were sufficient to cause declines in siscowet lake trout, and were likely related to low production potential of siscowet which grow slow and have a relatively late age at maturity.  Siscowet have high concentrations of omega-3 fatty acids, and interest has risen to exploit these fish as a source for fish oil.

These findings have implications for any re-developing fishery and suggest that over harvest is a real possibility.  This study on the historical relative abundance and harvest of siscowet will be published in the March issue of the Transactions of the American Fisheries Society.