Siscowet lake trout from Lake Superior. These fish are the most numerous type of lake trout in Lake Superior and occupy depths greater than 250 feet. - FWS photo

Historically, Lake Superior contained many morphologically distinct forms of lake trout that occupied specific depths, locations, and spawned at specific times of the year. 

Today, as was likely the case historically, the siscowet morphotype is the most abundant.  Recent interest in harvesting siscowets to extract omega-3 oil will require additional knowledge of the biology and stock structure of these lightly exploited populations. 

The objective of this recently published study in the Transactions of the American Fisheries Society was to determine if shape differences exist among populations of siscowet across Lake Superior and whether these shape differences can be used to imply stock structure.  Morphometric or shape analysis was used to differentiate among siscowet sampled from 23 locations in Lake Superior. 

In the study, 31 distance measurements among 14 anatomical landmarks taken from digital images of fish recorded in the field were analyzed.  Cluster analysis separated fish into three geographic groups: Isle Royale, Eastern Michigan and Western Michigan but finer scales of stock structure were also suggested. 

Further analysis demonstrated that head measurements contributed to most of the observed variation, and that about 70% were correctly classified to their region of capture based on the shape differences.  This is the first study to present shape differences associated with location within a lake trout morphotype in Lake Superior.  

These results suggest stock structure on a small geographic scale, which should be reflected in managing harvest at similar spatial scales to prevent sequential overfishing of stocks that has plagued other species in Lake Superior in the past