Harvest and relative abundance of siscowet lake trout in Michigan waters of Lake Superior, 1929-1961

Siscowet, a deepwater morphotype of lake trout Salvelinus namaycush and the top predator in Lake Superior, currently makes up most of the lake trout biomass in this lake. Anecdotal accounts indicate that siscowets made up some portion of the historical lake trout commercial fishery, but estimates of harvest and relative abundance are lacking. By using the location information provided by historical fishers on monthly catch reports and past and contemporary knowledge of the depth distribution of siscowets, we provide the first estimates of historical siscowet commercial harvest, fishing effort, and changes in relative abundance for Michigan waters of Lake Superior from 1929 to 1961. Siscowets made up about 27% of the historical yield of lake trout in Michigan waters during this period, but the composition varied greatly among management units. The relative abundance of siscowet in its principal habitat (waters deeper than 80 m) generally declined in most management units before the increase in fishing effort in the mid to late 1940s and the invasion of sea lamprey Petromyzon marinus during the 1950s. These factors led to the collapse of nearshore lean lake trout populations by the late 1950s. Modest levels of fishing effort (around 2,000 kin annually) before sea lamprey invasion were sufficient to cause declines in siscowet and were probably related to the low production rates associated with the k-selected life history attributes of this deepwater morphotype.