Photo 2. A cross sectional view of a thin sectioned lake whitefish otolith. This fish is estimated to be age 6.

Map 1. Lake whitefish management units and Green Bay NFWCO survey locations in Lake Michigan.

A lake whitefish entangled in the graded mesh gillnet is brought aboard the boat.

Biologist Ted Eggebraaten prepares to toss the buoy marking the end of the gillnet set.

Commercial fishers are very good at their jobs: catching fish.  In fact commercial catch rates may be high even when fish stock abundance is in decline, therefore, assessment surveys are needed to “ground-truth” trends observed in the commercial fishery.  In the 1836 Treaty Waters of the Upper Great Lakes, assessment surveys are a shared responsibility among the state of Michigan, Tribes, and the Service.  In Lake Michigan the Green Bay National Fish and Wildlife Conservation Office (NFWCO) is responsible for annual surveys at sites in three management units (Map 1).  These surveys employ graded mesh gillnets consisting of 2” to 6” mesh in 0.5” increments and have 2 purposes: 1) to determine the relative abundance of lake whitefish and lake trout in each management unit, and 2) to sample biological information (sex, lengths, weights, lamprey wounds, etc.) over a range of fish sizes.       

In 2008, a total of 61 lake whitefish and 248 lake trout were captured during the surveys.  Otoliths and\or scales were taken from each fish for ageing (Photo 2).  The relative abundance at each site was similar to recent years and averaged 1.4 to 3.3 lake whitefish captured per 1800’ survey net.  The biological data, including lamprey wounding data, from all assessment surveys across agencies are pooled by management unit and used in fish stock assessments.  The stock assessments incorporate both survey data and commercial fishery data into age structured models which generate estimates of fishing mortality, sea lamprey mortality, numbers of new fish entering the fishery, and the rate of fish growth.  Based on this information managers adopt harvest quotas that limit lake trout mortality to levels that enable populations to rebuild and, in time, become self sustaining.