The first step in the lake trout data collection assembly line is bringin the fish aboard using a gill-net lifter. - USFWS photo by Mark Holey

Fishery Biologists Stewart Cogswell and Charles Bronte remove lake trout from gill-nets. - USFWS photo by Rob Elliott

Biologists John Netto (right) and Dale Hanson (left) collect biological data from the netted lake trout. - USFWS photo by Rob Elliott

Biologists John Netto (left) and Rob Elliott (right) collect eggs from a ripe female lake trout. - USFWS photo by Mark Holey

A lake trout swims away after being interrogated by the GBFRO "assembly line." - USFWS photo by Rob Elliott

Staff from the Green Bay Fishery Resources Office (FRO) recently completed lake trout spawning surveys at Clay Banks and Northeim Reefs in western Lake Michigan.  Unlike the busy days of summer when recreational fishery and boating traffic is substantial, the FRO staff were often the lone enjoyer's of Lake Michigan's blustery days while they conducted surveys between Oct. 7 and 26.  These surveys, designed to coincide with the peak of lake trout spawning activity, take place as water temperatures dip below the lower 50's (~10 C).  This year Green Bay FRO biologists set 3,200' of gill-net at each reef to collect data on the spawning populations and the overall lake trout rehabilitation process in Lake Michigan. 

Generally speaking, gill-nets are set overtop spawning reefs in 15 - 40 feet of water and allowed to fish overnight.  Staff return early the next morning to lift nets and collect data on the catch.  The whole process resembles an assembly line.  Biologists can sample large numbers of fish and return them quickly back into the water.  Cool water temperatures and this efficient sampling scheme resulted in minimal survey-induced lake trout mortality. 

Specific objectives of the spawning surveys were wide-ranging.  First, biologists looked for evidence of natural reproduction in the lake.  All lake trout fingerlings and yearlings stocked by the Service are marked with a year-specific finclip, therefore the proportion of unclipped lake trout in the catch is used to assess the degree of natural reproduction contributing to current spawning activities.  This year, 16 of the 262 (six percent) lake trout sampled were unclipped.  Because fin-clipping efficiencies are generally 95-98 percent at the hatchery and clip reading error may occur when fins are regenerated, the six percent unclipped rate observed this year is not sufficient to suggest that significant natural reproduction of lake trout occurs at these reefs.   

Another survey objective was to continue the time-series of spawning data to identify general trends in spawning numbers.  Biologists compare year to year variation in the total number of lake trout sampled and the number of lake trout sampled per 1,000' of gill-net, i.e. catch per effort (CPE) as an indicator to determine if spawner densities are high enough to rebuild a naturally reproducing population.  Scales were sampled from each lake trout captured in the nets and coded wire tags (CWT's) were collected from adipose clipped fish.  The scales and coded wire tags are later read by biologists to determine the fish's age.  CWT's are especially valuable since each code is specific to a year-class, genetic strain and stocking location within the lake.  This information is used to determine lake trout movement patterns and to evaluate which strains of stocked lake trout exhibit the highest survival rate in the lake. 

Observations of sea lamprey wounding rates were another component of the data collection process.  Fish were measured for length and the number and severity of lamprey wounds were recorded.  This size specific lamprey wounding data is used to estimate an annual lamprey induced mortality rate on lake trout stocks for this management unit of Lake Michigan. 

Another objective accomplished in this survey was the collection of 50 lake trout in a specific size range (600-700 mm) for the Environmental Protection Agency's long-term contaminant monitoring program on the Great Lakes. 

Finally, and perhaps most importantly, egg samples from 31 spawning females were collected to assess the prevalence of Early Mortality Syndrome (EMS) on this year's lake trout reproduction efforts.  EMS is caused by a thiamine deficiency in the egg-laying females that comes from eating a thiaminase-rich food source, such as alewives.  Thiaminase is an enzyme that breaks down thiamin and its occurrence in alewife and other forage fish populations varies by time and location.  Although a thiamin deficient female will spawn, the deficiency is passed on to the eggs and affected eggs exhibit high mortality rates in larval and embryonic stages of development.  This year's collection of eggs by the FRO is the largest sample yet from Lake Michigan and the thiamin analysis will be run this winter at the USGS Northern Appalachian Research Laboratory.  Results will provide critical data to help future research efforts pinpoint the cause, location, and severity of the EMS problem and help assess its impacts to lake trout restoration in Lake Michigan. 

Green Bay FRO biologists will continue to process and analyze the data through the fall and winter so watch for more updates!