WESPEN Online Order Form print this page
US Fish & Wildlife Service FieldNotes

Alpena Fish and Wildlife Conservation Office Conducts 2012 Fishery Independent Juvenile Lake Trout Survey in Northern Lake Huron

Region 3, October 2, 2012
Length histogram of lake trout
Length histogram of lake trout - Photo Credit: n/a
Map of Northern Lake Huron showing management units
Map of Northern Lake Huron showing management units - Photo Credit: n/a

During May and June 2012, staff from the Alpena Fish and Wildlife Conservation Office conducted a spring juvenile lake trout survey in northern Lake Huron (Alpena north to Hammond Bay). This study was designed to 1) determine the depth strata juvenile lake trout inhabit during spring and early summer; and 2) increase biological data collection from juvenile lake trout for use in statistical-catch-at-age population models. This survey was instituted in 2010 to address a growing concern about the survival of stocked lake trout in northern Lake Huron. Beginning in the mid-2000s, few juvenile lake trout (age 5 and younger, also known as “recruits”) were being captured in the standard summer survey gear (2-6” mesh) and management agencies were growing concerned about the impact that reduced survival of juveniles would have on the population and the fisheries it supports. Accurate forecasts of recruitment are required to properly manage fisheries and the survey was instituted to discern whether juvenile survival had truly declined or whether other factors, such as reduced growth, was causing fish to show up in the survey at later ages.

 

During the 2012 survey, staff from Alpena FWCO set 30 small-mesh (2-4” mesh) gill nets at randomly selected sites in two lake trout management units: MH-1 and MH-2 (see map at right). Biological data were collected from all lake trout encountered, including length, weight, sex and maturity, diet, visceral fat index, lamprey wounding, aging structures, and notation of fin clips. Tissue samples were taken from unclipped fish for subsequent DNA analysis (the absence of a fin clip suggests that a fish is of wild origin). Similar biological data were collected from non-target species.

During the first two years of the survey, the following depth strata were surveyed: <100 ft, 100-200 ft, and > 200 ft. The depth range sampled during this time period was 42-238 ft. Twenty-four lake trout less than 500 mm (a commonly accepted size cutoff for juveniles) were captured during the 2010 and 2011 surveys combined. Seven of the 24 juveniles were of wild origin. Two of the 24 fish were less than 300 mm (one was wild). The shallow, middle, and deep strata contained 21%, 62.5%, and 17% of the total catch, respectively. Catch-per-unit effort (CPE) of juvenile lake trout was 0.26 fish per 1000’ of survey gear in 2010 and 0.10 in 2011.

During 2012, we altered our survey design to sample different depth strata: <100 ft, 100-150 ft, and >150 ft. Furthermore, we increased gang length from 1,000 to 2,000 ft in the middle and deep strata. Clodophera, a benthic algae, completely coated a few of the shallower nets early in the 2012 survey, forcing us to abandon the shallowest stratum. As a result, we increased effort in the 100-200 ft depth range to maximize coverage. Our average fishing depth ranged from 150 to 160 ft, with a minimum depth of 108 ft and a maximum depth of 212 ft. The 2012 survey produced 36 lake trout below 500 mm, of which 27 were wild. Of the 36 juveniles captured, seventeen were below 300 mm and 16 of these were wild. Total juvenile CPE (wild plus hatchery) was 0.72, while CPE of wild juveniles was 0.56 across all sites. Wild juvenile CPE was 1.0 in lake trout management unit MH-1 and 0.37 in unit MH-2 during 2012, compared to 0.083 in MH-1 and 0.11 in MH-2 during 2011.

The Alpena FWCO is considering modifying the survey in 2012 by 1) eliminating 4” mesh to reduce adult catch; 2) reducing the number of depth strata to two (>100 ft and >150 ft); and 3) increasing gang length to 2,400 ft. Alpena FWCO staff feel that such changes would allow us to more accurately index juvenile lake trout numbers in northern Lake Huron

The 2012 survey results (see figure below) indicate a large increase in wild two- and three-year old lake trout in the population. This suggests that natural reproduction in 2009 and 2010 has added significantly to the lake trout population in northern Lake Huron. The past three years have given us a better understanding of the abundance and distribution of juvenile lake trout in northern Lake Huron, where positive steps toward lake trout rehabilitation continue.

Contact Info: Adam Kowalski, 989-356-5102 ext13, adam_kowalski@fws.gov