| Summary: | I estimated population statistics, movement, and potential yield of burbot (Lota lota) in Wisconsin waters of Green Bay and Lake Michigan. After decimation by the sea lamprey (Petromyzon marinus), burbot have rebounded in the lake and are an abundant piscivore in Green Bay. In summers 1987 and 1988 6730 burbot in Green Bay and 195 in northwestern Lake Michigan were tagged and released. Additional burbot (4008) were collected from September, 1986 through October, 1989 and used for aging and calculating length-weight relations. Mean back-calculated lengths at age, determined from whole otoliths, showed curvilinear growth, rapid at early ages and nearing an asymptote by age 16 (66 cm). Females grew faster than males after age 2, and overlap in size ranges of each age class was evident for both sexes after age 1. Growth in length was similar throughout the lake and bay, but length-weight relations differed (ln(W)=3.021 ln(L)-11.82, Green Bay and Lake Michigan East of Door County, Wisconsin; ln(W)=3.27 ln(L)-13.26, Lake Michigan south of Door County, Wisconsin). In Green Bay, smaller burbot (<40 cm) were found in waters averaging 11-13°c, and larger burbot were found in waters averaging 9-10°c. Most of the tagged burbot were caught in waters deeper than 15 m, and bloated when the nets were brought to the surface. Bloated burbot were deflated by puncturing the gas bladder with the tagging needle. Handling mortality was low in both years of tagging (1-3%). Tag loss was 7 to 11% annually. Tag returns (431) revealed movement from Green Bay to Lake Michigan, but no burbot tagged in the lake were recaptured in the bay. Twenty-seven percent of the recaptured burbot had moved from the tagging areas. Mortalities in Lake Michigan (A=.52, u=.17 -- east of Door County, Wisconsin; A=.46, u=.08 -- east of Milwaukee, Wisconsin area) were calculated from catch curve analysis and mark/recapture data; in Green Bay mortalities (A=.59, u=.26) were estimated solely from mark/recapture data. The estimated size of the catchable burbot population in the Wisconsin waters of Green Bay in 1988 was 181,407 (±70,908 SD), a density of 0.82 burbot/ha. Burbot were most abundant in areas north of sturgeon Bay, Wisconsin, as dense as 75 (59 to 95) burbot/ha. Burbot were recruited into the drop net, trap net, and large-mesh gill net fishery at age 5 in Lake Michigan and age 6 in Green Bay. Yield and potential yield were low for both Lake Michigan and Green Bay, less than half of what was recruited. University of Wisconsin Sea Grant
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