Tim Desorcie | Geology Reports

USGS2023

Lake Trout population dynamics in the Northern Refuge of Lake Michigan: Importance of stocking rate

Objective The Northern Refuge of Lake Michigan was established under the auspices of the Great Lakes Fishery Commission in 1985 as an additional step toward restoring a self‐sustaining Lake Trout Salvelinus namaycush population to the lake. The overall goal of our study was to assess progress toward Lake Trout rehabilitation in the Northern Refuge through 2021. Methods We conducted annual (1992–2021) gill‐net surveys in the fall to assess the adult population and beam trawl surveys in the spring to assess naturally reproduced age‐0 Lake Trout. Result Spawner abundance averaged 45 fish • 305 m of gill net −1 • day −1 during 1992–1999, just 4 fish • 305 m of gill net −1 • day −1 during 2000–2014, and then 67 fish • 305 m of gill net −1 • day −1 during 2018–2021. Sea Lamprey Petromyzon marinus wounding rate decreased by nearly 90% between the 2000–2014 and 2015–2021 periods. The average annual mortality rate of adult Lake Trout decreased from 43% during the 1990s and 2000s to 22% during 2015–2021. All of these population dynamics were partly driven by changes in Lake Trout stocking rates, which declined by roughly 50% during 1994–1996 but then increased roughly twofold during 2006–2010 and remained at this higher level during 2010–2018. In 2021, early signs of natural recruitment of Lake Trout became evident. Beam trawling during June 2021 yielded a wild age‐0 Lake Trout, the first ever caught since the surveys began in 1992. In addition, 15 (4% of total catch) of the Lake Trout caught in the fall 2021 gill‐net survey were unclipped (wild) fish. Estimated ages of these wild fish indicated that natural recruitment had begun sometime around 2015. Conclusion With Alewife Alosa pseudoharengus abundance in Lake Michigan at record low levels, Sea Lamprey wounding rate greatly reduced, and Lake Trout spawner abundance at relatively high levels, the outlook for Lake Trout rehabilitation in the Northern Refuge holds promise.

Northern Refuge of Lake Michigan↗

USGS2020

Status and trends of pelagic and benthic prey fish populations in Lake Michigan, 2019

Lakewide acoustic (AT) and bottom trawl (BT) surveys are conducted annually to generate indices of pelagic and benthic prey fish densities in Lake Michigan. The BT survey has been conducted each fall since 1973 using 12-m trawls at depths ranging from 9 to 110 m and include 70 fixed locations distributed across seven transects; this survey estimates densities of seven prey fish species (i.e., alewife, bloater, rainbow smelt, deepwater sculpin, slimy sculpin, round goby, ninespine stickleback) as well as for age-0 yellow perch and large burbot. The AT survey has been conducted each late summer/early fall since 2004, and the 2019 survey consisted of 26 transects [513 km total (319 miles)] covering bottom depths ranging from 15 to 235 m and 30 midwater trawl tows covering bottom depths ranging 27 to 204 m; this survey estimates densities of three prey fish species (i.e., alewife, bloater, and rainbow smelt). The data generated from these surveys are used to estimate various population parameters that are, in turn, used by state and tribal agencies in managing Lake Michigan fish stocks. For the BT survey, total biomass density of prey fish equaled only 1.77 kg/ha, the 2nd lowest estimate of the time series and well below the long-term average total biomass of 35.7 kg/ha. For the AT survey, total biomass density of prey fish equaled 4.71 kg/ha, just above the long-term average total biomass of 4.25 kg/ha. Both surveys reported bloater to be the dominant species (by biomass) among prey fishes. Mean biomass of yearling and older (YAO) alewives in 2019 was 1.56 kg/ha in the AT survey and 0.07 kg/ha in the BT survey. Comparing the acoustic estimate to previous years, YAO alewife biomass was 76% lower than the 2018 estimate and less than the average from 2004-2019. Numeric density of age-0 alewife from the AT survey was only 35.1/ha in 2019, which is indicative of a poor year-class and only the fourth since 2004 with a density less than 100/ha. The alewife age distribution remained truncated, with age-2 fish dominating the population and only three alewife (out of 525 aged) that were older than age 3. Biomass density of YAO bloater was 3.08 kg/ha in the AT survey and 0.78 kg/ha in the BT survey- each at least an order of magnitude lower than what was estimated by the BT survey between 1981 and 1998. Numeric density of age-0 bloater was the lowest ever measured for each survey: 0/ha for the AT survey and 0.12/ha for the BT survey. Biomass density of YAO rainbow smelt was 0.03 kg/ha in the AT survey and 0.04 kg/ha in the BT survey, continuing the low rainbow smelt biomass that has been observed since 2001. Numeric density of age-0 rainbow smelt was 1.33/ha in the AT survey and 0.99 in the BT survey, indicating a weak year-class that follows three year-classes that exceeded 41/ha between 2016 and 2018. All four prey fish species sampled only by the BT survey indicated below average biomass densities. Deepwater sculpin was estimated at 0.47 kg/ha, which makes 9 of the past 10 years when biomass was <1 kg/ha. Slimy sculpin was estimated at 0.02 kg/ha, the second lowest density ever measured. Round goby was estimated at 0.39 kg/ha, which was below the average biomass of 0.96 kg/ha since 2008. Ninespine stickleback were only caught in one tow, and not surprisingly was estimated at a record low biomass. Burbot biomass remained near record low levels, and no age-0 yellow perch were caught, indicating a weak yellow perch year-class in 2019.

Lake Michigan↗