Dale Hanson | USGS Science

USGS2026

Status and trends of pelagic and benthic prey fish populations in Lake Michigan, 2025

Fall bottom trawl (fall BT) and lakewide acoustic (AC) surveys are conducted annually to generate indices of pelagic and benthic prey fish densities in Lake Michigan. The fall BT survey has been conducted each fall since 1973 using 12-m trawls at depths ranging from 9 to 110 m at fixed locations distributed across seven transects; this survey estimates densities of seven prey fish species [i.e., Alewife ( Alosa pseudoharengus ), Bloater ( Coregonus hoyi ), Rainbow Smelt ( Osmerus mordax ), Deepwater Sculpin ( Myoxocephalus thompsonii ), Slimy Sculpin ( Cottus cognatus ), Round Goby ( Neogobius melanostomus ), Ninespine Stickleback ( Pungitius pungitius )]. The AC survey has been conducted each late summer/early fall since 2004 (except 2020). The 2025 AC survey consisted of 26 transects [470 km total (292 miles)] covering bottom depths ranging from 5 to 259 m and 44 midwater trawl tows at 1.4 to 82.4 m fishing depth; 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 AC survey, total biomass density of prey fish equaled 9.3 kg/ha, continuing a recent trend of index values above the long-term average of 5.4 kg/ha. For the fall BT, total biomass density of prey fish equaled 3.4 kg/ha, close to values observed since 2014 and well below historic numbers and those observed earlier in the 2000s. Over the period both surveys have been conducted (2004-2025), the total biomass density index had trended downward in the fall BT through the mid-2010s and appears to have stabilized at low values, while the AC survey biomass density index has remained relatively stable over the time series. Mean biomass of yearling and older (YAO) Alewife was 4.3 kg/ha in the AC survey and 0.45 kg/ha in the fall BT. Since 2014, annual survey results suggest that the catchability of YAO Alewife for the fall BT is substantially lower than the AC survey. The 2025 AC survey YAO Alewife biomass density estimate was 57% higher than the average from 2004-2024. The Alewife population of Lake Michigan appears to be composed mostly of young fish and the proportion of age-4 and older Alewife was ~5% in both surveys. Age-0 Alewife numeric density from the AC survey was 259 fish/ha in 2025, lower than the long-term mean (487 fish/ha). Biomass density of large (≥120 mm) Bloater was 3.5 kg/ha in the AC survey and 1.9 kg/ha in the fall BT. The density of small (<120 mm) Bloater was 540 fish/ha in the AC survey, the second highest value in the time series. Meanwhile, small Bloater density estimated in the fall BT was only 6.1 fish/ha. Biomass density of large Rainbow Smelt (≥90 mm) was 0.69 kg/ha in the AC survey and 0.04 kg/ha in the fall BT survey. Numeric density of small (<90 mm) Rainbow Smelt was 541 fish/ha in the AC survey, the highest value in the time series, and 41 fish/ha in the fall BT. All four prey fish species indexed only by the fall BT had below-average biomass densities. Deepwater Sculpin biomass density was 0.21 kg/ha, which makes 15 of the past 16 years with biomass <1 kg/ha. Slimy Sculpin was estimated to be 0.03 kg/ha, an order of magnitude lower than the long-term average from the fall BT. Round Goby biomass density was 0.44 kg/ha and Ninespine Stickleback density was 0.20 kg/ha, the highest value since 2007.

Lake Michigan↗

USGS2025

Status and trends of pelagic and benthic prey fish populations in Lake Michigan, 2024

Fall bottom trawl (fall BT) and lakewide acoustic (AC) surveys are conducted annually to generate indices of pelagic and benthic prey fish densities in Lake Michigan. The fall BT survey has been conducted each fall since 1973 using 12-m trawls at depths ranging from 9 to 110 m at fixed locations distributed across seven transects; this survey estimates densities of seven prey fish species [i.e., Alewife ( Alosa pseudoharengus ), Bloater ( Coregonus hoyi ), Rainbow Smelt ( Osmerus mordax ), Deepwater Sculpin ( Myoxocephalus thompsonii ), Slimy Sculpin ( Cottus cognatus ), Round Goby ( Neogobius melanostomus ), Ninespine Stickleback ( Pungitius pungitius )] as well as age-0 Yellow Perch ( Perca flavescens ) and large (> 350 mm) Burbot ( Lota lota ). In recent years, wild juvenile (<400 mm) Lake Trout ( Salvelinus namaycush ) have also become more common in the fall bottom trawl. The AC survey has been conducted each late summer/early fall since 2004 (except 2020). The 2024 AC survey consisted of 24 transects [468 km total (291 miles)] covering bottom depths ranging from 16 to 173 m and 38 midwater trawl tows at 4 to 72 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. In spring of 2024, an additional spring bottom trawl survey (spring BT) was implemented across six of the transects sampled in the fall and sites ranged in depth from 9 to 237 m. The goal of the spring BT, conducted annually since 2021 with differing levels of effort, was to explore seasonal differences in biomass density and distributions of key prey species, most notably Alewife. Total prey fish biomass density from the spring BT was 5.7 kg/ha. For the AC survey, total biomass density of prey fish equaled 10.8 kg/ha, more than double the long-term average (20042023) of 5.1 kg/ha but 4.0 kg/ha lower than the 2023 estimate. For the fall BT, total biomass density of prey fish equaled 2.1 kg/ha, the lowest value since 2020 and 69% lower than the average from 2004-2023 (6.8 kg/ha). The 2024 fall BT biomass density was only 6.3% of the average over the entirety of the time series (1973-2023; 33.1 kg/ha). Over the period both surveys have been conducted (2004-2024), total biomass density has trended downward in the fall BT (despite a high 2022 estimate) and remained relatively stable in the AC survey. Deepwater Sculpin and Bloater were the most common species (by biomass) among prey fishes in the spring BT while the AC survey and fall BT reported co-dominance of Bloater and Alewife. Mean biomass of yearling and older (YAO) Alewife was 1.30 kg/ha in the spring BT, 4.7 kg/ha in the AC survey, and 0.68 kg/ha in the fall BT. Since 2014, annual survey results suggest that the catchability of YAO Alewives for the fall BT is substantially lower than the AC survey. Like previous spring surveys, Alewives were aggregated in deeper habitats, with 93% of biomass collected between 110 and 201 m. Results of the 2024 spring BT align with past spring surveys and do not suggest that spring bottom trawling provides a better index of age-2 and older Alewives than fall bottom trawling, even with adjustments for differences in habitat use. However, the spring BT does appear to index age-1 Alewives more effectively than the fall BT. The 2024 AC survey YAO Alewife biomass density estimate was 77% higher than the average from 2004-2023. The Alewife population of Lake Michigan appears to be composed mostly of young fish and the proportion of age-4 and older Alewives was <1.8% in each of the three surveys. Age-0 Alewife numeric density from the AC survey was 510 fish/ha in 2024, slightly higher than the long-term mean (486 fish/ha). Biomass density of large (≥120 mm) Bloater was 5.2 kg/ha in the AC survey and 0.76 kg/ha in the fall BT, while total Bloater biomass in the spring BT was 1.8 kg/ha - all three estimates were much lower than what was estimated by the fall BT between 1981 and 1998. The density of small (<120 mm) Bloater was 456 fish/ha in the AC survey, the second highest value in the time series and potentially reflective of an above average 2024 year-class. Meanwhile, small Bloater density estimated in the fall BT was only 16 fish/ha. Biomass density of large Rainbow Smelt (≥90 mm) was 0.21 kg/ha in the AC survey and 0.03 kg/ha in the fall BT survey, continuing the trend of low large Rainbow Smelt biomass observed since 2001. Numeric density of small (<90 mm) Rainbow Smelt was 31 fish/ha in the AC survey and 143 fish/ha in the fall BT. All four prey fish species indexed only by the fall BT had below-average biomass densities regardless of trawling season. Deepwater Sculpin biomass density was 0.26 kg/ha, which makes 14 of the past 15 years with biomass <1 kg/ha. Spring BT Deepwater Sculpin biomass density (2.0 kg/ha) was higher than any fall BT estimate since 2006, likely reflective of including bottom trawls at greater depths in the spring than the fall. Slimy Sculpin was estimated to be < 0.04 kg/ha in the spring and fall BT, an order of magnitude lower than the long-term average from the fall BT. Round Goby biomass density estimates were low and similar across seasons (0.43 kg/ha in the spring and 0.10 kg/ha in the fall). Ninespine Stickleback density was 3.9 fish/ha in the fall BT and no fish were collected in the spring BT.

Lake Michigan↗

USGS2024

Status and trends of pelagic and benthic prey fish populations in Lake Michigan, 2023

Fall bottom trawl (fall BT) and lakewide acoustic (AC) surveys are conducted annually to generate indices of pelagic and benthic prey fish densities in Lake Michigan. The fall BT survey has been conducted each fall since 1973 using 12-m trawls at depths ranging from 9 to 110 m at fixed locations distributed across seven transects; this survey estimates densities of seven prey fish species [i.e., Alewife ( Alosa pseudoharengus ), Bloater ( Coregonus hoyi ), Rainbow Smelt ( Osmerus mordax ), Deepwater Sculpin ( Myoxocephalus thompsonii ), Slimy Sculpin ( Cottus cognatus ), Round Goby ( Neogobius melanostomus ), Ninespine Stickleback ( Pungitius pungitius )] as well as age-0 Yellow Perch ( Perca flavescens ) and large (> 350 mm) Burbot ( Lota lota ). The AC survey has been conducted each late summer/early fall since 2004, and the 2023 survey consisted of 20 transects [450 km total (336 miles)] covering bottom depths ranging from 12 to 248 m and 29 midwater trawl tows above bottom depths ranging 16 to 246 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. A spring bottom trawl survey (spring BT) was implemented across 3 of the transects sampled in the fall and sites ranged in depth from 18 to 164 m. The goal of the spring BT is to explore seasonal differences in biomass density and distributions of key prey species, most notably Alewife. Additionally, we conducted acoustic sampling while bottom trawling to evaluate the vertical distribution of fish relative to the height of the trawl. The abbreviated spring BT survey results indicated that Alewives were primarily offshore with peak biomass density at the 91 m bottom depth. There was no evidence of higher acoustic density above the trawl at depths of 18, 73, 128, and 146 m. At 91 and 164 m, acoustic density above the trawl was >2x that in the trawl path, but sample size at these two depths was low. For the AC survey, total biomass density of prey fish equaled 14.8 kg/ha, 223% higher than the longterm average (2004-2022) of 4.6 kg/ha and 8.8 kg/ha higher than the 2022 estimate. For the fall BT, total biomass density of prey fish equaled 3.6 kg/ha, about 50% lower than the average value from 2004-2022 (6.9 kg/ha). The 2023 fall BT biomass was an order of magnitude lower than the average over the entirety of the time series (1973-2022; 33.7 kg/ha). Bloater was the dominant species (by biomass) among prey fishes in the fall BT, while the AC survey reported dominance of Alewife. Mean biomass of yearling and older (YAO) Alewife was 10.3 kg/ha in the AC survey, and 0.7 kg/ha in the fall BT. Since 2014, catchability of YAO Alewives for the fall BT has been substantially lower than the AC survey. While limited in scope, the results of the 2023 spring BT do not suggest that catchability is substantially higher in the spring than the fall, which aligns with the 2022 survey results. Comparing the AC estimate to previous years, YAO Alewife biomass was 359% higher than the average from 2004-2022. Numeric density of age-0 Alewife from the AC survey was 1,205 fish/ha in 2023, which is the third highest in the time series and well above the long-term mean of 428 fish/ha. Biomass density of large (≥120 mm) Bloater in 2023 was 3.5 kg/ha in the AC survey and 2.1 kg/ha in the fall BT - each at least an order of magnitude lower than what was estimated by the fall BT between 1985 and 1997. Following a record high year in 2021 (1,034 fish/ha), the numeric density of small (<120 mm) Bloater was 142 fish/ha in the AC survey, similar to the long-term mean of 120 fish/ha. Meanwhile, small Bloater density estimated in the fall BT was 2 fish/ha. Biomass density of large Rainbow Smelt (≥90 mm) was <0.05 kg/ha in the AC and fall BT surveys, continuing the trend of low Rainbow Smelt biomass that has been observed since 2001. Numeric density of small (<90 mm) Rainbow Smelt was 119 fish/ha in the AC survey and 7 fish/ha in the fall BT, indicating a weak year-class. All four prey fish species sampled only by the fall BT indicated below average biomass densities. Deepwater Sculpin biomass density was estimated at 0.4 kg/ha, which makes 13 of the past 14 years when biomass was <1 kg/ha. Slimy Sculpin was estimated at 0.02 kg/ha, only 5% of the long-term average. Round Goby was estimated at 0.3 kg/ha, below the average biomass of 0.85 kg/ha since 2008 but similar to intermittent low values observed throughout the dataset. Ninespine Stickleback density was 1 fish/ha. Only 35 small (<100 mm) Yellow Perch were caught, indicating a weak Yellow Perch year-class in 2023.

Lake Michigan↗

USGS2023

Status and trends of pelagic and benthic prey fish populations in Lake Michigan, 2022

Fall bottom trawl (fall BT) and lakewide acoustic (AC) surveys are conducted annually to generate indices of pelagic and benthic prey fish densities in Lake Michigan. The fall BT survey has been conducted each fall since 1973 using 12-m trawls at depths ranging from 9 to 110 m at fixed locations distributed across seven transects; this survey estimates densities of seven prey fish species [i.e., Alewife (Alosa pseudoharengus), Bloater ( Coregonus hoyi ), Rainbow Smelt ( Osmerus mordax ), Deepwater Sculpin ( Myoxocephalus thompsonii ), Slimy Sculpin ( Cottus cognatus ), Round Goby ( Neogobius melanostomus ), Ninespine Stickleback ( Pungitius pungitius )] as well as age-0 Yellow Perch ( Perca flavescens ) and large (> 350 mm) Burbot ( Lota lota ). The AC survey has been conducted each late summer/early fall since 2004, and the 2022 survey consisted of 26 transects [570 km total (354 miles)] covering bottom depths ranging from 5 to 255 m and 37 midwater trawl tows above bottom depths ranging 5 to 232 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. In spring of 2022, an additional spring bottom trawl survey (spring BT) was implemented across six of the transects sampled in the fall and sites ranged in depth from 9 to 236 m. The goal of the spring BT was to explore seasonal differences in biomass density and distributions of key prey species, mostly notably Alewife. Total prey fish biomass density from the spring BT was 2.1 kg/ha. For the AC survey, total biomass density of prey fish equaled 6.2 kg/ha, 37% higher than the long-term average (2004-2021) of 4.5 kg/ha and 0.43 kg/ha higher than the 2021 estimate. For the fall BT, total biomass density of prey fish equaled 8.7 kg/ha, the highest value since 2013 and 21% higher than average value from 20042021 (6.8 kg/ha). The 2022 fall BT biomass density was still well below the average over the entirety of the time series (1973-2021; 34.3 kg/ha). Over the period both surveys have been conducted (2004-2021), total biomass density has trended downward in the fall BT (despite a high 2022 estimate) and remained relatively stable in the AC survey. Bloater was the dominant species (by biomass) among prey fishes in both the spring and fall BT, while the AC survey reported co-dominance of Bloater and Alewife. Mean biomass of yearling and older (YAO) Alewife was 0.38 kg/ha in the spring BT, 3.0 kg/ha in the AC survey, and 0.10 in the fall BT. Alewife were aggregated in deepwater habitats in the spring of 2022 (> 110 m). Since 2014, catchability of YAO Alewives for the fall BT has been substantially lower than the AC survey. Results of the 2022 spring BT do not suggest that catchability is substantially higher in the spring than the fall. Comparing the acoustic estimate to previous years, YAO Alewife biomass was 40% higher than the average from 2004-2021. An age-7 fish was recorded for the first time since 2009. Despite the rare catches of older fish, the Alewife age distribution still appears truncated, with age-1 fish as the most represented age class in all three surveys. Numeric density of age-0 Alewife from the AC survey was 7 fish/ha in 2022, which is the third lowest in the time series and well below the longterm mean of 452 fish/ha. Biomass density of large (≥120 mm) Bloater was 2.7 kg/ha in the AC survey and 4.4 kg/ha in the fall BT - each at least an order of magnitude lower than what was estimated by the fall BT between 1981 and 1998. Following a record high year in 2021 (1,037 fish/ha), the numeric density of small (<120 mm) Bloater was only 15 fish/ha in the AC survey. Meanwhile, small Bloater density estimated in the fall BT was 261 fish/ha, the highest value since 1990 and likely partially reflective of a large 2021 year-class. Biomass density of large Rainbow Smelt (≥90 mm) was 0.29 kg/ha in the AC survey and 0.12 kg/ha in the fall BT survey, continuing the trend of low Rainbow Smelt biomass that has been observed since 2001. Numeric density of small (<90 mm) Rainbow Smelt was 21 fish/ha in the AC survey and 2.7 fish/ha in the fall BT, indicating a weak year-class. All four prey fish species sampled only by the fall BT indicated below average biomass densities. Deepwater Sculpin biomass density was estimated at 0.41 kg/ha, which makes 12 of the past 13 years when biomass was <1 kg/ha. Slimy Sculpin was estimated at 0.10 kg/ha, the highest estimate since 2016 but still only 25% of the long-term average. Round Goby was estimated at 1.3 kg/ha, above the average biomass of 0.82 kg/ha since 2008 but similar to intermittent high values observed throughout the dataset. Ninespine Stickleback density was 1.5 fish/ha. Burbot biomass remained near record low levels, and no age-0 Yellow Perch were caught, indicating a weak Yellow Perch year-class in 2022.

Lake Michigan↗

USGS2022

2021 Lake Michigan lake trout working group report

No abstract available.

Lake Michigan↗

USGS2022

Status and trends of pelagic and benthic prey fish populations in Lake Michigan, 2021

Lake wide acoustic (AC) 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 includes 70 fixed locations distributed across seven transects; this survey estimates densities of seven prey fish species [i.e., alewife ( Alosa pseudoharengus ), bloater ( Coregonus hoyi ), rainbow smelt ( Osmerus mordax ), deepwater sculpin ( Myoxocephalus thompsonii ), slimy sculpin ( Cottus cognatus ), round goby ( Neogobius melanostomus ), ninespine stickleback ( Pungitius pungitius )] as well as for age-0 yellow perch ( Perca flavescens ) and large (> 350 mm) burbot ( Lota lota ). The AC survey has been conducted each late summer/early fall since 2004, and the 2021 survey consisted of 25 transects [507 km total (315 miles)] covering bottom depths ranging from 15 to 235 m and 42 midwater trawl tows covering bottom depths ranging 13 to 215 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 2.4 kg/ha, the 5th lowest estimate of the time series and well below the long-term average total biomass of 34.28 kg/ha. For the AC survey, total biomass density of prey fish equaled 6.61 kg/ha, 50% higher than the longterm average total biomass of 4.28 kg/ha. The AC survey reported bloater to be the dominant species (by biomass) among prey fishes, while the BT survey reported co-dominance of alewife, bloater, and round goby. Mean biomass of yearling and older (YAO) alewives in 2021 was 1.71 kg/ha in the AC survey and 0.504 kg/ha in the BT survey. Catchability of YAO alewives continues to be substantially lower for the BT survey since 2014. Comparing the acoustic estimate to previous years, YAO alewife biomass was 10% higher than the 2019 estimate and less than the average from 2004-2019. Numeric density of age-0 alewife from the AC survey was 352 fish/ha in 2021, which is 71% of the long-term mean of 499 fish/ha. The alewife age distribution remained truncated, with age-0 fish and age-1 fish dominating the population. Biomass density of YAO bloater was 3.7 kg/ha in the AC survey and 0.43 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 highest ever measured for the AC survey at 1,037 fish/ha while for the BT survey, it was 20 fish/ha. Biomass density of YAO rainbow smelt was 0.13 kg/ha in the AC survey and 0.005 kg/ha in the BT survey, continuing the trend of low rainbow smelt biomass that has been observed since 2001. Numeric density of age-0 rainbow smelt was 84 fish/ha in the AC survey and 1.9 fish/ha in the BT survey, indicating a weak year-class. All four prey fish species sampled only by the BT survey indicated below average biomass densities. Deepwater sculpin was estimated at 0.45 kg/ha, which makes 11 of the past 12 years when biomass was <1 kg/ha. Slimy sculpin was estimated at 0.05 kg/ha, the sixth lowest density ever measured. Round goby was estimated at 0.63 kg/ha, which was below the average biomass of 0.84 kg/ha since 2008. Ninespine stickleback density was < 1 fish/ha. Burbot biomass remained near record low levels, and only three age-0 yellow perch were caught in all trawls, indicating a weak yellow perch year-class in 2021.

lake Michigan↗

USGS2020

2019 Lake Michigan Lake Trout Working Group Report

This report provides a review on the progression of lake trout ( Salvelinus namaycush ) rehabilitation towards meeting the Salmonine Fish Community Objectives (FCOs) for Lake Michigan (Eshenroder et. al. 1995) and the interim goal and evaluation objectives articulated in A Fisheries Management Implementation Strategy for the Rehabilitation of Lake Trout in Lake Michigan (hereafter the "Strategy"; Dexter et al. 2011).

Lake Michigan↗

USGS2019

Status of pelagic prey fishes in Lake Michigan, 2018

Acoustic surveys were conducted in late summer/early fall during the years 2004-2018 to estimate pelagic prey fish biomass in Lake Michigan. Midwater trawling during the surveys as well as acoustic target strength provided a measure of species and size composition of the fish community for use in scaling acoustic data and providing species-specific abundance estimates. The 2018 survey consisted of 33 acoustic transects 648 km total (403 miles) and 52 midwater trawl tows. Bottom depth at sampling sites ranged from 5 to 245 m (16-804 ft). Mean prey fish biomass density was 8.5 kg/ha, which was 1.9 times higher than in 2017 and 2.4 times the long-term (15 years) mean. The numeric density of the 2018 alewife year-class was 52% of the time series average and 1.8 times the 2017 density. The 2018 cohort was 7% of total alewife biomass (5.2 kg/ha). In 2018 alewife comprised 61% of total prey fish biomass, while rainbow smelt and bloater were 2% and 37% of total biomass, respectively. Small bloater were extremely rare in 2018 and were only caught near Frankfort, Michigan. Their density (< 1 fish/ha) in 2018 was the lowest observed in the 2004-2018 period. Biomass density of rainbow smelt and bloater remain well below observed in the 1980s-1990s. Cisco are infrequently caught in this survey, including the past two years. In 2018 three adult fish were caught (> 400 mm), with two in Grand Traverse Bay and one south of Manistique, Michigan. These results indicate that cisco density is very low at the lake level.

Lake Michigan↗

USGS2018

Status of pelagic prey fishes in Lake Michigan, 2017

Acoustic surveys were conducted in late summer/early fall during the years 1992-1996 and 2001-2017 to estimate pelagic prey fish biomass in Lake Michigan. Midwater trawling during the surveys as well as target strength provided a measure of species and size composition of the fish community for use in scaling acoustic data and providing species-specific abundance estimates. The 2017 survey consisted of 34 acoustic transects [711 km total (442 miles)] and 40 midwater trawl tows. Mean prey fish biomass was 7.99 kg/ha [38.9 kilotonnes (kt = 1,000 metric tons)], which was 46% higher than in 2016 and 35% of the long-term (22 years) mean. The numeric density of the 2017 alewife year-class was 27% of the time series average and 0.6 times the 2016 density. This year-class contributed 15% of total alewife biomass (4.4 kg/ha). In 2017, alewife comprised 55% of total prey fish biomass, while rainbow smelt and bloater were 32% and 14 % of total biomass, respectively. Rainbow smelt biomass in 2017 (1.0 kg/ha) was 29% of the long-term mean and increased for the second time since 2008. Bloater biomass in 2017 was 2.5 kg/ha and 32% of the long-term mean. Mean density of small bloater in 2017 (120 fish/ha) was 80% of the long-term mean. Biomass density of large bloater increased to 2.2 kg/ha in 2017. This remains much lower than in the 1990s but likely shows evidence of recruitment of small fish observed in the past 5 years. Although prey fish biomass remains low relative to the 1990s, it did increase in 2017. This increase, along with higher-than-average survival of two recent alewife year classes, are likely a response to reduced predation pressure stemming from a reduction in the abundance of Chinook salmon.

Michigan, Wisconsin↗

USGS2017

2016 Lake Michigan Lake Trout Working Group Report

This report provides a review on the progression of lake trout rehabilitation towards meeting the Salmonine Fish Community Objectives (FCOs) for Lake Michigan (Eshenroder et. al. 1995) and the interim goal and evaluation objectives articulated in A Fisheries Management Implementation Strategy for the Rehabilitation of Lake Trout in Lake Michigan (Dexter et al. 2011); we also include data describing lake trout stocking and mortality to portray the present state of progress towards lake trout rehabilitation.

Report↗

USGS2016

Status of pelagic prey fishes in Lake Michigan, 2015

Acoustic surveys were conducted in late summer/early fall during the years 1992-1996 and 2001-2015 to estimate pelagic prey fish biomass in Lake Michigan. Midwater trawling during the surveys as well as target strength provided a measure of species and size composition of the fish community for use in scaling acoustic data and providing species-specific abundance estimates. The 2015 survey consisted of 27 acoustic transects (580 km total) and 31 midwater trawl tows. Four additional transects were sampled in Green Bay but were not included in lakewide estimates. Mean prey fish biomass was 4.2 kg/ha [20.3 kilotonnes (kt = 1,000 metric tons)], equivalent to 44.8 million pounds, which was 36% lower than in 2014 (31.7 kt) and 17% of the long-term (20 years) mean. The numeric density of the 2015 alewife yearclass was 25% of the time series average and nearly 9 times the 2014 density. This year-class contributed 8% of total alewife biomass (3.4 kg/ha). In 2015, alewife comprised 82.5% of total prey fish biomass, while rainbow smelt and bloater were <1% and 16.9% of total biomass, respectively. Rainbow smelt biomass in 2015 (0.02 kg/ha) was 74% lower than in 2014, <1% of the long-term mean, and lower than in any previous year. Bloater biomass in 2015 was 0.7 kg/ha and 8% of the long-term mean. Mean density of small bloater in 2015 (489 fish/ha) was slightly lower than peak values observed in 2008-2009 but was more than three times the time series mean (142 fish/ha).

Report↗

USGS2015

Status of Pelagic Prey Fishes in Lake Michigan, 2014

Acoustic surveys were conducted in late summer/early fall during the years 1992-1996 and 2001-2014 to estimate pelagic prey fish biomass in Lake Michigan. Midwater trawling during the surveys as well as target strength provided a measure of species and size composition of the fish community for use in scaling acoustic data and providing species-specific abundance estimates. The 2014 survey consisted of 27 acoustic transects (603 km total) and 31 midwater trawl tows. Four additional transects were sampled in Green Bay but were not included in lakewide estimates. Mean prey fish biomass was 6.5 kg/ha [31.7 kilotonnes (kt = 1,000 metric tons)], equivalent to 69.9 million pounds, which was similar to the estimate in 2013 (29.6 kt) and 25% of the long-term (19 years) mean. The numeric density of the 2014 alewife year-class was 3% of the time series average and was the lowest observed in the 19 years of sampling. This year-class contributed <1% of total alewife biomass (4.6 kg/ha). Alewife ≥age-1 comprised 99.5% of alewife biomass. Numeric density of alewife in Green Bay was more than three times that of the main lake. In 2014, alewife comprised 71% of total prey fish biomass, while rainbow smelt and bloater were 1% and 28% of total biomass, respectively. Rainbow smelt biomass in 2014 (0.08 kg/ha) was 66% lower than in 2013, 2% of the long-term mean, and lower than in any previous year. Bloater biomass in 2014 was 1.8 kg/ha, nearly three times more than the 2013 biomass, and 20% of the long-term mean. Mean density of small bloater in 2014 (122 fish/ha) was lower than peak values observed in 2007-2009 but was similar to the time series mean (124 fish/ha). In 2014, pelagic prey fish biomass in Lake Michigan was 71% of that in Lake Huron (all basins), where the community is dominated by bloater.

Michigan, Wisconsin, Indiana, and Illinois↗

USGS2014

Status of pelagic prey fishes in Lake Michigan, 2013

Acoustic surveys were conducted in late summer/early fall during the years 1992-1996 and 2001-2013 to estimate pelagic prey fish biomass in Lake Michigan. Midwater trawling during the surveys as well as target strength provided a measure of species and size composition of the fish community for use in scaling acoustic data and providing species-specific abundance estimates. The 2013 survey consisted of 27 acoustic transects (546 km total) and 31 midwater trawl tows. Mean prey fish biomass was 6.1 kg/ha (relative standard error, RSE = 11%) or 29.6 kilotonnes (kt = 1,000 metric tons), which was similar to the estimate in 2012 (31.1 kt) and 23.5% of the long-term (18 years) mean. The numeric density of the 2013 alewife year class was 6% of the time series average and this year-class contributed 4% of total alewife biomass (5.2 kg/ha, RSE = 12%). Alewife ≥age-1 comprised 96% of alewife biomass. In 2013, alewife comprised 86% of total prey fish biomass, while rainbow smelt and bloater were 4 and 10% of total biomass, respectively. Rainbow smelt biomass in 2013 (0.24 kg/ha, RSE = 17%) was essentially identical to the rainbow smelt biomass in 2012 and was 6% of the long term mean. Bloater biomass in 2013 was 0.6 kg/ha, only half the 2012 biomass, and 6% of the long term mean. Mean density of small bloater in 2013 (29 fish/ha, RSE = 29%) was lower than peak values observed in 2007-2009 and was 23% of the time series mean. In 2013, pelagic prey fish biomass in Lake Michigan was similar to Lake Huron, but pelagic community composition differs in the two lakes, with Lake Huron dominated by bloater.

Lake Michigan↗

USGS2012

Status of pelagic prey fishes in Lake Michigan, 2012

Acoustic surveys were conducted in late summer/early fall during the years 1992-1996 and 2001-2012 to estimate pelagic prey fish biomass in Lake Michigan. Midwater trawling during the surveys as well as target strength provided a measure of species and size composition of the fish community for use in scaling acoustic data and providing species-specific abundance estimates. The 2012 survey consisted of 26 acoustic transects (576 km total) and 31 midwater tows. Mean total prey fish biomass was 6.4 kg/ha (relative standard error, RSE = 15%) or 31 kilotonnes (kt = 1,000 metric tons), which was 1.5 times the estimate for 2011 and 22% of the long-term mean. The increase from 2011 resulted from increased biomass of age-0 alewife, age-1 or older alewife, and large bloater. The abundance of the 2012 alewife year class was similar to the average, and this year-class contributed 35% of total alewife biomass (4.9 kg/ha, RSE = 17%), while the 2010 alewife year-class contributed 58%. The 2010 year class made up 89% of age-1 or older alewife biomass. In 2012, alewife comprised 77% of total prey fish biomass, while rainbow smelt and bloater were 4 and 19% of total biomass, respectively. Rainbow smelt biomass in 2012 (0.25 kg/ha, RSE = 17%) was 40% of the rainbow smelt biomass in 2011 and 5% of the long term mean. Bloater biomass was much lower (1.2 kg/ha, RSE = 12%) than in the 1990s, and mean density of small bloater in 2012 (191 fish/ha, RSE = 24%) was lower than peak values observed in 2007-2009. In 2012, pelagic prey fish biomass in Lake Michigan was similar to Lake Superior and Lake Huron. Prey fish biomass remained well below the Fish Community Objectives target of 500-800 kt, and key native species remain absent or rare.

Lake Michigan↗