Status of pelagic prey fishes in Lake Michigan, 2018

Abstract

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.