The benthic fish assemblage of the soft-bottom community of the Duluth-Superior Harbor before and after round goby invasion (1989-2011)

The soft-bottom benthic fish community of the St. Louis River and Duluth-Superior Harbor was examined yearly (20 of 23 years) from 1989 through 2011 using bottom trawling. Catch per unit effort (CPUE) and fish lengths were used to assess the impact of the round goby, which appeared in trawl records in 1998, on benthic native fishes. Logperch (Percina caprodes), spoonhead sculpin (Cottus ricei), johnny darter (Etheostoma nigrum), channel catfish (Ictlarus puncatus), tadpole madtom catfish (Noturus gyrinus) and bullhead catfish (Ictalurus spp.) plus the invasive Eurasian ruffe (Gymnocephalus cernuus) populations were assessed. After detection in the trawl record in 1998, round goby continued to increase in abundance through 2006. Logperch showed a 65% decline in median CPUE from 2.4 to 0.85, while johnny darter increased almost three-fold from 0.24 to 0.68 CPUE following round goby detection. Bullheads dropped from 7.5 CPUE to being virtually extirpated from the study area. However, their decline preceded round goby appearance, and was linked to the stocking of predator fish to combat the Eurasian ruffe. It is hypothesized that the presence of the Eurasian ruffe contributed to depressing the native fish population to the extent that the round goby appearance had less impact on the community. However, after 20 years of decline, the native fish appear to be rebounding due to declines in the Eurasian ruffe and round goby populations and improvements in water quality.