Sculpin and round goby assessment, Lake Ontario 2012

Historically slimy sculpin Cottus cognatus were the most abundant native, benthic prey fish in
Lake Ontario and important prey for juvenile lake trout. Over the past 34 years, slimy sculpin
abundance has fluctuated, but generally decreased, with a substantial decline occurring in the
past 10 years. The 2012 slimy sculpin mean density (0.005 ind.·m^-2, sd=0.012, n=62) and mean
biomass density (0.058 g·m^-2 , s.d= 0.120, n=62) were the lowest recorded in the 27 years of
sampling using the original bottom trawl design. An absence of slimy sculpin less than 50mm
(age-0) in the past 10 years suggests population declines are the result of reduced recruitment
potentially due to predation or reduced reproductive effort. Over the entire time series, the depth
of maximum slimy sculpin abundance has steadily increased from 65m to 125m. Depthassociated
sculpin behavior may be a result of water clarity changes that intensify predation risk
at shallower depths or a food related response where sculpin have moved deeper to habitats that
still support low densities of Diporeia, a favored food source. In the fall of 2012, round goby
density (0.526 individuals·m^-2) was two orders of magnitude greater than slimy sculpin,
suggesting round goby are now the dominant benthic prey fish in Lake Ontario. Invasive
species, piscivory, and declines in native benthic invertebrates are likely important drivers of
slimy sculpin population dynamics.