Great Lakes Prey Fish Populations: A Cross-Basin Overview of Status and Trends, 1978-2014

The assessment of Great Lakes prey fish stocks have been conducted annually since the 1970s. Although the surveys are conducted using bottom trawls, they differ among the lakes in the proportion of the lake covered, seasonal timing, trawl gear used, and the manner in which trawls are towed (across or along bottom contours). Because each assessment is unique, population indices were standardized to the highest value for a time series within each lake for coregonids Cisco (Coregonus artedi) and Bloater (C. hoyi), Rainbow Smelt (Osmerus mordax), Alewife (Alosa pseudoharengus), and Round Goby (Neogobius melanostomus). There was basin-wide agreement in trends of age-1 and older biomass for all prey species, with the highest concordance occurring for coregonids and Rainbow Smelt, and weaker concordance for Alewife. In general, trends in year-class strengths were less concordant across the basin and only coregonids showed statistical agreement across the upper Great Lakes. Although there was general basin-wide agreement in downward trends in prey fish biomass, factors likely driving these declines (top-down vs. bottom-up) appear to vary for each Lake. These cross-lake comparisons serve as a starting point for exploring differences in population dynamics of prey fishes across the Great Lakes basin.