Standard Operating Procedures for Great Lakes Hydroacoustics: Importance of Recommendations Across Lakes

Hydroacoustic estimates of fish density are an important aspect of fisheries monitoring and research.  A Standard Operating Procedure for collection and analysis of hydroacoustic data in the Laurentian Great Lakes (GLSOP) was published in 2009 to facilitate comparisons across lakes.  Here we evaluate the importance of three particular recommendations from the GLSOP on density estimates of target species: 1) noise subtraction, 2) setting volume backscattering strength (Sv) thresholds from user-defined minimum target strength (TS) of interest (TS-based Sv threshold), and 3) calculations of an index for multiple targets to identify and remove TS from areas where these values are likely biased (Nv index). Eliminating noise had the predictable effect of decreasing density estimates in most lakes.  Using the TS-based Sv threshold decreased measured density in the middle and lower layers in the deepest lakes with abundant invertebrates (e.g., Mysis diluviana).  Correcting for biased in situ TS increased density up to 86% in the middle and lower layers of the shallower lakes, which had the highest fish densities.  The current recommendation by the GLSOP significantly influences acoustic density estimates, but the degree of importance of these recommendations is lake dependent.  Even so, applying the SOP recommendations as a standard across lakes, whether in the Laurentian Great Lakes or elsewhere, will improve ability to compare results among lakes.  We recommend further development of standards including  minimum TS, pulse duration, and analytical cell size for reducing effect of biased in situ TS on density estimates.