Comparison of Methods for Estimating Atlantic Cod Biomass from Acoustic and Trawl Surveys

Synoptic acoustic and trawl surveys were used to estimate the biomass and spatial distribution of spring-spawning Atlantic cod (Gadus morhua) in the Gulf of Maine cod spawning protection area.  A fishing vessel equipped with pole-mounted Simrad 38- and 120-kHz split-beam EK60 echo sounders surveyed once monthly from dusk to dawn along ten 8.2‑km parallel transects that covered a 80.8 km² area during April-July 2011.  During each survey, two bottom trawl vessels (one with a small mesh and one with a large mesh net) each made ten tows in parallel behind the acoustic survey vessel.   Acoustic estimates of cod biomass were compared among combination of factors: (1) species apportionment based on trawl catch vs. manual acoustic classification of echoes, (2) predicted vs. in situ target strength, (3) size of elementary distance sampling unit, and (4) dead zone correction methods.  The biomass and target strength estimates were also compared between the two frequencies for the survey where both were simultaneously collected.