Synoptic acoustic and trawl surveys of spring-spawning Atlantic cod in the Gulf of Maine cod spawning protection area

Abstract

Repeated acoustic and trawl surveys were performed in the Gulf of Maine cod spawning protection area (GOMCSPA) to: (a) describe the spatial and temporal distribution of spring-spawning Atlantic cod (Gadus morhua); (b) estimate their abundance and biomass; and (c) evaluate precision of the survey methods. A fishing vessel equipped with 38- and 120-kHz split-beam echo sounders surveyed once monthly from dusk to dawn along ten parallel transects that covered a 80.8km2 area during April–July 2011. During each survey, two bottom trawl vessels (one with a small mesh net and one with a large mesh net) each made ten tows in parallel behind the acoustic survey vessel. Cod abundance and biomass was derived from acoustic backscatter by a combination of methods: (1) species apportionment based on trawl catch vs. echo classification; (2) in situ vs. predicted target strength; (3) size of elementary distance sampling unit (EDSU) and statistical approach; and (4) with and without dead zone correction. The mean cod density based on echo classification and a 100-m EDSU resulted in a substantially lower coefficient of variation when the variance was estimated by geostatistics compared to any other method used. Based on echo classification, semivariogram modeling revealed that 67–77% of the variance in cod biomass density was explained by a spatial structural component at a range (correlation length) of 2.0–2.4km. Density maps, produced by ordinary kriging, showed cod were relatively widespread in the survey area in May, but congregated at higher densities in areas adjacent to two elevated bathymetric features. Most cod converged to a single location in June, and were at a higher concentration compared to the highest densities observed in May. This congregation decreased in size and density in July. The survey estimates of cod biomass were 184–494 mt in May, 138–617 mt in June, and 39–135 mt in July. Based on echo classification, the biomass for the GOMCSPA, extrapolated from these survey estimates, were 260–466 mt in May, 196–513 mt in June, and 91–198 mt in July. These results provide some evidence that adult Atlantic cod in spawning condition congregated within the GOMCSPA during the seasonal fishing closure, and that the biomass being protected by the closure may have represented 4–5% of the GOM cod spawning stock biomass at the time of the study.