To trawl or not to trawl: Questioning core assumptions of trawl placement choice in fisheries acoustics surveys

Abstract

In acoustic-trawl surveys, acoustic echosign needs to be biologically characterized, and if done correctly, will lead to accurate biomass estimates, robust stock assessments, and ensuing healthy fish stocks. However, methods for validating the core assumption that trawl placement choice yields a representative sample of the associated echosign (given an ideal net) are rarely described. Therefore we detail several such methods, using Pacific hake as a case study, exploiting both historic survey data as well as additional field experimental data. Specific methods focused on validation of (1) trawl effort spatially matching backscatter distributions, (2) trawled-location backscatter amounts matching all-location backscatter amounts, (3) trawled depth matching backscatter depth, and (4) spatial homogeneity of fish length within an aggregation. Application of the methods to the adult Pacific hake survey generally validated the assumption that trawl placement choice yielded a representative sample of the associated echosign, except for two instances. The backscatter from fished aggregations was greater than that from general aggregations in one of the two survey years analyzed. In addition, experimental field data detected a slight but significant trend of longer fish in the offshore portion of an aggregation. Neither of these occurrences are expected to yield a bias in the biomass estimate of Pacific hake.