Factors Influencing By-catch and Discard Rates: Analyses from Multispecies/Multifishery Sea Sampling

Abstract

Factors influencing the species composition and magnitude of landings and discards were evaluated based on data from at-sea observations of 4 533 otter trawl tows. Data were collected from the USA mixed species otter trawl fisheries of the Georges Bank-Southern New England region, sampled during 1989–92. General linear models for main effects have related discard rates, total catch, and indices of species richness, diversity and evenness to temporal, spatial and operational variables associated with the fishing process (year, month, statistical reporting area, primary species sought, cod-end mesh size, vessel size, tow duration, total catch, total discards and depth). Discarding rates (proportion of the catch discarded) varied significantly both for individual species and for aggregated species by year, area, month, and target species. The effects of cod-end mesh size were variable, and confounded with year-class strength, particularly in the case of yellowtail flounder. Fisheries regulated by minimum fish and mesh sizes (e.g. for flounders and large gadoids) generally exhibited higher average discard rates and more variation than fisheries directed to species without such restrictions (e.g. for small pelagics, skates and others). The species composition and diversity of catches were significant functions of area, year, target species and month, as well as mesh size and tow duration. Year effects were more important for explaining variation in discard rates than total (multispecies) catches. This result is probably because large, partially-recruited year-classes differentially attract effort from other species targets, but most of the fleet landings are composed of mixed catches of species at moderate abundance levels. Multivariate approaches to analysis of sea sampling data offer important insights into the potential effectiveness of technological and area/time management measures for reducing fishery discards.