Estimation of bycatch in shrimp trawl fi sheries: a comparison of estimation methods using fi eld data and simulated data

Bycatch, as used in the present study, is the incidental catch of nontarget organisms that occurs to some extent in almost all commercial fi sheries (Alverson, 1994). Some of these incidentally caught organisms may be protected species—such as marine mammals, marine turtles, and seabirds—or they may be fish or invertebrates that are either harvested as target species by other fi sheries, or species that fi shermen call “trash fi sh” because they have little or no economic value. Bycatch in most commercial fi sheries has only been a major issue since the 1980s—primarily because individuals caught as bycatch have historically been discarded at sea, leaving fi shery managers and the general public unaware of the extent of bycatch mortality. For many organisms, bycatch may be a signifi cant source of mortality, and inclusion of bycatch mortality in stock assessments or management plans may be critical for effective management. Because bycatch species are not usually landed, quantifying bycatch poses a very different problem from that of quantifying the catch of a target species. Several methods of quantifying bycatch have been tried, including the requirement that fishermen record catch and bycatch in logbooks (Walsh and Kleiber, 2001), use of research vessel surveys to model commercial fi shing (Nichols et al.1), and the placement of observers aboard fi shing vessels (Julian and Beeson, 1998). Although direct observation is the most accurate method, unless observer coverage of the fl eet is complete, estimation of bycatch from observation data requires sampling of the fl eet and then extrapolating from the samples (the observations) to the Estimation of bycatch in shrimp trawl fi sheries: a comparison of estimation methods using fi eld data and simulated data