The efficiency and selectivity of spring-toothed scallop dredges: a comparison of direct and indirect methods of assessment

摘要

The efficiency and selectivity of spring-toothed scallop dredges was assessed using a concurrent depletion experiment and diver survey of dredge tracks on a north Irish Sea fishing ground. Two size classes of the scallop, Pecten maximus (L. 1758), were examined: below minimum legal landing size (MLLS), (90-109 mm shell length (SL)) and above MLLS (>109 mm SL). Estimates of efficiency from the depletion experiment (24.3 percent and 29.5 percent respectively) were consistently lower than those from the diver surveys (38.0 percent and 40.7 percent respectively). This difference appeared to be due to inherent variation in the efficiency of scallop dredges rather than bias from either technique. This emphasizes the need for error terms to be built intoestimates of dredge efficiency. The diver survey also found that dredges were highly selective toward scallops greater than 90 mm SL, catching only 3.0 percent or less of individuals below this size. Consequently, the diver survey provided a much more accurate assessment of scallop size and age composition than dredge surveys. Dredge efficiency was also assessed for four species of benthic fauna commonly taken as by-catch in the local, fishery. Estimates of efficiency from the depletion experiment werefound to include a considerable amount of indirect fishing mortality. When efficiency was defined as total mortality due to fishing (the combination of catch and indirect fishing mortality), estimates from the depletion experiment and diver surveys werein close agreement. For two species, Luidia ciliaris (Phillipi 1837) and Cancer pagurus (L. 1758), these efficiency or total mortality estimates were approximately 45 percent and 68 percent respectively, emphasizing the impact scallop dredging might have on non-target species. In summary, we recommend that if possible, depletion experiments should be combined with diver surveys when assessing scallop dredge performance. Diver surveys provided additional information on dredge selectivity along with an improved measure of the variance in dredge efficiency estimates.