The role of fisheries closures in population assessments and management of marine benthic invertebrates: a Dungeness Crab case study

Date

2018-09-14

Authors

Dunham, Jason S.

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Abstract

Effective management of marine invertebrate populations is imperative to ensure fisheries remain sustainable. Spatial closures with unfished invertebrate populations can serve as scientific reference sites for evaluating impacts of fisheries management measures, and the extent of shifted baselines. There are two Dungeness crab fisheries closures (Vancouver Harbour and English Bay) in Burrard Inlet (BC) where crab harvesting is not permitted. From 2009-13, biannual fishery independent standardized trap surveys were conducted in closed and fished areas during the spring and fall. Crab biological metrics that were examined included: trap Catch Per Unit Effort, size, injuries, shell condition, discard ratios, sex ratio, and proportions of old males and sublegal males near the minimum size limit. Crabs in closed areas were tagged to provide movement information. Time series data collected since the early 1990s in two of the same areas were analyzed. A Remotely Operated Vehicle (ROV) was used to collect video imagery to estimate crab density and describe benthic habitats, and to collect physical data on water parameters. Field measures of handling injuries were obtained from commercial vessels. Trap soak studies were conducted to quantify injuries inflicted on crabs while captured in traps. Crab shell condition data collected from the commercial fleet were analyzed. Only Vancouver Harbour is an effective closure. Large males, the target of the fishery, were more abundant and bigger there, and these measures tended to increase between seasons. In contrast, in English Bay, similar to fished areas, large males were less abundant and smaller, especially at the end of fishing seasons. Vancouver Harbour is an effective closure because it is sufficiently large to retain adult crabs, has less edge habitat, and better enforcement. The unfished crab population in the Vancouver Harbour closure was used as a reference against which to compare characteristics of the exploited population to evaluate impacts of the main management measures: the minimum size limit, non-retention of females and soft crabs, and a seasonal soft shell closure. The exploited population exhibited lower abundance of large old males, smaller males, higher removals of, and injuries to, the biggest sublegal males, and higher rates of non-lethal injuries and mortality to all crab classes. In contrast, positive consequences of the management measures include sublegal male and female abundances, and female size not being affected by the fishery. Moreover, sublegal males were injured the least and had low injury-related mortality. The exploited crab population never recovered after the seasonal closure to the level of abundance achieved in the permanent closure. The unfished crab population provided: • insights into population dynamics not influenced by fishing pressure, • insights into population dynamics influenced by fishing pressure and context around the magnitude of changes that have occurred, • a means whereby biological and/or environmental influences can be separated from fishery impacts, • the foundation for challenging the accepted definition of ‘healthy’ populations as currently used in the precautionary approach policy. Unfished invertebrate populations should be formally incorporated into fisheries management by redefining the Healthy Zone to include two population states, exploited and unfished.

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Keywords

fisheries closures, marine invertebrate fisheries management, Dungeness crab, shifting baseline, precautionary approach

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