Discard Mortality of Sea Scallops Following Capture and Handling in the Sea Scallop Dredge Fishery - Final Report

The focus of sea scallop, Placopecten magellanicus, management over the past 20 years has been to encourage the harvest of larger animals. This has been accomplished through a series of management measures including gear modifications, effort controls, crew size limitations and spatial management to protect juvenile scallops. While these measures have been effective in reducing the harvest of small scallops, their capture does still occur. Central to fully understanding the impact of the fishery on the resource, is a comprehensive estimate of the non-harvest mortality associated with commercial operations. Non-harvest mortality can be broken down into a number of different processes, with discard mortality being a major category. Discard mortality (DM) is the rate of mortality associated with animals that are captured and subsequently released due to primarily market factors. The latest stock assessment for sea scallops assumes that 20% of all animals discarded will die. There is considerable uncertainty associated with this estimate that is based on a single older tagging study and studies examining a non-Placopecten species under different biotic and abiotic conditions

Discard Mortality of Sea Scallops Placopecten magellanicus Following Capture and Handling in the U.S. Dredge Fishery

Discard mortality can represent a potentially significant source of uncertainty for both stock assessments and fishery management measures. While the family Pectinidae is considered to be robust to the capture and handling process, understanding species-specific discard mortality rates is critical to characterize both population dynamics and to develop regulatory measures to meet management objectives. The discard mortality rate for the U.S. dredge fishery of sea scallop Placopecten magellanicus was estimated empirically via a retention study aboard industry vessels under commercial conditions. Over 16,000 sea scallops were assessed via a composite index of scallop vitality that consisted of semiqualitative measures of both overt trauma (shell damage) and response to stimuli. Results indicate that overall sea scallop discard mortality was 21% and consistent with the values currently assumed in the stock assessment. Survival mixture models support the utility of a simple metric of physical trauma as an effective predictor of mortality. Exposure time was also identified as a positively correlated factor that was important in describing the discard mortality process. Application of experimental results highlight the need to consider some operational characteristics of the fishery to reduce potential discard mortality

Novel Tools and Techniques to Investigate and Reduce the Impacts of Capture-and-Handling

Fish have been an important food resource for humans throughout history, but with growing populations increasing the demand for this resource, many fish stocks have become overfished. In response, management has traditionally addressed overfishing by establishing regulations that reduce the directed fishing mortality associated with harvesting. However, these practices often do not take into account the mortality associated with fish that are incidentally captured and discarded (as bycatch). This discard mortality (DM) can represent a potentially large source of removals for species that are particularly susceptible to the stressors of capture-and-handling and/or discarded at high rates. It is therefore vital to understand overall DM in a fishery and the factors associated with capture-and-handling driving mortality. But this information can be very challenging to collect, with many species/fisheries having yet to be addressed. In light of these difficulties, my dissertation aimed to develop novel tools and techniques that improve our ability to evaluate and/or reduce the impacts associated with fisheries-interactions. In particular, this included (1) the development of a technique that addresses capture behavior as an underlying driver of mortality, (2) a novel application of electronic tags to better deduce fate in a family of frequently discarded fish, and (3) a new tool for forecasting where fisheries-interactions (i.e., incidental capture) are most likely to occur in both time and space. The information provided by these tools and techniques will help inform stock assessments and bycatch mitigation strategies, which in turn will help to promote the sustainability of bycatch species. Moreover, two species of elasmobranch [thorny skate (Amblyraja radiata) and oceanic whitetip shark (Carcharhinus longimanus)] were used as case study species in this dissertation. Both have been heavily overfished, but the impact of capture-and-handling remains poorly understood for each species. Thus, my dissertation contributes critical capture-related information for thorny skate and oceanic whitetip shark, which could aid in the development of future mitigation strategies to promote their recovery