Abstract
Substantial effort has been devoted to improving fisheries management for conservation of elasmobranchs (sharks, skates, and rays) over the last two decades in response to worldwide population declines. Porbeagles (Lamna nasus), are considered a data-limited species for stock assessments, and the goal of this dissertation was to fill research gaps for improving fishery management, conservation, and stock assessments for North Atlantic porbeagle. In Chapter 1management and conservation efforts related to North Atlantic porbeagle are reviewed, and available data sources (i.e., trade statistics, economic value, recreational fisheries, observer programs and electronic monitoring) that are not currently being used in conservation or stock assessments of this data-limited species are identified. By taking advantage of available data, efforts for new data collection may be more cost-effective for this data-limited species. Chapter 2is an interdisciplinary review of all available information (i.e., life history, genetics, and movement) of North Atlantic porbeagle to evaluate population structure. The review reveals different interpretations regarding the number of biological populations and stock boundaries among the organizations involved with the conservation and management of porbeagle in the North Atlantic. These inconsistencies can pose impediments to conserving porbeagle populations and achieving management objectives. In Chapter 3 the Northeast Fisheries Observer Program data is used to characterize porbeagle bycatch in seven fleets that have not previously been included in stock assessments and management of porbeagle (bottom otter trawl, haddock separator trawl, bottom otter trawl ruhle, midwater otter trawl, paired midwater trawl, fixed gillnet, and drift gillnet). Porbeagles had low at-vessel mortality in trawl gears (3 to 32%) and relatively high mortality in gillnets (81 to 93%).The Standardized Bycatch Reporting Methodology was adapted to estimate annual dead discards from the fleets that caught most porbeagles (bottom otter trawl, haddock separator trawl, fixed gillnet, and drift gillnet). Dead discards were variable and fluctuated between 47 and 758 mt for 2010-2018, primarily from fixed gillnets and bottom trawls. These catches are not included in porbeagle stock assessments or catch limits, but these estimates of recent discards are substantially greater than U.S. catch limits. Chapter 4 used Pop-up satellite archival transmitter tags to estimate the vertical susceptibility of juvenile porbeagle to bycatch in the Gulf of Maine region (i.e., the time porbeagle spend at depths Gulf of Maine fishing gear operates in). The porbeagles spent 42% of their time near the surface (0 to 25 m deep, where pelagic longlines and midwater gillnets generally fish), 51% in midwater (26 m deep to 12 m off the bottom, where some midwater trawls and purse seines fish), and 7% at the bottom (bottom to 12 m off the bottom, where bottom trawls and most gillnets fish).The findings of this dissertation suggest that North Atlantic porbeagle is not as data-limited as previously thought. Some available data is not being considered in stock assessment and fishery management, and monitoring frameworks are in place to improve the conservation status and fisheries management of porbeagle. New information on discards, vertical susceptibility toby catch, and other identified sources of data (e.g., recreational fishing, economic value, and trade statistics) can improve fisheries management and conservation of porbeagle if included in future stock assessments and conservation assessments.