Research Document 2024/060
Estimating Total Mortality among Fisheries Affecting Porbeagle Shark (Lamna nasus) in Atlantic Canadian Waters
By Bowlby, H.D., Yin, Y., Wilson, M., Simpson, M.R., and Miri, C.M.
Abstract
Porbeagle Sharks (Lamna nasus) in the northwest Atlantic are currently being considered for listing under the Canadian Species at Risk Act (SARA), and Fisheries and Oceans Canada (DFO) Science was asked to estimate total annual fishing mortality. This would come from landings, and at-vessel mortality (AVM) or post-release mortality (PRM) of discards from fisheries in the Maritimes (MAR), Gulf (GULF), Quebec (QC) and Newfoundland and Labrador (NL) Regions. This evaluation considers 2015 to 2021, representing a time period following the closure of the commercial Porbeagle fishery.
Total landings remained low from Atlantic Canadian fisheries, dropping from 3.8 mt in 2015 to less than 200 kg in 2021. The vast majority of landings in 2015–2021 came from longline gear in MAR, primarily benthic longline in the Atlantic Halibut fishery with lower amounts from the pelagic longline fishery for Swordfish and Other Tunas.
At-sea observer (ASO) coverage was variable among different fisheries and could not be estimated for fisheries in NL, GULF or QC. When coverage was low (< 5% annually), fisheries observed discards of Porbeagle substantially underestimated fishery-wide totals and needed to be scaled up to annual discard estimates. Also, several fisheries that would be expected to interact with Porbeagle had no ASO coverage and thus could not be considered in this assessment. A suite of statistical estimators was evaluated to model fishery-wide discards for pelagic longline in MAR. However, the data were not sufficient for quantitative models and thus these approaches were not applied. Simple scalars of either the proportion of observed trips (MAR) or proportion of observed target catch (NL) were used to approximate annual fishery-wide bycatch from individual fisheries.
Although estimates of total mortality were derived from scenarios assuming different AVM and PRM rates for various fisheries, they lacked precision and were predicated on numerous assumptions. Also, there were several factors that would have caused annual mortality to be underestimated but could not be corrected in advance of this assessment, so the magnitude of underestimation was unknown. Given demonstrated challenges and limitations of the available data, it is not possible to derive meaningful estimates of total annual fishing mortality of Porbeagle throughout Atlantic Canadian waters. Interpretation of the implications of observed increases or decreases in annual fishing mortality is not possible without information on underlying abundance and status of infrequently observed, discarded bycatch species (such as Porbeagle). This limits the utility of estimates of fishing mortality to address conservation or management goals and warrants consideration of an alternate framework to quantify threats to bycatch species from fisheries.
Accessibility Notice
This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.
- Date modified: