September 27, 2021 — The following was released by the Science Center for Marine Fisheries:
Marine mammal conservation is one of the top goals of U.S. ocean management. That is why it’s particularly important for regulators to have an accurate estimate of how fisheries and other ocean users may impact marine mammal populations. A new study looks at ways of strengthening and fine-tuning existing marine mammal management and assessing the impacts on one marine mammal population in the western North Atlantic, the gray seal.
The study, from Drs. André Punt, John R. Brandon, Doug DeMaster, and Paula Moreno, is the culmination of a 3-year research project led by Dr. DeMaster and conducted in collaboration with the scientists at NOAA’s Northeast Science Center, with funding from the Science Center for Marine Fisheries. It specifically examines a key variable in marine mammal management, the Potential Biological Removal (PBR) level.
Under the Marine Mammal Protection Act, marine mammal populations are managed according to their Potential Biological Removal (PBR), which determines the level of mortality (and serious injury) that is sustainable for each marine mammal population. When bycatch of a marine mammal population nears or exceeds its PBR level, restrictions are often imposed on nearby fisheries as a precautionary measure to prevent the population from becoming depleted.
In the case of gray seals, assessments of human-caused mortality levels are complicated by the fact that the gray seal population exists on both sides of the U.S.-Canada border. Determining when the gray seal PBR is exceeded is subject to errors due to incomplete information, meaning that certain management decisions could unnecessarily trigger fishing restrictions (termed by scientists as ‘false positives’) or needed restrictions are not imposed (known as ‘false negatives’). These restrictions often include catch limits of commercially valuable species, limits on what kinds of gear can be used, and other regulations that can be costly for the fisheries forced to adopt them.
To address this, the authors assembled the best available data on the western North Atlantic grey seal population and Canadian fisheries. This allowed them to produce two key estimates when assessing whether gray seal bycatch exceeds the PBR level. It used a more realistic, species-specific rate of gray seal maximum net production (i.e., 14.1% per year), rather than using a generic default value for other seal species (12% per year). If adopted by the National Marine Fisheries Service (NMFS), this would increase gray seal PBR level by 18%. The study also used an extrapolated bycatch estimate for adjacent Canadian waters, which had not previously been attempted.
The study found that, in comparison to the base model currently used by NMFS, this approach is more robust to transboundary movements and uses a more accurate estimate of maximum net production of gray seals, allowing managers to adopt more appropriate management measures on fisheries while still achieving precautionary conservation goals for the grey seal population. The results can provide a blueprint for other assessments of marine mammal-fishery interactions for similar transboundary marine mammal stocks.