| Summary: | Atlantic mackerel (Scomber scombrus) has long been a significant marine resource in the northwest Atlantic, supporting commercial and recreational fisheries over two centuries. As a small pelagic fish feeding on planktonic organisms, mackerel serve a critical role in the marine food web as prey for higher trophic level species, including large predatory fish, marine mammals, and sea birds. Significant harvest pressure and recent low abundances and landings have led to questioning whether such removals have jeopardized the fishery and ecosystem’s sustainability. Further complicating management, mackerel populations throughout the North Atlantic have been significantly influenced by climate change, represented principally through shifts in population distribution. With forage fish like Atlantic mackerel particularly sensitive to oceanographic and environmental conditions, shifts in the ecosystem’s state may pose issues for the species’ future growth, survival, and recruitment. This dissertation aims to provide tools for future northwest Atlantic mackerel stock assessments through better description of population trends, both contemporary and historical, and to quantify habitat changes for the stock to inform current knowledge on the stock’s spatial structure. The first chapter aimed to provide an additional abundance index for future northwest Atlantic mackerel benchmark stock assessments. Given conflicting information provided by currently used fishery-independent trawl survey data and commercial landings information, a larval abundance index using long-term federal ichthyoplankton data was constructed for the stock’s southern contingent. The larval index captured peaks in years with believed strong recruitment, and significantly correlated to estimated annual egg production and spawning stock biomass. However, catchability corrections conducted likely still underestimate earlier years’ larval abundances. Thus, when using the larval index in future assessments, we recommend the time series without catchability corrections be split and each have their own correction factor q estimated within the overall stock assessment model. The second chapter estimated how Atlantic mackerel larval habitat suitability has changed over the last four decades using species distribution models. Physical (temperature) and biological (zooplankton) variables that have been reported to influence larval survival were included to determine how such relations influence habitat suitability in the Northeast U.S. Shelf. Atlantic mackerel larval densities correlated with sea temperature and copepod abundances, suggesting that larval survival may be sensitive to specific temperatures and zooplankton prey. Since the 1970s, suitable habitat located in the Mid-Atlantic Bight has decreased, as southern New England and the western Gulf of Maine regions have become more suitable ecoregions, highlighting an overall northeast habitat shift. While total Northeast U.S. Shelf habitat suitability has decreased since the 1970s, the time series’ declining trend was not statistically significant. The third and final chapter uses stochastic stock reduction analysis (SSRA) to infer northwest Atlantic mackerel population trends over the last two centuries, using historical landings, data on mackerel biology, and descriptions of the fisheries’ evolution. Population trends were estimated from 1804 through 2016. Population trajectories highlighted many of the major population decreases through time from harvest, with results suggesting the stock in 2016 could be as low as 11% of the 1804, unfished stock size. The SSRA developed could benefit from additional model development, but should be considered for inclusion in future stock assessments as part of an ensemble approach. The research in this dissertation aims to provide scientists and managers with a better understanding of Atlantic mackerel ecology, population dynamics, the fishery, and improve future management for one of the most historically significant marine species of the North Atlantic. The application of the tools transcends Atlantic mackerel, and can be applied to other fish stocks. This dissertation serves as an example of how fisheries science can be conducted to inform and improve fisheries management.
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