Evaluation of a spatiotemporal index standardization method for coastal shark species; implications for future stock assessments

Kaitlyn O'Brien^1*John K. Carlson²Enric Cortés²William B. Driggers³Bryan S. Frazier⁴Robert J. Latour¹

• ¹Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA, United States
• ²Panama City Laboratory, NOAA Fisheries, Southeast Fisheries Science Center, Panama, FL, United States
• ³Mississippi Laboratories, National Marine Fisheries Service, Southeast Fisheries Science Center, Pascagoula, MS, United States
• ⁴South Carolina Department of Natural Resources, Marine Resources Research Institute, Charleston, SC, United States

Methods combining data from spatially limited, independently conducted surveys indicate a preliminary recovery for coastal shark species along the Atlantic. However, anthropogenic climate change is expected to shift distributions and alter migration timing for these highly migratory species, potentially affecting survey catchability and interpretation of abundance indices. This study investigates the use of an observation-level spatiotemporal model to evaluate abundance trends and possible species-specific distribution shifts for coastal sharks. Vector autoregressive spatiotemporal (VAST) models were applied to data from six fishery-independent surveys of six coastal shark stocks to generate area-weighted indices of abundance. Area-weighted indices, trends in density over space and time, and analysis of density anomalies were used to evaluate changes in a stock's spatial distribution across the U.S. southeast Atlantic. In addition to VAST, generalized linear mixed effect models were used to generate indices of abundance for each survey, which were further analyzed using dynamic factor analysis (DFA) and Bayesian hierarchical analysis (Conn). The index standardization methods, particularly VAST and Conn, largely agreed with one another and appeared robust to spatial patterns. Only two of the six shark stocks showed increasing trends by the end of the time series, with indices for multiple species plateauing or declining. Positive trends in density and increased variability in density anomalies in the VAST models across the northern extent of the surveyed spatial domain suggests a northward expansion or a timing discrepancy between migration onset and sampling efforts for multiple species. Overall, the VAST models provided evidence of spatial changes that could impact each survey's catchability, thus complicating the interpretation of abundance trends and affecting future stock assessments and management measures.