Hidden markov mark-recapture modeling highlights transboundary exchange for northeast pacific sablefish Fisheries Research

Developing robust fisheries management advice requires understanding fish population distribution and movement in relation to harvesting and management boundaries. Traditional stock assessment models often assume homogeneous populations with negligible transboundary movement, an assumption that may lead to undesirable management outcomes when violated. We developed a hidden Markov movement model and fitted it to almost forty years (1979-2018) of conventional tagging data for northeast Pacific sablefish (Anoplopoma fimbria), a commercially valuable species distributed across the northeast Pacific. This is the first time that sablefish movement has been evaluated at this spatial scale and included transboundary migration. Estimated movement rates were then combined with stock assessment abundance estimates to quantify the magnitude of exchange between management regions. Several sensitivity analyses were conducted to evaluate the robustness of the model assumptions regarding size structure, time-at-liberty, size-transition and parameter uncertainty. Sablefish exhibited long-distance movements. Notably, British Columbia acted as a transition zone for sablefish movement between regions and a net recipient for sablefish abundance, receiving net immigration from both Alaska and the California Current, a pattern influenced by regional abundance differences. Differences in movement rates between size classes were modest, although larger sablefish in Alaska were more likely to move into British Columbia compared to smaller fish. Our findings, consistent with previous smaller-scale sablefish movement research, uniquely quantify the demographic exchange across management regions, highlighting a large contribution of sablefish from Alaska and the California Current to British Columbia. This study emphasizes the need to account for the transboundary nature of sablefish within assessment and management frameworks. Collaborative, international efforts in monitoring and data sharing will be essential for ensuring the sustainable harvest of transboundary stocks like sablefish.