Variability of Pacific herring (Clupea pallasii) spawn abundance under climate change off the West Coast of Canada over the past six decades

Pacific herring (Clupea pailasii) is an ecologically- and commercially-important species in the northeast Pacific Ocean. Detailed records of herring spawn off the coast of British Columbia (BC), Canada, collected annually since 1928, indicate that spawn distribution has gone through systematic changes over time. The cessation of spawning in some areas has led to concerns that commercial herring fisheries have resulted in serial depletion of local spawning. However, previous studies suggested that changes in spawn distribution may not necessarily be fisheries induced. Hitherto, there have been no satisfactory explanations for these changes. In this study, we developed boosted regression tree (BRT) models for four major spawning regions in BC (the Prince Rupert District, Haida Gwaii, the Central Coast, and the West Coast of Vancouver Island) to explore the nonlinear relationships between herring spawn abundance and environmental variables, including two indices of Pacific Ocean basin-scale variability (the PDO: Pacific Decadal Oscillation; and the NPGO: North Pacific Gyre Oscillation) and regional scale wind-stress off the BC coast. We find a "bowl-shaped" relationship between herring spawn abundance and two environmental variables (the PDO and NPGO) across all spawning regions and a "dome-shaped" relationship between spawn abundance and upwelling-favorable wind-stress, the latter indicating that there is an optimal range of wind stress values that favor herring spawn abundance. Results also suggest that, in Haida Gwaii, herring spawn abundance has been primarily governed by the NPGO and downwelling-favorable alongshore wind-stress; in contrast, the PDO and upwelling-favorable alongshore wind-stress are top contributors to spawn fluctuations in all other regions. These results help facilitate our understanding of how changes in physical environmental conditions affect herring spawn abundance as well as subsequent population dynamics.