Evaluating factors affecting the distribution and timing of Pacific Herring Clupea pallasii spawn in British Columbia

Abstract

Pacific herring Clupea pallasii spawn in nearshore areas in late winter to early spring, but factors influencing the timing and spatial distribution of spawning are not well known. We modeled the temporal and spatial distribution of spawning for 5 herring stocks in British Columbia from egg deposition surveys conducted from 1988-2018 using different sets of environmental predictors and modeling methods. Random forest modeling showed that the timing of spawning in each year was mainly influenced by the number of daylight hours being >10.5, cumulative degree days >100 and salinity at 30.5. The spatial distribution of spawning tended to occur at consistent locations over time. Results showed that the probability of spawning occurring at a transect in a given year was largely determined by the biomass of herring and location of the transect relative to the center of spawning. Environmental factors at individual transects played a much smaller role in determining spawn distribution. There was mixed evidence for spatial expansion of spawning distribution in years of high biomass, with some stocks, such as Haida Gwaii, showing no expansion of the spawning area in years of higher biomass. Since Pacific herring recruitment has been linked to their ability to time larval hatch to spring bloom timing, future warming temperatures may result in earlier herring spawning relative to the spring bloom. This will increase the probability of mismatch with prey, impacting larval herring starvation, growth and perhaps mortality, leading to reductions in recruitment to these important stocks.