Pacific Herring Spawns Provide Temporal Subsidies to Nearshore Ecosystems

Pacific herring (Clupea pallasi), low trophic level fish at the base of many pelagic food webs, are integral to coastal social-ecological systems in North America. This migratory species provides a temporal subsidy to coastal systems by moving from offshore to nearshore waters for annual spawning events. Spawning migrations support subsistence and commercial fisheries, and provide an ecological pulse in prey availability. In coastal British Columbia, we quantified changes in rockfish (Sebastes maliger and caurinus) diet composition surrounding spawn events to understand the temporal importance of this subsidy. We found that the percentage of fish in rockfish diets switched from 30% fish tissue pre spawn to 34% herring roe post spawn, and that this shift peaked at 2-3 weeks post spawn events. This dietary change is most pronounced in females, where herring roe accounted for an average of 29% of stomach contents through the month following a spawn event. This may be due to differences in energy requirements for gravid females. Stable isotope concentrations of fish tissues (muscle, heart, liver, gonads) also showed evidence of herring nutrient assimilation. Determining these ecosystem-level associations is key to understanding both the social and ecological consequences of herring spawn declines