Factors driving spatial variation in egg survival of an ecologically and culturally important forage fish

Low trophic‐level forage fish are experiencing global declines, influencing coupled human–ocean systems worldwide. Along the northwest coast of North America, declining trajectories of Pacific herring (Clupea pallasii) have prompted interest in improving the understanding of its population and community dynamics to better guide future conservation and management strategies. To improve future population estimates and understanding of the ecological factors governing herring egg survival, the magnitude, spatial variation and mechanisms driving herring egg loss rates were quantified. This was achieved by way of repeated observational field surveys and a predator exclusion experiment. Observational surveys revealed that regional egg loss rates (Z) were substantial and ranged from 0.101 ± 0.019 to 0.134 ± 0.028, the equivalent of 88–94% egg loss over a 21 day incubation period, or 50–60% egg loss over 6.8 days, the average time lag between spawn deposition and annual egg surveys. Furthermore, spatial variation was high, with egg loss rates varying 5‐fold among study sites. Depth, time since spawn, and spawn area were primary spatial drivers of egg loss, but predator abundance and exposure were secondarily important. Experimental evidence showed that benthic predation and habitat type were strong drivers of egg loss, suggesting that a high proportion of eggs, particularly those spawned on benthic substrates, are consumed by predators. These results have important conservation implications for managers and ecologists seeking to estimate herring biomass and to understand the environmental influences on predator–prey interactions that affect herring dynamics.