Experimental determination of key factors affecting herring egg survival

Pacific herring play pivotal roles in coastal social-ecological systems, yet our understanding of the factors controlling spawning events, and the survivorship of deposited herring eggs is lacking. Traditional ecological knowledge (TEK) from British Columbia First Nations puts forward two factors as key contributors to herring egg survivorship: predation and freshwater outflow. In addition, diver-based observations suggest that herring adhere differentially amongst macrophyte substrates. We hypothesized that predation and freshwater outflow would have negative effects on herring egg density and maturation, with differential effects amongst substrates. We tested this experimentally using herring eggs naturally deposited on 4 substrate types: Zostera marina, Macrocystis pyrifera, Saccharina latissima, and Egresia menziesii. These were suspended ~1m above the benthos at stream outflow and non-outflow locations, using predator exclusion treatments (cage vs. un-caged). We quantified change in percent cover and wet weight of herring eggs to evaluate effects of these factors on egg survivorship. Counter to our hypotheses, freshwater outflow had very little effect on egg survivorship. Predation effects were site-specific, and we observed potential herring egg predators including sea stars and urchins contributing to egg loss. Substrate type accounted for the greatest difference in egg loss rates we observed. Parameterization of egg loss rates is important to Pacific herring stock assessments, and our study re-emphasizes that this substrate-level variability can be large, suggesting that herring egg survivorship may benefit from substrate choice. However, preliminary experimental work on substrate selectively and preference is inconclusive and demands further research