Investigating Surf Scoter Predation on Herring Eggs

Investigators: Scott Pearson, Todd Sandell, and Kyle Spragens (WDFW); Kathryn Sobocinski and Marco Hatch (WWU)

Pacific herring (Clupea pallasii) are a top food source for many higher trophic level species in the Salish Sea, including marine mammals, several species of birds, and larger fish like Chinook salmon. Understanding salmon prey resources is important for understanding marine survival. The Cherry Point herring population, once the largest spawning population of herring in the southern Salish Sea, has declined significantly (96% since 1973) and the causes of decline are unclear despite investigations into multiple mechanisms. Because the Cherry Point herring population spawns later than other stocks in the region, it is thought to be an important food source for outmigrating Chinook salmon. But this stock may also provide valuable egg foraging for surf scoters (Melanitta perspicillata, a sea duck), especially those with later migration timing corresponding to the later spawn timing, creating additional pressure on a declining resource.

This project explores two approaches for improving egg survival: (1) deploy artificial spawning rafts to encourage expanded spawning at the site and (2) investigate interactions between feeding sea ducks and herring eggs at Birch Head, the region of recent spawning for Cherry Point herring. Project investigators are estimating scoter abundances to understand potential predation pressure, collecting scoters for stomach content analysis to directly quantify egg consumption and understand overall diet, and using cages to experimentally exclude surf scoters from foraging in a portion of the spawning region to evaluate whether egg mortality is higher in areas available to scoters. High predation mortality on herring eggs has implications for salmon marine survival because fewer herring recruit to the juvenile stage, which is an important food resource for outmigrating salmon, especially Chinook.

Juvenile Puget Sound herring. Photo credit Megsie Siple.