Causes and consequences of Pacific herring (Clupea pallasii) deep spawning behavior

Shifts in the reproductive strategies of marine species can result from ecological disturbance and often lead to either harmful or adaptive population−level effects. Pacific herring (Clupea pallasii) can exhibit remarkable plasticity in spawn density and spatial distribution, shifting in response to both climatic and anthropogenic pressure. To test alternative factors leading to recently observed and previously uninvestigated deep spawning events (−30 m, 8 x the preceding 25-year mean), we surveyed spawn sites varying in motorized boat traffic, predator density, and sea surface temperature, and conducted a field experiment to test depth effects (at −3, −15, and −30 meters) on the survival rates of herring eggs exposed and protected from predation. We found herring spawn to −44 m, and strong evidence for a positive relationship between depth of suitable habitat and maximum spawning depth (with a possible link to surface temperatures), which was magnified when spawner density was high. This result is consistent with historical records of fisheries independent survey data collected from 1989 to 2015, showing an increase in maximum spawning depth with greater biomass of spawners. Finally, experimental evidence indicated that egg survival decreased, on average, by 20 % at −30 m relative to −3 m depths. If declining trends in spawning distribution continue as sea temperatures rise, the prevalence of deep spawning events may expand as herring become further concentrated into deep fjords and smaller geographic areas, adding further risk to already declining stocks.