Pacific herring spawn and macroalgae subsidize intertidal detritivores

Background/Question/Methods

Each year along the Pacific coast of North America, Pacific herring (Clupea pallasi) aggregate and spawn in the nearshore and intertidal. Although ephemeral, these spawn events often represent extremely large aggregations of biomass and energy, a portion of which is transferred to intertidal ecosystems. This input of herring resources, mainly in the form of eggs, represents a spatial subsidy to intertidal and possibly terrestrial ecosystems, but little is known of these relationships. Using fatty acids and stable isotopic signatures of carbon and nitrogen, we (1) traced the sources of production for detritivorous sand hoppers (Traskorchestia traskiana and T. georgiana) and (2) tested the hypothesis that herring resources subsidize sand hoppers. To this end, sand hoppers and potential sources of production (macroalgae, seagrass and herring eggs, when present) were collected from five mixed substrate beaches (two control, two high density and one low density spawn) before and after a herring spawn event in Quatsino Sound, British Columbia. 

Results/Conclusions

Spatial and temporal results from fatty acid and stable isotopic analyses indicate that herring resources are consumed by sand hoppers, providing the first evidence of a cross-ecosystem subsidy by herring. Differences in mean δ13C signatures were detected in sand hoppers at high density spawn beaches following the spawn (Δδ13C = 0.43 and 0.56, p<0.05) but not at the low density spawn or control beaches. NMDS ordination separated polyunsaturated fatty acid information into two different clusters (Global R = 0.67, p<0.05), with one cluster representing sand hoppers from high density spawn beaches. Brown algae (Phaeophyceae), a dominant wrack component, were also identified as a source of production and like herring, also represent a marine subsidy. Although herring spawn events are short-lived, sand hoppers receive and store lipids, thus acting as a reservoir of herring resources. Because sand hoppers are also important prey for several terrestrial predators (e.g. song birds and bears), this research identifies previously unknown marine-terrestrial trophic linkages. British Columbian populations of Pacific herring, after recovering from a collapse in the 1960’s, are again experiencing major declines. Our research traces the likely diminishing ecosystem influences of one of the most ecologically and economically important marine fishes in British Columbia.