Feeding ecology of forage fishes in the San Juan Archipelago: Diet composition and variation in Ammodytes hexapterus, Hyperprosopon ellipticum, and Clupea pallasii

Abstract

Forage fish play an important role in wasp-waist ecosystems like that of the San Juan Archipelago. These small fish, including the Pacific sand lance, silver surf perch, and Pacific herring, regulate the flow of energy through the trophic levels of this marine ecosystem, from the plankton to the larger predatory fish, sea birds, and marine mammals. Gut content analyses reveal that Pacific sand lance and Pacific herring feed primarily on calanoid copepods, while the surf perch has a more varied diet. Pacific sand lance and Pacific herring had low values on Levins’ standardized measure of niche breadth, indicating a narrow dietary breadth characteristic of a specialized diet. Surf perch had a higher value, indicating a wider dietary breadth and more generalist feeding approach. Morista’s measure of niche overlap was used to quantify dietary overlap between the species. Pacific sand lance and Pacific herring had a high rating, which could indicate competition for the same prey items. Both of these species’ dietary overlap with surf perch was much lower, suggesting little competition for prey items. The diet composition of surf perch exhibited significant temporal variation. The diet composition of Pacific sand lance did not exhibit significant temporal variation, but did exhibit significant spatial variation between Jackson Beach and the sand wave channel. By quantifying these measures of compositional, temporal, and spatial variation in forage fish diets, we can now better understand the types of resource partitioning that are influencing trophic exchange in the San Juan Archipelago.