MEPS 596:181-198 (2018)  -  DOI: https://doi.org/10.3354/meps12598

ABSTRACT: Ocean entry for salmon (Oncorhynchus spp.) is a critical period during which recruitment to the adult population is likely set. During this period, predation risk will be modulated by availability of suitable prey at the time and location of out-migration. Therefore, identifying variables affecting the distribution of prey coast-wide facilitates ecosystem-based management of Chinook salmon O. tshawytscha in the California Current. In this study, we quantified distributions of salmon forage assemblages relative to biogeographic breaks and ocean conditions along the California Current shelf ecosystem from Monterey Bay, California (36.5°N), to Willapa Bay, Washington (46.5°N). Epipelagic micronekton samples were collected during late springs of 2011, 2013, 2014, and 2015. We characterized (1) abundance of salmon forage taxa north and south of geographic boundaries, (2) spatial gradients in forage assemblages, and (3) relationships between environment and spatiotemporal variability of forage assemblages. We found higher abundances of market squid Doryteuthis opalescens, rockfishes (Sebastes spp.), and sanddabs (Citharichthys spp.) south of Cape Mendocino, while pandalid shrimp (Pandalus spp.), rex sole Glyptocephalus zachirus, and smelt (Osmeridae) were more abundant in the north. Multivariate analyses demonstrated a latitudinal gradient in the relative contribution (rank order) of individual taxa to salmon forage assemblages, and further analyses revealed the presence of 4 distinct multi-species assemblages associated with regional and meso-scale oceanographic dynamics. Our findings indicate that distributions of salmon forage assemblages and the oceanographic characteristics associated with those assemblages is similar to the spatial coherence of Chinook salmon population survival observed over longer time series.

KEY WORDS: Biogeography · Ecosystem oceanography · Forage fishes · Salmon · Spatial ecology · California Current