River Delta Eelgrass Supports Extended Estuary Residence and Foraging by Outmigrating Chinook Salmon

Abstract

Eelgrass can enhance marine survival of juvenile salmon by providing food and refuge but data to quantify these functions are scarce. Eelgrass on river deltas may be particularly important as the first eelgrass encountered during outmigration. We used a lampara net to capture juvenile Chinook salmon from a range of eelgrass and non-eelgrass habitats during April-September, monthly 2008-2010 and 2015 in the Skagit River delta and biweekly 2010-2015 in the Nisqually River delta and reach. At Nisqually a subsample of Chinook was retained for otolith, coded-wire tag and diet analysis, and in 2014 and 2015 we sampled potential prey items (infauna, epifauna, and neuston) from the same eelgrass sites and on the same schedule as fish. High catch rates and unique diets indicated the importance of eelgrass growing on the outer edge of deltas (delta front eelgrass) to Chinook salmon in summer. At Nisqually in July-August in delta front eelgrass catch rates were at least twice as high as in other delta flat and nearshore habitats and diets were distinctive in having relatively high percentages of shrimp zoea, mysid shrimp, and crab megalopa. During the peak of outmigration in May-June Nisqually Chinook were abundant in non-eelgrass as well as eelgrass and diets were more homogenous among habitats. At Skagit Chinook also frequented delta front eelgrass in July-August and were more widespread earlier on. Changes in habitat-specific prey abundance or size of prey needed may account for Chinook shifting from a wide distribution in May-June to concentrating in delta front eelgrass later on. Eelgrass epifauna suggested a correspondence between diets and invertebrates produced in eelgrass but infauna and neuston results are needed to fully characterize prey availability. Knowing where, when, and how eelgrass benefits salmon is relevant to both salmon and eelgrass restoration. Our data begin to answer these questions