Abstract
AbstractThe early marine period is a critical life history stage for growth and survival of anadromous juvenile Pacific salmon. The integrity of nearshore ecosystems where juvenile salmon reside and the capacity of these habitats to provide prey can thus influence overall salmon returns. Eelgrass Zostera marina beds in particular are considered critical nearshore habitat. By examining how juvenile salmon use this habitat during their early marine life, we gain insight into the trophic dynamics in these ecosystems and help gauge the effects of accelerating eelgrass loss and nearshore habitat development on juvenile salmon. This project investigated the role of eelgrass as juvenile salmon foraging habitat. We compared juvenile Chum Salmon Oncorhynchus keta and juvenile Chinook Salmon O. tshawytscha diets to prey availability in zooplankton tows and in epifaunal eelgrass samples across a gradient of eelgrass density in the Comox Estuary, British Columbia. Harpacticoid copepods dominated the diets of both juvenile Chum Salmon and Chinook Salmon and were found in abundance in eelgrass blades. We complemented diet analysis with carbon (δ13C) and nitrogen (δ15N) stable isotope analysis to examine the relative contribution of zooplankton, eelgrass epifaunal invertebrates, and terrestrial invertebrates to the diet of juvenile Chum Salmon. Juvenile Chum Salmon isotope ratios closely reflected those of eelgrass invertebrates, suggesting that eelgrass invertebrates made up approximately 80% of the diets of juvenile Chum Salmon. Our results highlight the value of eelgrass in providing nearshore foraging opportunities for juvenile salmon and suggest that eelgrass habitat protection and restoration may provide critical support for growth, thereby easing the transition of juvenile salmon from freshwater to the marine environment.
Highlights
Rapid growth of Pacific salmon Oncorhynchus spp. during their early marine phase correlates with survival to adulthood (Beamish and Mahnken 2001)
Due to the benthic nature of juvenile Chum Salmon feeding, we hypothesized that the majority of items found in juvenile Chum Salmon prey would be found in eelgrass habitats
We expected that juvenile Chum Salmon would have δ13C signatures that would reflect a diet of eelgrass invertebrates, indicating predation on eelgrass invertebrates
Summary
Rapid growth of Pacific salmon Oncorhynchus spp. during their early marine phase correlates with survival to adulthood (Beamish and Mahnken 2001). Salmon display high growth rates in estuaries, Subject editor: Kenneth Rose, University of Maryland Center for Environmental Science, Cambridge. In the face of large-scale eelgrass loss in many Pacific estuaries (e.g., Tallis et al 2009; Thom et al 2012) and drastically diminished returns of Pacific salmon across the West Coast of North America (Noakes et al 2000), it is critical to understand how eelgrass habitat affects juvenile salmon prey provision. We expected that juvenile Chum Salmon would have δ13C signatures that would reflect a diet of eelgrass invertebrates, indicating predation on eelgrass invertebrates
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