Cooperative Fish and Wildlife Research Units Program: Washington
Education, Research and Technical Assistance for Managing Our Natural Resources

Veggerby K, Scheuerell MD, Sanderson B, Kiffney P. Stable isotopes reveal intertidal fish and crabs use bivalve farms as foraging habitat in Puget Sound, Washington.


Bivalves such as oysters and clams have been farmed in the Puget Sound region of the Salish Sea for thousands of years. Indigenous peoples have traditionally used systems called clam gardens to modify stretches of shoreline habitat to create ideal conditions for edible bivalves. More recently, oysters, clams, and other bivalves have been grown by both First Nation and non-First Nation farmers using a variety of gear types in intertidal zones across Puget Sound and the coast of Washington. These gear types create complex vertical structure and attachment points for aquatic epiphytes and invertebrates which increases habitat structural complexity, but may alter eelgrass cover in areas where bivalve farms and eelgrass meadows overlap. Eelgrass meadows are highly productive and ecologically foundational nearshore habitats that provide valuable ecosystem services including sediment deposition and stabilization, and the provision of nursery, refuge, and foraging habitat throughout much of the Puget Sound. Eelgrass meadows overlap with bivalve farming activities across many intertidal areas of Washington State. Many nearshore species of fish and crab inhabit both bivalve farm and eelgrass habitats, but how they are using the farm habitat and the importance of bivalve farm habitat for foraging and predator refuge relative to unfarmed eelgrass meadows is unclear. We used stable isotope mixing models to estimate, for several species of nearshore fish and crab in two areas of North Puget Sound, Washington, the percent diet originating from eelgrass meadows, pelagic planktonic sources, and oyster farm habitats. Our results indicate that several species of nearshore fish derive a significant proportion of their diets from farm areas, while others derive most of their diets from eelgrass habitat or planktonic sources. The results indicate that foraging habitat uses are species specific, and that modifying habitat will likely change both species composition and species usage of an area. Having an adequate amount of habitat diversity is needed to support the community of fishes and invertebrates that inhabit these intertidal areas.