On the edge: assessing fish habitat use across the boundary between Pacific oyster aquaculture and eelgrass in Willapa Bay, Washington, USA

Ka Muething,Br Dumbauld,F Tomas,G Waldbusser

doi:10.3354/aei00381

Abstract

Estuaries are subject to diverse anthropogenic stressors, such as shellfish aquaculture, which involve extensive use of estuarine tidelands. Pacific oyster Crassostrea gigas aquaculture is a century-old practice in US West Coast estuaries that contributes significantly to the regional culture and economy. Native eelgrass Zostera marina also commonly occurs in intertidal areas where oyster aquaculture is practiced. Eelgrass is federally protected in the USA as ‘essential fish habitat’, restricting aquaculture activities within or near eelgrass. To contribute scientific information useful for management decisions, we sought to compare fish habitat use of oyster aquaculture and eelgrass, as well as the edges between these 2 habitats, in Willapa Bay, Washington, USA. Furthermore, given a recent shift towards off-bottom culture methods, in part to protect seagrasses, long-line and on-bottom oyster aquaculture habitats were compared. A combination of direct (underwater video, minnow traps) and indirect (predation tethering units, eelgrass surveys) methods were employed to characterize differences in fish habitat use. Eelgrass density declined within both aquaculture habitats but less so within long-line aquaculture. Most fish species in our study used long-line oyster aquaculture and eelgrass habitats similarly with minimal edge effects, and on-bottom aquaculture was used less than either of the other 2 habitat types. These results are consistent with previously observed positive relationships between fish abundance and vertical habitat structure, but also reveal species-specific behavior; larger mesopredators like Pacific staghorn sculpins were sighted more often in aquaculture than in interior eelgrass habitats.

Highlights

As intersections of terrestrial, freshwater, and marine systems, estuaries provide a wide array of ecosystem services and have helped to support flourishing human populations for centuries (Costanza et al 1997, Lotze et al 2006)
We explored the similarities and differences between fish habitat use of oyster aquaculture and eelgrass by addressing 2 main questions: (1) Do oyster aquaculture and eelgrass habitats support different abundances of fish, and is there an associated effect at the edge between these habitats? (2) Does the aquaculture method affect the difference seen amongst habitats? Together, these questions were designed to provide an ecological basis for an integrated framework of management regulations related to the overlap of eelgrass and oyster aquaculture in US West Coast estuaries
This research sought to compare the use of habitat created by 2 different oyster aquaculture methods with that of eelgrass, which is an important natural habitat for fish and mobile invertebrates in a US Pacific coast estuary

Summary

Introduction

Freshwater, and marine systems, estuaries provide a wide array of ecosystem services and have helped to support flourishing human populations for centuries (Costanza et al 1997, Lotze et al 2006). The native eelgrass Zostera marina of the US West Coast provides habitat for early life stages of commercial species like salmonids, Dungeness crab, rockfish, and English sole (Rooper et al 2003, Holsman et al 2006, Dumbauld et al 2015, Olson et al 2019). This clear but indirect connection between eelgrass and the economic success of fisheries is the reason for its protection in the USA as ‘essential fish habitat’ under the Magnuson-Stevens Fishery Conservation and Management Act (16 USC §§1801– 1891d). Under current regulations implemented by the Pacific Fishery Management Council, existing aquaculture is generally permitted to continue as practiced, but new aquaculture is prohibited within 25−30 feet (7.6−9.1 m) of existing eelgrass beds (Pacific Fishery Management Council 2014)

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