The effect of oyster aquaculture on seagrass (Zostera marina) at the estuarine landscape scale in Willapa Bay, Washington (USA)

Authors: Dumbauld, Brett; McCoy, Lee

Submitted to: Aquaculture Environment Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2015
Publication Date: 6/8/2015
Citation: Dumbauld, B.R., Mccoy, L.M. 2015. The effect of oyster aquaculture on seagrass (Zostera marina) at the estuarine landscape scale in Willapa Bay, Washington (USA). Aquaculture Environment Interactions. 7:29-47.

Interpretive Summary: Seagrasses provide valuable services like nursery habitat for juvenile fishes and crabs in estuaries around the world. They are also declining in many locations due to a variety of human impacts and thus are also protected by laws including a no-net loss policy in the US, which results in a very precautionary and protective approach by planners and estuarine managers that avoids any direct impacts including those potentially caused by shellfish aquaculture. Oyster aquaculture has been shown to have some direct impacts on the native seagrass (eelgrass, Zostera marina) in US west coast estuaries, but these have generally been documented at small spatial scales (on individual beds or small experimental plots) and over short time scales (one to four years). We quantified the effect of oyster aquaculture practices on eelgrass at a much larger estuarine landscape scale in Wilapa Bay, Washington where 17% of US oysters are produced. First, we created a predictive model for eelgrass density using a number of factors including distance to mouth, distance to channel, salinity, elevation, and cumulative wave stress. We then used this model to predict the amount of eelgrass that would occur in oyster aquaculture areas and compared this with the amount actually present in 3 different years based on aerial photography. The amount of eelgrass observed was always less than the predicted amount but the impact was small representing less than 1.5% of the total eelgrass predicted in any year. The type of oyster culture bed (off bottom longlines, on-bottom juvenile seed, on bottom adults fattening) did not affect this relationship, but there was less eelgrass present on mechanically harvested beds than those harvested by hand. Oyster culture beds in Willlapa Bay had 65%-145% of the predicted amount of eelgrass present with relatively low variability between years however , suggesting that culture could even enhance eelgrass presence and that oyster aquaculture as a disturbance does not cause persistent effects on eelgrass at the landscape scale.

Technical Abstract: Both seagrasses and bivalve shellfish provide valuable ecosystem services including nursery habitat in estuaries worldwide. Seagrasses are protected by no-net-loss provisions in US federal and state regulations resulting in a precautionary approach by managers that avoids any direct impacts from development activity including shellfish aquaculture. Recent research suggests that oyster aquaculture has direct impacts to the native seagrass (eelgrass, Zostera marina) in U.S. West coast estuaries, but these occur over relatively small spatial and short temporal scales. We quantified the impacts of oyster aquaculture on Z. marina at the larger estuarine landscape scale in Willapa Bay, Washington which produces approximately 17% of US oysters. A model of Z. marina density outside of aquaculture was created using distance to mouth, distance to channel, salinity, elevation, and cumulative wave stress as predictors. These factors were then used to predict Z. marina distribution for aquaculture beds and compared to an inverse distance interpolation of points outside of aquaculture. We found the amount of Z marina observed within oyster aquaculture beds was less than the amount predicted by either method, but compared to the total amount of Z. marina in Willapa Bay, the impact of oyster aquaculture represented less than 1.5% of the total predicted values in any year. Type of oyster culture bed (off- bottom longline, seed = juveniles only, fattening = adults only and mixed types) did not contribute to explaining observed variation, but mechanically harvested beds had significantly lower Z. marina than hand and mixed harvest beds. The majority of beds (all harvest types) had 65-145% of the model predicted Z. marina and exhibited relatively low variability between years, suggesting that Z marina as habitat is resilient and that oyster aquaculture as a disturbance does not result in persistent effects at the landscape scale in this estuary.