Submitted to: Journal of Shellfish Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2008
Publication Date: 4/30/2009
Citation: Tallis, H.M., Ruesink, J.L., Dumbauld, B.R., Hacker, S., Wisehart, L.M. 2009. Oysters and aquaculture practices affect eelgrass density and productivity in a Pacific Northwest estuary. Journal of Shellfish Research. 28: 251-261. Interpretive Summary: While it is widely recognized that bivalve aquaculture can have significant effects on seagrass through disturbance there is little quantitative documentation of this for oyster aquaculture and eelgrass (Zostera marina) in U.S. West coast estuaries where some aquaculture practices have recently been restricted or banned. The influence of three oyster farming activities (dredge harvested on-bottom culture, hand picked on-bottom culture and longline off-bottom culture) on eelgrass were examined in this study. Both physical and biological components of aquaculture operations were found to alter eelgrass measurements. Eelgrass density was lower under all aquaculture practices relative to that found in uncultivated eelgrass meadows and was negatively correlated with oyster cover. Dredge-harvested areas had the lowest eelgrass density generally followed by longline culture and hand-picked on-bottom culture. Experimental dredge harvest treatments led to significant declines in eelgrass density that were detectable until 1 to 4 years after dredging depending on site and initial disturbance intensity. Eelgrass growth also varied by aquaculture type and with oyster density, however patterns were location specific. Total above ground plant production declined with oyster density and was 70% lower in aquaculture beds than in uncultivated eelgrass meadows. These findings do not support across the board regulations such as a ban of all on-bottom culture operations and instead argue for context specific studies and a landscape scale approach.
Technical Abstract: It is widely recognized that bivalve aquaculture can have negative impacts on eelgrass through disturbance. In some bays in the Pacific Northwest (USA), certain oyster (Crassostrea gigas) aquaculture practices have been restricted to protect native eelgrass (Zostera marina). We argue that aquaculture, like all food production systems, has tradeoffs with natural biodiversity, but that the magnitude of those tradeoffs depends on the ecological details of the production system. We identified the influence of biological (oyster-eelgrass interactions) and physical (disturbance associated with culture practices) components of oyster aquaculture on eelgrass in Willapa Bay, WA (USA). Capitalizing on aquaculture farms as large scale “manipulations”, we explored three different aquaculture practices (dredged on-bottom, hand picked on-bottom and long line off-bottom). We found that both the biological and physical components of aquaculture were related to altered eelgrass measures. Eelgrass density was lower under all aquaculture practices relative to uncultivated eelgrass meadows in 2002 – 2004. However, dredged beds had the lowest eelgrass densities (~30% of uncultivated) while hand picked on-bottom and long line beds were intermediate. Experimental dredging led to significant declines in eelgrass density (p < 0.0001) that were detectable until 1 – 4 yrs after dredging, depending on site and initial disturbance intensity. Additionally, eelgrass density was negatively correlated with oyster density even though oysters were sparsely planted (avg. 20% cover). Eelgrass growth, measured in 2004 and standardized by plant size (relative growth rate), also varied by aquaculture type and with oyster density. However, patterns in this individual-level indicator were context-specific. At Long Island, eelgrass growth increased with oyster density, perhaps due to fertilization, but Nemah showed the opposite pattern. Total above-ground production, which is a function of plant density and growth, declined with oyster density and was 70% lower in aquaculture beds than uncultivated areas. Our findings do not support recent regulations banning all on-bottom aquaculture systems because hand picked on-bottom beds often showed the least impact on eelgrass. We have shown that tradeoffs between aquaculture practices and eelgrass populations are measurable and that several management options may allow the continued co-existence of oyster aquaculture and eelgrass.