Evaluating Trophic and Non-trophic Effects of Shellfish Aquaculture in the Central Puget Sound
Bridgett Ferriss et al 2015
This article appears within the larger work, Shellfish Aquaculture in Washington State, Final Report to the Washington State Legislature December 2015.
The study took existing data from the Geoduck Aquaculture Research Project (GARP) by primary author Sean McDonald and put it through computer models to predict what would happen if the mass of geoduck planted in Central Puget Sound was increased by 120%.
They found that there was likely plenty of phytoplankton in the open waters of the Central Sound, and did not predict any effect from reduced phytoplankton on other sea life. However, they did not assess for possible impacts in closed estuaries, where there is less turnover of water, such as Willapa Bay or Burley Lagoon. The results could be far different in estuaries with relatively restricted water exchange.
They also are evaluating a region starting with low aquaculture density, estimated at 2% of the State’s aquaculture. The South Puget Sound has far greater density of plantings, approximately 12% of beaches used for aquaculture, and effects might be much more extensive.
They also did not assess the potential impacts on zooplankton. Although not well studied, it is likely that geoduck planted in dense intertidal plantations will ingest the larval stages of other marine animals, especially forage fish and out-migrating salmon. Note that Chum salmon spawn in Woodland Creek and smolts move to salt water in Henderson Inlet where they remain in their early growth stages.
This study predicts “a notable impact on the food web” with up to 20% in increased and decreased biomass of species. There was major reduction of wild salmon, Walleye pollock, resident eagles, resident birds, migratory eagles, Great blue herons and predatory gastropods. “The biomasses of 9 of the 10 functional groups changed substantially.”
“the impact of antipredator structure (PVC tubes and nets) placed on geoduck plots had a larger influence on the surrounding food web by providing predation refuge or by changing foraging opportunities. In turn, these effects propagated throughout the food web.”
“The demersal Fish and small crustacean functional groups were sensitive to increased cultured geoduck biomass and subsequently induced biomass changes throughout the food web. The species’ substantial bottom-up influences is due to the aggregation of multiple key prey species into single functional groups and their multiple trophic connections across the food web.”
“The substantial decrease of most bird groups in the model is important to note as these are important ecologically, culturally, and socio-economically.”
This evaluation of the net effect of geoduck aquaculture on the ecosystem of Central Sound suggests that there is a significant shift of the previous balance between various organisms including birds, fish and crustaceans with an overall distinctly negative impact. They recommended further empiric research, but to the best of my knowledge, none has been done.