Predicting the effect of multi-species shellfish aquaculture on food resources for local fauna using carrying capacity models

Background/Question/Methods

This talk will describe an example of using theoretical carrying capacity models to predict the effect changing shellfish aquaculture cultivation will have on the environment to inform management decisions. I conducted a carrying capacity analysis to predict the theoretical cumulative impact on food resources available to both farm-raised shellfish and other filter-feeding organisms in Burley Lagoon. Burley Lagoon is a 410 acre, 2-mile long and seven-tenths of a mile-wide estuary located at the northern-most point of Carr Inlet in South Puget Sound, Washington, USA. The purpose of the carrying capacity analysis was to determine whether the proposed increase in acreage of farmed shellfish production was possible without significantly changing the food supply available naturally to wild shellfish that inhabit the estuary. Specifically, the aquaculture operation proposed the conversion of up to 25.5 acres of intertidal leasehold to Pacific geoduck (Panopea generosa) aquaculture. At the time of the analysis, the aquaculture area within Burley Lagoon, was dedicated to oyster, mainly Pacific oyster (Crassostrea gigas), and Manila clam (Venerupis philippinarum) cultivation.

Results/Conclusions

I conducted the carrying capacity analysis consistent with the approach described by Gibbs (2007). Three shellfish production scenarios were examined – (a) continue the current 80 acres dedicated to cultivation without Pacific geoduck; (b) maintain the current acreage dedicated to cultivation and include 25.5 acres of new area dedicated to growing Pacific geoduck; and (c) maintain the current acreage dedicated to cultivation and include 17 acres of new area dedicated to growing Pacific geoduck. I calculated three performance indicators specified by Gibbs (2007) – clearance efficiency, filtration pressure and regulation ratio for each cultivated species separately. I then applied these indicators to the shellfish growing alternatives presented. Our results indicated that shellfish aquaculture in Burley Lagoon would have an insignificant influence on the availability of food resources to indigenous filter-feeding organisms that inhabit the estuary.