The effects of shellfish aquaculture on chlorophyll-a in the north east Pacific Ocean




Ford, Helen

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Food production systems need to keep pace with the rising global population. Food from aquatic environments comes from both capture fisheries and aquaculture. Industrial fishing pressure has caused a global loss of more than 90% of large predatory fishes and 80% of the world’s fish stocks are reported as fully exploited or overexploited. Global finfish, shellfish and aquatic plant aquaculture has been steadily increasing to meet the global demand for seafood. In British Columbia, aquaculture is primarily marine, with salmon and shellfish accounting for the majority of species cultured. Although shellfish aquaculture accounts for significantly less production and value compared to salmon aquaculture, the amount of foreshore dedicated to farming shellfish is nearly half (44%) the total area utilized by all aquaculture in the Province. Introduced Pacific oysters (Crassostrea gigas) (74%) dominate shellfish aquaculture in British Columbia. Pacific oysters are known to be very efficient generalist filter feeders that can grow faster and larger than native species. Extensive aquaculture is a form of aquaculture, where farmed animals feed exclusively on naturally occurring food in the surrounding water column. The goal of this research was to determine if there was a measureable depletion of phytoplankton around shellfish farms along the west coast of Canada and the United States. Chlorophyll-a, a pigment found within phytoplankton, was used as a proxy for phytoplankton abundance for this study. In field season one, two bays were studied, one exposed to shellfish culture (Westcott Bay) and one not exposed to shellfish culture (Fisherman Bay). The concentration of chlorophyll-a was measured in each bay at three locations at two depths (0.5 and 3 meters) and at two tidal heights (high and low). Chlorophyll-a concentration was found to be related to either depth or tide, with location in a bay showing no difference in either of the bays studied. In addition to water column measurements, 100 Pacific oysters were placed at two locations within Westcott Bay Seafarm to test for local differences in oyster growth. The results from this experiment showed that Pacific oysters grown in the center of a shellfish farm were smaller than oyster grown at the farm’s periphery. Field season two tested for spatial patterns between chlorophyll-a concentration and proximity to a shellfish farm in three different bays (Westcott Bay, Trevenon Bay and Gorge Harbour). A measureable depletion footprint of chlorophyll-a concentration was detected in the two sheltered shallow bays tested (Westcott Bay and Gorge Harbour), whereas no depletion footprint was detected in the exposed, deep bay (Trevenon Bay). Tide height played a significant role in predicating chlorophyll-a concentration in all three of the bays studied. These results suggested that some areas may be more suitable for shellfish culture than others. Taken together, this research demonstrated a measureable gradient of phytoplankton in sheltered shallow bays exposed to shellfish culture with depletion closest to the farm site, as well as greater oyster growth at the periphery of shellfish farms where phytoplankton would be predictably in greater abundance.



Aquaculture, Fluorescence, Chlorophyll-a, Pacific oyster