Research Document - 2013/001
Assessing potential benthic habitat impacts of small-scale, intertidal aquaculture of the geoduck clam (Panopea generosa)
By L. Sauchyn, C.M. Pearce, J. Blackburn, L. Keddy, and S. Williams
Abstract
The geoduck clam (Panopea generosa) is the largest burrowing clam in the world, adults living up to a metre below the sediment surface. In order to extract them, harvesters use high-volume water hoses to liquefy the surrounding sediment. High-density culture of clams and/or harvesting to a depth of a metre or slightly more could have profound effects on the local benthic environment, but little research has examined the possibility. We seeded a small-scale (3 x 20 m) intertidal plot at 0.5 m above chart datum with juvenile clams at a commercial density and harvested them a year later using industry-standard techniques. We took sediment samples within the harvest zone (0 m) and at varying distances (5, 10, 25, 50 m) along three transects (onshore, parallel to shore, and offshore) from the area of impact at various times (ranging from a month prior to seed out-planting through to 12 months post-harvest). We examined various sediment qualities (i.e. grain size, percent organics, total carbon, total nitrogen, sulphide concentration, and redox) as well as infaunal diversity and quantity. Results showed that many of the measured variables were not significantly negatively affected by either the culture or harvesting processes. Those that were affected include the following. The silt and clay fraction of the sediment increased significantly immediately after (1 day) harvesting, but only within the culture plot (0 m) and the impact was short-lived, returning to baseline values within 123 days after harvesting. The sulphide concentration decreased significantly after out-planting at distances up to 25 m from the culture plot, but the values remained within the normal to oxic zones (described in Wildish et al. 1999), indicating little to no ecological impact. There was a significant increase in total carbon and redox potential 123 days after harvesting. These changes, however, were not seen 1 day after harvest and they occurred at all distances along all transects, suggesting that they were likely due to an external event, not the harvest per se. In addition, the variations in total carbon and redox were not great enough to have significant ecological implications. There was a lack of seasonal increase in infaunal abundance and richness after harvesting, but only in the harvest zone (0 m) and not at 10 m. The study, unfortunately, cannot assess the rate of recovery of the infaunal community after harvesting due to the subsequent seasonal decline in abundance and richness and lack of long-term sampling. Although there were few ecologically significant, long-term impacts of small-scale, short-term intertidal geoduck culture/harvest in the present study, changes in habitat, size of the culture plot, frequency of culture, and seasonal timing of out-planting and harvest may alter the degree of impact on, and rate of recovery of, the marine environment. The interpretation of the results of the study should be used with caution until further research (currently underway) validates findings for larger-scale operations and over a broader range of potential ecological indicators.
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