54 DECEMBER 2017 • WORLD AQUACULTURE • WWW.WAS.ORG The five epibenthic groups that were most numerous in each substrate were selected and their relative proportions calculated (Fig. 10). These groups were the same at each substrate except the swept-net plots, which had acari or mites as one of the top five, rather than foraminifera. Harpacticoid copepods were the most abundant group, representing the majority of all taxa at each substrate. Harpacticoid and cyclopoid copepods appeared to be the most variable between substrates, ranging between 50-86 percent and 6-27 percent respectively while other groups were more consistent. Farming activities such as net sweeping and harvesting had a minimal effect on species richness, composition or diversity. Salmonid prey species such as harpacticoid copepods were present before and after net sweeping. Benthic fauna displayed only slight differences after mechanical harvest and when compared with manual harvest. Fish and Crabs — A Video Perspective We gathered video imagery using tripod mounted digital point-of-view cameras between one day and one month after harvest or net sweeping. Video camera sampling was challenging in the dynamic and sometimes detritus-filled environment of Samish Bay; nevertheless, informative video imagery was obtained to characterize activities of large mobile macrofauna in unfarmed, netted areas, and recently harvested tracts at multiple dates from July through September 2011-13. Comparisons of activity by fish and crustaceans between habitat types were expressed as an average count and percentage of one or more animals per one hour observation period. During each sampling period four cameras were placed on two fixed platforms 15 cm and 45 cm off the bottom in oblique and vertical alignments. Immediately following harvest, birds, usually gulls (Larus spp.) and great blue heron (Ardea herodias) were attracted to the exposed clam tracts. Within 30 minutes of tidal inundation, an increasing number of aquatic predators began to assemble, primarily dominated by starry flounder (Platichthys stellatus), shiner perch (Cymatogaster aggregata) and Dungeness crab (Metacarcinus (Cancer) magister). Other common taxa included Cancer productus, Hemigrapsus oregonensis, Lumpenus sagitta, Pholis ornate, Leptocottus armatus and other unidentified fish and invertebrates. Figure 11 illustrates these differences for the most abundant taxa seen on the site. Fish and crabs fed on small bivalves, polychaetes and other animals exposed by the action of the harvester. Feeding activity increased for the first 2-4 hours after harvest and declined within one day following harvest to levels seen on unharvested tracts. In sum, disturbance of the harvest leads initially to increased foraging by birds during tidal exposure, followed by a pronounced and intense foraging response by fish and crabs attracted to the harvest site after tidal inundation. These foragers and predators targeted only larger organisms disturbed by harvesting. Density and species richness of smaller epibenthic and benthic organisms were not affected by harvesting. Key Aspects of Farm Operations Farming methods used at the Chuckanut Shellfish farm site are unique in several key ways. The site has a sandy substrate whereas most other farms in the region culture Manila clams in gravel, crushed shell, or mixed sand and gravel substrates. The farm is more intensively managed than traditional clam farms, with predator netting used throughout the production cycle, regular net maintenance to exclude predators, and monthly sweeping to reduce net fouling. Harvest is accomplished with a modified tulip bulb harvester during low tides when the farm is exposed, whereas the majority of Manila clams on the west coast of the US and Canada are currently harvested manually. The Chuckanut growing grounds, however, present unique challenges for the farmer. It is in a relatively isolated location in the middle of Samish Bay and accessible only by boat, and then only during medium to high tidal ranges. The wide tide range (about 7 m) and seasonality of workable low tides means the time on the clam beds is limited to windows of 3-4 hours, and markedly constrained during winter months, when the best low tides occur late at night and sea conditions can be challenging. Are Predator Nets Needed? At Chuckanut, predator nets are deployed and remain in place for the whole culture period, and are then reused in the FIGURE 10 (LEFT). Percent composition of benthic fauna (top) and percent composition of top five epibenthic taxa from each substrate type (bottom). FIGURE 11 (ABOVE). Comparison of activity by fish and crustaceans expressed as an average count per minute observation period on harvested, netted, and eelgrass plots. Similar activity by a variety of bird species was observed on the exposed tracts immediately after harvest.
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