WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2017 53 Benthic and Epibenthic Invertebrates Invertebrates on and around Chuckanut farm are largely small organisms of little economic value to the farm but of great value as food and prey for a wide range of ecologically and economically important resident and migratory fish and shellfish in Samish Bay. The following briefly summarizes results and interpretation of samples collected at the Chuckanut farm in 2011 and 2012. Detailed results of this sampling effort are reviewed and presented in Kralj (2017). The densities and taxa richness of benthic fauna at off-farm reference sites were significantly greater than those of harvested sites. This difference between reference and farm sites is related, at least partially, to the somewhat lower elevation of reference sites and their close proximity to adjacent seagrass meadows. It was difficult to find a reference site that exactly replicated the farmed area because all the bare sand substrate on the farm was cultivated. There is a significant decrease in taxa richness after manual harvest but not after mechanical harvest (Kralj 2017). Taxa richness before manual harvest was consistently greater than after manual harvest but density or taxa richness was not different before and after mechanical harvest. There were no significant differences between swept and non-swept plots for either density or taxa richness of epibenthic invertebrates. This is surprising considering the macroalgae cover on non-swept plots. Macroalgae density comparisons between net-covered plots and non-farmed seagrass and sand areas were inconsistent. In July 2011, swept and non-swept plots had significantly greater densities of epibenthic organisms compared to the seagrass substrate but in May 2012, seagrass had greater densities than all other substrate types. A total of 172 and 125 unique taxa were identified in benthic and epibenthic samples respectively. Each taxon was assigned to a larger group to more broadly quantify proportions of each group in samples. In benthic samples, 99 percent of taxa were polychaetes, crustaceans, nematodes, bivalves or foraminifera (Fig. 10). Remaining organisms included insects, gastropods, echinoderms (Holothuroidea and Ophiuroidea), flatworms, phoronids, and an unknown non-annelid worm. About half of all animals in benthic samples were nematodes and about a quarter were worms (11 percent oligochaetes, 12 percent polychaetes). TABLE 2. Results of analyses of seasonal water sampling for chlorophyll, particulate organic carbon, total suspended solids, phosphate, silicate, nitrate and ammonium, at the NW (outgoing) and SE (incoming), and center of the farm in 2011. All samples were taken during an ebbing or falling tide (shoreward margin) shortly before the farm tract was exposed. Sample Date and Location [Chl] [POC] [TN] [TSS] [PO4-P] [SiO4-Si] [NO3+2-N] [NH4-N] µg/l mg/l mg/l mg/l µg/l µg/l µg/l µg/l Late Spring (May) Outgoing 3.7 0.9 0.14 7.2 51 1,775 7.5 11 Center 3.9 0.9 0.13 7.8 54 1,746 13.7 10 Incoming 5.1 1.1 0.15 7.7 53 1,737 12.2 15 Mid-Summer (July) Outgoing 5.8 1.3 0.18 4.3 39 1,272 2.1 10 Center 4.3 0.9 0.13 4.7 40 1,299 1.3 13 Incoming 4.7 1.0 0.15 3.8 42 1,326 0.8 12 Late Summer (August) Outgoing 2.4 0.7 0.08 19.8 34 816 2.6 30 Center 2.5 0.6 0.08 10.9 38 855 2.5 16 Incoming 2.7 0.6 0.07 13.5 42 895 3.1 25 Early Fall (October) Outgoing 4.1 0.6 0.08 16.7 43 567 32.7 25 Center 1.4 0.6 0.07 10.9 41 574 34.0 32 Incoming 1.5 1.5 0.19 13.5 34 634 22.6 17 TABLE 3. Estimated total wet and dry weight of macroalgae (Ulva, Sargassum, etc.) colonizing predator nets in 2011 and 2012. Macroalgae on nets 6/15/11 7/13/11 8/10/11 10/1/11 5/6/12 6/3/12 7/16/12 7/29/12 10/16/12 Wet Weight (t) 17.52 10.33 9.08 3.33 28.95 19.57 12.85 10.91 2.11 Dry Weight (t) 2.14 1.26 1.11 0.41 3.53 2.39 1.57 1.33 0.26 (CONTINUED ON PAGE 54)
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