WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2017 55 following cycle. What are the effects of this culture practice? These netted habitats contained greater densities and taxa richness of invertebrates compared to adjacent bare sand but lower mean richness than eelgrass covered sand sampled off the farm. Predator netting causes sediment accumulation beneath nets (Kaiser et al. 1996, Spencer et al. 1997, Bendell-Young 2006, Dumbauld, Ruesink et al. 2009). Simenstad and Fresh (1995) found greater amounts of sediments in the <1-2mm grain size on gravel substrate under netting than on un-netted plots and suggested that nets may prevent resuspension. In similar studies (Spencer et al. 1996, 1997, 1998), organic enrichment and increased infauna densities occurred within net covered areas. Kaiser et al. (1996) examined the effects of the growing phase of Manila clam cultivation on the intertidal benthic community at a commercial clam farm in southeast England. Predator netting, not the presence of clams, was the main cause of sediment deposition and community changes. In this case, although the density of benthic fauna was greater within netted plots than in control plots, there was no significant difference in species diversity between netted and control plots. The increased abundance of benthic fauna may have been related to predator exclusion and hydrographic changes associated with the netting, which increased food supply and the likelihood of larval settlement. As indicated by video observations taken immediately after harvest, there is a large pulse of prey availability after predator nets are removed and clams are harvested. Many prey items existing under netting were protected from predation for up to three years. Luckenbach et al. (2016) reported decreases in the abundance and biomass of infauna (exclusive of cultured clams) on clam farms, including in harvested areas, relative to natural uncultivated areas. This was accompanied by substantial increases in epibenthic macroalgae, which in some cases supported increased epifaunal species richness and abundance relative to uncultivated areas. Habitat use by finfish, crustaceans and turtles was largely unaffected by the presence of clam farms. These reports indicate site-specific conditions have a major influence on benthic and epibenthic usage and abundance. Using predator nets in Manila clam cultivation can result in lower or higher densities and taxa richness of invertebrates compared to control sites. However, sediment accumulation does not occur at the Chuckanut farm, at least from spring to fall. This is likely the result of high wind waves and moderate currents that act to sweep fines from nets and occasionally expose buried net margins. Munroe et al. (2007) also investigated possible sedimentation on netted clam ground and did not find increased sedimentation in clam farms in Baynes Sound, British Columbia. Biofouling and Net Maintenance Predator netting creates a hard substrate and becomes fouled with macroalgae and other flora and fauna in a community that is different from what might typically be found at that location. The production site was previously bare sand substrate with no structured habitat. Macroalgae production may be stimulated by the presence of clams (Bendell et al. 2014). Similar net fouling is associated with the use of predator nets at other farm locations (Spencer et al. 1997, Powers et al. 2007). Habitat provided largely by macroalgal growth on protective bottom mesh of clam leases supports elevated densities of mobile invertebrates and juvenile fishes similar to that of natural seagrass habitat, thereby representing a previously undocumented ecosystem benefit of bivalve aquaculture (Powers et al. 2007, Luckenbach et al. 2016). This ecological role for structural habitat rising above clam aquaculture growing beds is consistent with a broader recognition that artificial reefs, plastic seagrass, oyster shell mounds, and other emergent bottom structures provide significant habitat services. Macroalgae fouling was routinely swept from the nets during spring and summer. If allowed to accumulate, net biofouling resulted in markedly increased juvenile clam mortalities. While most algae swept from nets drifted away with tidal flow, some algae remained in aisles between rows or prevented water from draining from the farm on ebb tides. However, algae removal by sweeping did not impact epibenthic organisms associated with the netting. Do Nets Protect Clams? There are contrasting views regarding the value of predator nets as a farm practice and their ecological impacts. Bendell (2015) concluded there was little evidence that nets protect farmed clams from predators mainly because they do not effectively exclude epibenthic predators such as crabs and fish (or are an attractant for these bivalve predators), poor husbandry of the nets results in gaps in the nets allowing for predation, and infaunal predation rates are high and infaunal predators can not be excluded (Cigarria and Fernandez 2000). A companion study (Whiteley and Bendell-Young 2007) conducted in the same region observed few effects of Manila clam culture on benthic bivalve diversity and abundance at multiple sites in Baynes and Barkley, and Desolation Sounds, British Columbia. Manila clam abundance and 25 other bivalve species densities were no different between sites in the mid-intertidal areas. Although farm sites had similar characteristics in species composition, differences between farm sites and references sites were small. Munroe et al. (2015) provided data based on a review of more than 35 peer-reviewed articles and their own research that demonstrated the importance and efficacy of predator protection for clam farms in various locations around the world. A randomized side-by-side comparison of Manila clam survival and harvest between protected (netted) and unprotected control (non-netted) plots, showed clam survival and yield were consistently improved by using nets. What Works for Chuckanut Shellfish? Chuckanut Shellfish relies on several measures to improve clam survival. The netting is made of a relatively stiff and heavy polypropylene material and is not easily disturbed by waves or currents. Mechanical net deployment involves burying the margins of the net row and securing ends of netting material to exclude access for potential predators. Once the net is in place, clam seeding occurs during early tidal exposure when most potential predators are absent. This allows sufficient time for small seed clams to burrow beneath the surface covered by netting. Finally, netting is monitored throughout the culture cycle to ensure edges of the material are not exposed and to repair holes or damage to the net as a result of floating debris. These aspects of net type and installation and careful net husbandry are essential elements of the farm operation. (CONTINUED ON PAGE 56)
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