56 DECEMBER 2017 • WORLD AQUACULTURE • WWW.WAS.ORG On the Chuckanut Shellfish farm, predator nets are essential for survival of a marketable crop of Manila clams. Culture over multiple growing cycles has demonstrated that, without net protection or without timely net maintenance and oversight, the entire clam crop can be consumed by predaceous fish, crabs and diving ducks. Chuckanut Shellfish has observed 100 percent predation by crabs in 24 hours on year-old clams when nets have become dislodged during certain times of year. Video imagery clearly revealed the extent and intensity of predator activity on exposed and harvested tracts. What are the Differences between Mechanical and Manual Harvest Effects? There were few major or consistent differences in the benthic community before and after harvesting, no difference with mechanical harvest, and a modest decrease after manual harvest. The degree of sediment disturbance was similar for each method. In contrast with mechanical harvest methods such as suction harvest (Kaiser et al. 1996, Spencer et al. 1998), hydraulic dredging (Mercaldo-Allen et al. 2011), and “Rusca” harvest in Italy (Pravoni et al. 2004), the modified tulip-bulb harvester did not penetrate the substrate to a great depth, or remove or displace material, and therefore disturbed a relatively small volume of sediment. Recently a research team in British Columbia, Canada employed a track driven walk-behind Manila clam harvester to compare mechanical with manual harvest. This harvester was similar to the machine employed by the Chuckanut farm in Washington. To determine the feasibility and potential benthic impacts of using this harvester, a comparative environmental assessment and operational performance of both mechanical and manual (i.e. hand rake) harvesting techniques were undertaken. In July 2008, assessments were conducted at three study sites in Baynes Sound, British Columbia. Each of the three sites contained a mechanical and manual harvest plot. No major differences were observed between the effects of each harvest method. Sulphide, redox potential and sedimentation was highly variable within treatment plots and transects, within all samples for each beach, and among samples for each beach. Despite the variability in results, sedimentation from mechanical and manual harvesting was negligible in comparison to the sediment flux occurring during natural processes (e.g., storm events) (Stirling 2011, Stirling and Cross 2013). At first glance, the mechanized harvest method could be perceived as having a greater impact than manual harvest (Fig. 12). There are minimal physical differences between methods in harvest depth and redeposition of soils to the beach. The lack of increase or variations in sediment disruption could explain why this and earlier studies found no significant benthic fauna impacts between the two methods. Mechanical vs Manual Harvest Costs The main purpose of using mechanical tools to culture and harvest shellfish is to improve efficiencies and increase profitability. The current average cost to harvest Manila clams manually in Washington State is about $0.45/lb. The cost estimate of harvesting Manila clams with a mechanical harvester is $0.06/lb. Based on cost comparisons accounting for amortization of equipment and marginal costs, the harvest at Chuckanut Shellfish is more efficient (over 10 times faster) and cost‐effective than manual harvesters using short‐handled rakes. As of the date of this publication, Chuckanut Shellfish’s modified tulip bulb harvester has dug one million pounds of clams over the past 16 years. Although designed for use in a tulip greenhouse, with routine cleaning, maintenance and occasional parts replacement, it works very well as a clam harvester. The mechanical harvester performs the same work as eight manual clam diggers, with a comparable impact on harvest beds and in a shorter time. The relatively small size of Chuckanut Shellfish farm, coupled with a desire to minimize staffing requirements, and the ability of the grower to properly modify, maintain and operate culture and harvest equipment are key aspects of this farming practice. These are offset by the relatively high upfront costs of the mechanical equipment, the need for constant maintenance and repairs, and challenges of working within a narrow tidal window on a farm sometimes exposed to extreme weather conditions. Despite significant upfront costs, intensive production and harvesting methods have gained the attention of other clam growers and similar tools are now being deployed at several other locations in Washington State and British Columbia, Canada. Acknowledgments All environmental sampling/monitoring and data analyses/modeling was funded with a grant (NOAA Award NA10NMF4270309) from the Saltonstall-Kennedy Program, National Marine Fisheries Service. Field support and logistics was generously provided by the Chuckanut Shellfish farm operator and owner, Mr. William (Bill) F. Dewey. FIGURE 12 (TOP AND BOTTOM). Mechanical Manila clam harvester operating next to a traditional manual harvester in Samish Bay, Washington.
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