Effects of Net Dips on Mariculture-Reared Hard Clam Depredation, Grow-out Performance, and Production Costs.

Commodities

Practices

The mariculture production of hard clams is affected by a variety of natural factors; however many growers report that none are as damaging and persistent as predation. In South Carolina, depredation from the cownose ray (CNR) is particularly concerning for growers as the rays not only feed on the crop, but cause significant damage to grow-out gear in the process, contributing to direct crop loss and additional labor and supply costs to replace the gear.

Several strategies have been developed and adopted by growers to deter predation. In South Carolina, many growers have adopted the use of polyester mesh bags for grow-out due to their relatively low cost, ease of handling and harvesting, and effectiveness to deter predation compared to other gear types. These clam bags are commonly coated with a variety of net dips, which helps reinforce the integrity of the bags. While clam bag coatings offer further protection against predation, the use of net dip is expensive, and the increased stiffness of the coated bags can lead to additional biofouling on gear and prevent clams from digging into the substrate, slowing growth and contributing to mortality on farms. Therefore, optimal gear selection for clam farmers using mesh bags requires consideration of predator protection, affordability of gear, and performance of the clam crop itself within the selected gear type.

To help inform optimal gear selection that balances cost and performance, the project team proposes to compare 3 different types of net dip treatments and a control: (1) untreated mesh bag; (2) Latex-based net dip treated mesh bag; (3) acrylic net dip; and (4) Silicone-based foul-release net dip. Gear designs will be compared on 4 commercially-active hard clam leases in SC that vary in site conditions and husbandry techniques. Working with the growers, the project team will assess the survival and growth rates (both in terms of shell height and tissue weight) of hard clams and the levels of biofouling and bag damage associated with each of these gear designs. A cost-analysis will also be conducted for each treatment, providing valuable economic data that farmers can consider during the gear selection process. Results will be published in a peer-reviewed scientific journal, incorporated into an extension one-pager to provide guidance to farmers, demonstrated in two YouTube videos, and presented in a workshop and webinar to relay results to the broader hard clam mariculture community.

The overall goal of this project is to increase farm profitability and operational resiliency for clam mariculturists using polyester mesh bags by informing effective gear selection. As net dip treated clam bags become more popular in southern USA clam operations, data from field studies is required to build best management practices for net dip selection that balances cost and longevity of gear with biological performance of the clam crop. Below are the objectives that are proposed to accomplish to reach this goal:

1. Engage with hard clam mariculturists throughout the duration of the project. They will have significant influence over project planning, mariculture lease site preparation and sampling, and dissemination of results to inform future best management practices. At the point of submission of this full proposal, all net dip treatments to be trialed in this study were proposed by SC growers as treatments that have great potential to deter predation and optimize crop performance while remaining cost-effective and implementable on current operations.

2. Conduct a field comparison of 4 commercially available net-dip treatments on current SC operations. The project team, including growers, will collaboratively assess crop performance (survival, growth), bag performance (percent fouling and damage), and economic viability following the use of these 4 treatments.

3. Disseminate results to stakeholders through tailored communication products. Target audiences include the scientific community, extension specialists, gear manufacturers, and the clam farming community.

4. Evaluate the impact of this project through grower participant surveys. Surveys will be created and disseminated to project participants to track the potential impact and incorporation of new best management practices following this project.