Evaluation of Elevated Rack Height to Control Biofouling on an Intertidal Oyster Farm: Efficacy and Economics

Commonly referred to as mud worms, members of the genus Polydora are ubiquitous polychaetes that are recognized as important pest on shellfish farms worldwide. Along the east coast of the U.S. two species – Polydora cornuta and Polydora websteri are widespread. Polydora cornuta settles on the exterior of oysters and forms thick mats of mud that inhibit oyster growth and cause oyster mortality.  Polydora websteri burrows through the oyster’s shell, weakening the shells and causing unsightly blisters on inner shell surfaces that are unappealing to consumers and decrease marketability. 

Oyster farming methods vary greatly from place to place largely dictated by the particulars of the farm’s environment. In the southern Delaware Bay, New Jersey farmers employ rack and bag oyster culture methods, rearing hatchery-raised seed in mesh bags that are secured to low lying rebar racks. The oyster racks are situated on intertidal flats that are exposed to the air at low tide.  The proportion of time the oysters are exposed to the air is dependent on rack height relative to the tide and the tidal amplitude. The time the oysters are aerially exposed will impact their ability feed, as the oysters must be submerged to have access to the naturally occurring plankton in the water. With increased aerial exposure one would assume growth may be reduced as potential feeding time is limited. Regulations to protect horseshoe crabs require farms in the Delaware Bay to maintain racks at 14” off bottom, hence racks are typically constructed with 14-15” legs.

The Delaware Bay is well suited for P. cornuta and P. websteri.  Farm management strategies involve washing oysters using  high-volume trash pumps. During the summer months “mudding” associated with P. cornuta requires oysters to washed on a weekly basis. Labor costs associated with Polydora mitigation for a midsize farm (250,000 market oysters) are approximately 700 person-hours, and equipment and supplies cost about $2,000 annually - nearly 30% of production costs.

Control of Polydora biofouling on oyster farms has been the focus of numerous studies including NESARE Farmer Grant projects in New Jersey and Maine (Haskin 2013 and Leach 2011).  Saturated brine, ice slurries, heat, and 2-day drying episodes have shown some success in reducing infestations; however, the logistical challenges of implementing these strategies precludes practical farm-scale application (Cox 2012, Martinelli et al 2022, Morse 2015, Littlewood et al 1992, Hadley et al 1997, Hooper 2011, Nel et al 1996).

In 2021, Zoe Schaedle, a student intern on our farm conducted a three-week experiment to explore rack height as potential control of Polydura sp. Zoe placed clean oysters on racks at four heights – 15, 20, 25, and 30” and after two weeks she assessed relative fouling by weighing the mud on the oysters. Zoe’s results were interesting as the amount of fouling decreased with rack height (Figure 1).  This trial has inspired the proposal submitted here in.

The questions addressed in this proposal are: (1) can I reduce biofouling, and the cost of mitigating biofouling by using higher racks? And (2) what are the other production costs and or benefits that might be associated with employing higher racks?  As it is critical to understand these tradeoffs from an economic perspective, an important aspect of this study is the development of an economic model to help visualize how a change in gear could affect farm profits.

Mud worm biofouling has been the most significant problem for my farm.  In the height of the growing season biofouling control requires 2-4 workers and multiple pumps. The costs of labor are significant and as the farm has expanded production, it has been impossible to keep the fouling at bay. New strategies for managing this biofouling are essential to the future growth and sustainability of the farm. Effective new strategies will allow us to realize improved productivity, reduced labor costs and increased net farm income.  A Farmer Grant will enable us to evaluate potential new management strategies with rigor. It will also enable us to engage an expert statistician to help us understand the results from an economical viewpoint, which will better inform our decisions as to how we can best manage this issue and achieve our long-term production goals.  This problem is not unique to our farm and we believe this study will be relevant to farms here in New Jersey, across the northeast region, and around the world.