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

Progress report for FNE23-038

Project Type: Farmer
Funds awarded in 2023: $20,088.00
Projected End Date: 02/28/2025
Grant Recipient: Sweet Amalia Oyster Farm
Region: Northeast
State: New Jersey
Project Leader:
Lisa Calvo
Sweet Amalia Oyster Farm
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Project Information

Project Objectives:
  1. Evaluate the efficacy of elevated rack height in controlling biofouling of Polydora in oysters grown on a mid-Atlantic intertidal oyster farm
  2. Evaluate the effect of rack height on survival, growth, disease, and condition of oysters grown on a mid-Atlantic intertidal oyster farm
  3. Examine the economic costs and benefits of elevated rack height for the control of Polydora biofouling on a mid-Atlantic intertidal oyster farm
  4. Share the results of this project with shellfish farming, extension, and research communities.

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.

Figure 1. Results of preliminary studies on the effect of rack height on mud worm biofouling. Y-axis is mud (g) and x-axis is rack height (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.


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  • Dr. Daphne Munroe - Technical Advisor
  • Kevin Sullivan - Technical Advisor


Materials and methods:

1. Evaluate the efficacy of elevated rack height in controlling biofouling of Polydora in oysters grown on a mid-Atlantic intertidal oyster farm

Three rack height treatments, 15”, 25” or 30”, will be evaluated for efficacy in controlling mud worm biofouling.  The 15” height represents the standard rack height currently used on the farm.  Racks will be constructed using ½” rebar. For each treatment four replicate two-rack units will be deployed in paired rows in one research block section of the farm.  The treatment units will randomly be assigned a location within the research block.  Five bags containing 150 like-age (approximately 1-year old), size, and stock oysters will be secured to each of the racks in May 2023. The racks will be monitored weekly through August for P. cornuta biofouling/mud-build-up. Photos of one randomly selected bag from each replicate of each treatment will be taken to document degree of fouling. The selected bags will also be qualitatively scored for the degree of mud present. Treatments showing moderate fouling will be washed using a high-volume trash pump dispensing ambient seawater.  The time required to wash each replicate treatment unit will be recorded. This will provide a basis for estimating labor costs associated with reducing biofouling. Water temperature data loggers (Hobo Pendant MX 2201) will be deployed on one replicate rack for each treatment. Relative time out of water/air exposure will be inferred from the temperature data. Statistical analyses will include chi-square test for percentage data (fouling scores) and ANOVA for comparison of total washing hours. 

2. Evaluate the effect of rack height on survival, growth, disease, and condition of oysters grown on a mid-Atlantic intertidal oyster farm

In early October 2023, samples will be collected to assess oyster performance metrics. One bag of oysters from each replicate will be sampled at the initiation the shell height of 30 oysters will be measured using digital calipers. At the end of the study the number of dead and live oysters will be counted, shell height will be measured as above, and 10 oysters from each treatment replicate will be selected for condition assessments. Condition will be assessed using the methods described by Riisgård 1988.  Disease analyses (histological and FTM) on 30 oysters per treatment group will be performed by Rutgers University experts. Shells of the 10 oysters from each treatment replicate will be qualitatively scored for degree of blistering associated with P. websteri.

Shell height, condition, disease weighted prevalence, and survival performance metrics will be statistically analyzed using one-way ANOVAs.  Statistical analysis of P. websteri infestation data will employ chi-square tests.

3. Examine the economic costs and benefits of elevated rack height for the control of Polydora biofouling on a mid-Atlantic intertidal oyster farm

Economic assessment will include a producer-level enterprise cost of production analysis (Ahearn and Vasavada 1992) for each of the three rack height treatments.  During the course of the study records will be kept on gear costs, labor costs, and yield associated with production at each rack height. This is intended to compare benefits that might be gained with reduced labor associated with reduced biofouling, with potential losses such as reduced growth rate associated with reduced feeding opportunity associated with elevated racks that remain exposed to air for longer durations. The economic model will be developed in consultation with Kevin Sullivan, Rutgers University.

4. Share the results of this project with shellfish farming, extension, and research communities.

Results will be shared in a report/article format that will be posted to various email lists targeting New Jersey and east coast shellfish growers. Hard copies will be shared with growers at a New Jersey Aquaculture Association meeting.


Research results and discussion:

Due to a delay in securing materials and constructing racks, as well as in bringing on summer staff we were unable to meet the May 1 start date and implement the experiment at proposed for the 2023 summer season. A no-cost extension for the project was requested and the experiment will be conducted in 2024.  We did; however, construct racks for the experiment and conduct a trial experiment which ran from July through October 2023.  Four rack height treatments (15", 20", 25", and 30") were evaluated, rather than three as proposed.  Wash times were measured on two occasions and oyster growth (shell height (mm)) was measured at the start on July 18, 2023 and at the end of the trial on November 7, 2023.  Biofouling varied among the treatments increasing with decreasing rack height.  Reflecting this difference in biofouling, the time to wash the oysters to remove biofouling also increased as rack height decreased (Figure 1).  Compared to the standard 15" rack, wash time was reduced by 8%, 35%, and 48% at 20", 25", and 30" racks heights respectively.  Growth also varied with rack height. Growth rate (mm per the 110 period) was 10.8 mm at 15", 11.3 mm at 20", 7.2 mm at 25", and 5.5 at 30". At the end of the experiment average shell height was 75.4 mm, 75.8, 71.7, and 69.9 mm at 15", 20", 25", and 30" rack heights respectively (Figure 1).

Figure 1

This preliminary experiment informed several changes to methodology for our planned trial in the upcoming 2024 field season as follows:

  1. As originally proposed three rack heights will be evaluated; however, 20" will be substituted for 30" making the tested treatment array 15", 20", and 25".
  2. Biofouling will be scored according to a ranking system of percent coverage.
  3. Wash times will be recorded 5 times over the course of the season rather than weekly.
Research conclusions:

To be reported at a later date.

Participation Summary
1 Farmers participating in research

Learning Outcomes

1 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Project is ongoing, however, an initial trial of the experiment informed experimental design modifications for planned trial for 2024 field season. 

Project Outcomes

Project outcomes:

Project ongoing, none to date.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.