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

Biofouling is an ever-present problem for shellfish farmers. Among the many biofouling agents are mud worms, genus Polydora. Mud worms can cause significant economic loss on oyster farms. This project evaluated the efficacy of elevated rack height as a strategy to control mud worm biofouling on an intertidal oyster farm employing rack and bag culture methods. During the study oyster production and cost data were collected for oysters grown at each rack height (15″, 20″, and 30″) in an experimental farm field trial. This information, along with past business records for the rack and bag farm operation, was used to inform an economic cost model. Rack height had a significant effect on oyster growth, mortality, and mud worm fouling during the experiment with higher growth, mortality, and fouling with decreasing rack heights. Oyster condition, shell strength, shell height, shell width, and shell depth did not significantly differ among rack heights. Economic analyses indicated that each rack height is practicable and can be financially viable. Some labor cost was saved by using elevated racks because of reduced biofouling; however, labor increased overall because of the increased time to grow-out on the elevated racks. Higher capital and labor costs, and lower growth rates associated with higher rack heights, were more than offset by increases in survival, thus increasing overall revenue and profits when using higher rack heights. The economic model presented here suggests that even small improvements in survival can greatly improve profitability. Conversely, small reductions in survival could lead to consequential cash flow problems. This study combined experimental results and economic modeling to demonstrate that for this intertidal farm increasing the height of oyster racks might prove an effective strategy to control fouling pests.