Projects

Maximizing the return on investment of oyster aquaculture by managing mud blister worm infestation

End Date: 01/31/18

Kelly Dorgan of Dauphin Island Sea Lab and Bill Walton of Auburn University will conduct lab and field experiments with mud blister worms, which can bore into oyster shells and affect appearance and taste of the meat. The study will look at the worms under varying conditions at oyster farms to better predict when the worms are likely to attach themselves to oysters. This information can advise oyster growers on how often treatments are needed to control worm populations.

Abstract

Off-bottom oyster farming is relatively new on the Gulf Coast and may provide a large economic boon to the region, both to producers and the support industry, and helps watermen continue working on the water even when fisheries close. As oyster aquaculture is focused on the half shell market, it does not compete with the wild harvest intended for the shucked market.

A key challenge to this industry, noted by farmers and chefs, is that whereas periodic air-drying controls most bio-fouling, mud blisters formed by polychaete worms such as Polydora websteri seem to be resistant to current methods.

This proposal aims to develop methods to cost-effectively control mud blister worms in the northern Gulf of Mexico by targeting specific questions about the life history of the worms and growth rates of oysters and worms.

Understanding the factors contributing to mud blister worm infestations will enable farmers to target specific times and conditions for labor- or cost-intensive treatments and to better assess the costs and benefits of treatment. We plan to integrate laboratory experiments with a 1.5-year field experiment in which oysters are deployed at 4 different farms along the coast of Alabama. Different stocking densities and ploidies will be tested to vary growth rates, and oyster condition, infestation, larval worm abundances and distributions, and environmental conditions will be sampled to better understand these factors contributing to worm infestations.

Throughout this project, we will work closely with farmers to share results, get updates from the field, and discuss strategies to maximize the return on investment for this growing industry.

Objectives

The overarching goal of this project is to develop methods to cost-effectively control mud blister worm populations in oyster farms in the northern Gulf of Mexico. A major challenge is that methods used worldwide to control mud blister worms are typically reactive, labor- intensive and often detrimental to the oysters. Thus we propose an approach that integrates field and laboratory methods and includes surveying the extent of the problem locally and targeting specific questions about the life history of the worms to develop new pro-active solutions to avoid or reduce the infestations. Specifically, we aim to better predict when and under what conditions worms are likely to settle onto oysters and how frequently treatments should be administered to be most effective.

Our specific objectives are

  1. To survey mud blister worm infestation rates seasonally and under varying growing conditions
  2. To determine whether and how larval recruitment affects rates of infestation
  3. To inform growers of the relative benefits of costly strategies to treat mud blisters

 

Methodology

We propose a combination of field and laboratory experiments to relate infestation rates of mud blister worms on oysters to larval worm abundances and distributions, growth conditions of oysters, and environmental parameters to address the following hypotheses:

H1: Mud blister occurrence and impact on oyster condition depends on non-local sources of larvae;

H2: Mud blister occurrence and impact on oyster condition depends on local larvae from worms inhabiting the outer shell; H3: Mud blister occurrence and impact on oyster condition is strongly inhibited by high growth rates of oysters.

Specifically, we will deploy triploid and diploid oysters at different stocking densities to manipulate growth rates at 4 farms along the coast of Alabama. Oyster condition, infestation with mud blister worms, larval abundances, and environmental parameters will be sampled seasonally over 1.5 years. Laboratory experiments in conjunction with field sampling will be used to determine the extent of brooding behaviors (and thus whether local worms may be an important source of larvae) and settlement behaviors and durations. Additionally, population genetic analyses of adult and juvenile worms will be conducted to determine relatedness of populations within a farm.

Rationale

Off-bottom oyster farming, focused on the premium, half shell market, is relatively new on the Gulf Coast. With funding from Mississippi-Alabama Sea Grant and National Sea Grant, Alabama and Louisiana have developed nascent industries, with permits pending in Mississippi and Florida. In Alabama, there are currently nine commercial oyster farms, with over 4 million oysters in production currently (Walton, pers. obs.) and three in Louisiana. Oyster farming may provide a large economic boon to coastal communities in the North Central Gulf of Mexico region, both to the producers as well as the local support industries. This can be a critical component when fisheries have slowed or been closed.

Moreover, oyster farming complements the wild-harvest oyster fishery, as the majority of wild-harvested product is intended for the shucked market rather than the half-shell market. In addition, shellfish farming may help watermen maintain traditional ways of life, keeping them working on the water even when wild harvest declines or the fishery is closed as it is currently in Alabama. Shellfish farming has widely been recognized as providing important environmental benefits, particularly improving water quality.

 

The primary finding from prior funded research identified periodic air-drying (where frequency and duration of exposure are controlled) as a cost-effective means of controlling most bio-fouling, and air-drying has been implemented with considerable success. Fouling by mud blister worms, however, has remained problematic, resisting the current fouling control practices; mud blister worm infestation can negatively affect taste and consumer perception, as well as make the oysters difficult to shuck. These blisters are formed by several species of polychaete worms, including Polydora websteri, that settle as metamorphosed larvae on the inside of an oyster shell and begin to bore a U-shaped tunnel.

Worms are not directly toxic to the oysters, but do cause oysters to divert energy to shell production, weaken shells (potentially contributing to increased mortality rates), and store bacteria (potentially including pathogens) within the oyster shell. Meat of heavily infested oysters has been reported as more watery, and oysters show reduced gamete volume fractions and other indications of poor health. For oyster farmers, these effects can lead to loss of potential customers, loss of current customers, and weaken the producer's negotiating position on price. Efforts to control these infestations add significant production costs and risks through increased handling time and stress to the oyster (e.g., through dramatic temperature or salinity changes). Critically, for oyster farming to be successful and sustainable within the region, producers will need to produce oysters that consistently are perceived by consumers to be of superior quality.