Reducing mortality and increasing oyster farm productivity through water grading techniques

Abstract

This project is designed to test the impact of submerging oysters during the grading process on survival and growth in Crassostrea virginica, and the resulting outcomes for commercial oyster growers. Mortality is a critical issue for commercial oyster aquaculture and the primary focus of this research. Many farms prepare for significant annual losses and can experience devastating crop losses in mass mortality events. This is complicated by differing and sometimes conflicting recommendations for management to reduce mortality under normal and high stress conditions. Less stressful grading techniques have the potential reduce mortality and increase growth rates while reducing the amount of time farmers have to pause handling activities during periods of high stress. This project consists of two studies designed to monitor the effects of this innovation on grading at a farm scale and a smaller experimental scale. We will subject oyster seed planted early and late in the growing season to two different grading treatments, submerged and not submerged grading. Measurements of each treatment line will be taken as they are graded. Treatment groups will be assessed for changes in survival and growth between treatments. Continuous monitoring of water quality conditions will allow us to make assumptions about environmental stressors impact on survival. Measurements of oyster shell shape and condition will provide insights on the different grading methods impacts on quality. Analysis of the grader’s performances will allow us to generate estimates of return on investment and cost benefit analysis for each machine. We expect to find an increase in survival and grow rate in our oysters managed with submerged grading. We expect our analysis to demonstrate a favorable return on investment due to greater gear utilization, higher survival, and low cost of goods sold. Our results will help farmers make educated decisions on equipment purchases and inform them on the efficacy of alternative grading methods. Our findings will also provide equipment manufacturers with academic research on the effectiveness of submerged  oyster grading. This project will also provide an existing farm in Alabama a submerged grading system which, in addition to being used for the project, will also be available for demonstration. New Reef Oyster Company is a subrecipient and matching contributor on this project. They will be responsible for rearing, treating, and sampling oysters throughout the project. New Reef will also procure any necessary equipment or supplies for the project.

Objectives

1. To compare differences in mortality rates between oyster seed managed with submerged tube grading and shaker grading.
2. To compare differences in mortality rates between oyster juveniles managed with submerged tube grading and tube grading.
3. To compare differences in growth rates between oyster seed managed with submerged tube grading and shaker grading.
4. To compare differences in growth rates between oyster juveniles managed with submerged tube grading and tube grading.
5. To identify the impacts of submerged grading on overall farm productivity.

Methodology

The project will operate at two scales simultaneously: farm-scale and experimental scale. All oysters will be reared in Portersville Bay for the project starting with seed raised in an on shore nursery from R1 to R4 where after oysters will be moved to the Bayou Sullivan Oyster Park (BSOP) for the remainder of the project. Seed on shore will be raised in an upweller system. All oysters in BSOP will be raised in floating, 6-bay cages.

The farm-scale project will follow 4 cohorts of ~125,000 oysters. 2 cohorts will be planted early in the growing season, likely May, and 2 cohorts will be planted late in the growing season, likely November. The seasonal cohorts will be divided into two treatment groups: submerged grading or non-submerged grading. Cohorts will be confined to their respective rows on the farm plot to prevent accidental mixing. 

The experimental scale project will follow the same seasonal cohorts, but it will add an experimental group to act as a control for immersion. Each experimental group will have 4 replicants, in randomized bag positions on the end of each of our 4 rows. 

Frequency of grading treatment will be based on the size of the oysters. Between R1 and R4, oysters will be graded weekly. Between R4 and R10, oysters will be graded bi-weekly. Between R10 and R44, oysters will be graded every 6 weeks. Measurements of representative samples will be taken at grading intervals. Representative counts will allow us to approximate total counts and calculate % mortality (Objectives 1 and 2). The grading treatment will produce size classes; change in size class between treatments will allow us to calculate growth rates (objective 3 and 4). 

These results as well as estimates of cost and productivity will allow us to identify submerged grading's economic impact.

Rationale

Mortality is one of the single largest issues farmers on the Gulf Coast face. Farmers experience significant annual losses during normal conditions with the potential for near total loss during Sudden Unusual Mortality Syndrome (SUMS) or other mass mortality events. Recommendations to industry for morality management often leave farmers in a position where they must decide which stressor they would rather subject their crops to, especially during periods of extended chronic stress. This is typically a choice between subjecting crops to stressors related to handling or stressors related to neglect. If a farmer does choose to handle their crop, they may choose to forgo grading and just reduce densities. This has the drawbacks of not identifying product that would retain on a larger mesh growing container and not identifying marketable product. Market or financial pressures may incentivize farmers to grade their oysters for this reason during periods of heightened environmental stress. It is often impractical for farms to go without sales for extended periods of time.

Farms need a less stressful mechanism to grade their crops to benefit from the reductions to mortality seen in normal conditions from higher frequency handling. Significant reductions to mortality under normal conditions would be significantly beneficial to farmers. Lessening handling stress prior to acute or chronic stress events may also increase survival through mass mortality events, especially those that are sudden or unexpected.

Submerged grading has the potential to do this. Keeping oysters submerged during the grading process should lessen the trauma the animal suffers, particularly on the growth edge of the bill. Less chipping should also reduce moralities from other handling activities such as desiccation or long emmersion times as part of the handling process.