Using tidal energy to clean and tumble oysters

Using tidal energy to clean and tumble oysters

Summary

The removal of epiphytic (fouling) pests is one of the most labor, time, and cost intensive tasks in oyster aquaculture. Current methods of oyster tumbling and cleaning often require specialized motorized equipment that burns gas and is expensive to both procure and maintain. This equipment can be loud- creating conflict with nearby landowners and disturbance of wildlife. The movement of tides presents a potential free-and-green power source for oyster pest-maintenance.

This project tested two new suspended subtidal growth methods that used tidal flux to passively clean and tumble oysters. Oyster growth rates and the degree of fouling were measured on a monthly basis over the course of the 2016 growing season and were compared to a control group grown using a conventional float-bag system.Dana Morse of UMaine Cooperative Extension/Seagrant served as a technical advisor to the project, providing input on experimental design/layout and help in the field. UMaine grad student Sean O’Neil performed the project’s fieldwork while the P.I. was sidelined due to an injury. Pemaquid Oyster Company provided a loan of seed-stock to the project. UMaine’s Darling Marine Center served as a field site for the experiment, allowing the research to take place without a loss of leased growing space at Winnegance Oyster Farm.

Objectives/Performance Targets

The time-frame of the project was shifted one month late due to back-ordered supplies and an injury suffered in early June by the primary investigator. The full experiment was run, ending in late November.

 Oyster cages of three designs were constructed in the spring of 2016. Four cages were built of each treatment. Conventional oyster cages consisting of two rigid plastic oyster bags suspended by pontoons served as a control treatment. The control treatment cages needed to be manually flipped out of the water on a weekly basis, killing epiphytic pests by exposing them to the air and direct sunlight. After 24 hours, these cages needed to be flipped back into the water. The experimental cages were not designed to need regular manual adjustment.

Experimental cylindrical cages were designed to spin/tumble with tidal action. These cages constructed using recycled 20 gallon soda-syrup barrels and large trawl-floats.

A second type of experimental cage was designed to pivot on an axis, periodically exposing oysters to air. These cages were constructed using aquaculture-floats and plastic-coated wire grating sourced from a lobster trap company.

Oyster cages were installed on longlines at the Darling Marine Center’s subtidal aquaculture lease and stocked with oysters in the last week of June and first week of July. The experiment was monitored twice a week (when control cages were flipped). Oyster growth (length, width, and depth), the degree and type of fouling present on each cage, and the mortality seen in each treatment were measured each month- ending in late November. All cages were manually cleaned on a monthly basis, so new fouling could be accurately measured.

The experiment was removed from the Darling Center lease after the final measurement period. A project overview and preliminary data were presented at the Kennebec Estuary Land Trust’s lecture series (Aquaculture Night) to a crowd of around 40. Over 250 unique users have visited the research-project related pages/posts on the Winnegance Oyster website. At the time of writing, data is largely analyzed and the final report and figures are being prepared. Final results will be presented during posters sessions at the Northeast Aquaculture Conference and Exhibition in Providence, RI on Jan. 12^th. An account of this outreach will be detailed in the final report.

A view of the study in early November

Accomplishments/Milestones

Preliminary Results:

The cylindrical/barrel cages remained largely free of fouling through the entire experimental trial. The cages did not spin as expected with tidal flow, but instead turned very slowly in response to wave action. This slow rotation provided ample time for air and sunlight dry the cages, preventing the colonization of both algae and epiphytic invertebrates.    The pivoting cages suffered the worst fouling mostly due to colonization by invertebrates on the underside of the cages. Balance proved a critical issue. Over-sized fins could throw the cages off-balance, holding half the oysters out of the water with the potential for catastrophe crop loss. Undersized fins didn’t tip the cages far enough to reap the benefit of full air drying, resulting in increased fouling. Even with several adjustments of fin size, buoyancy, and material, the pivot cages tipped less than originally intended.    Control cages fouled as expected, with algal colonization on sun-facing submerged surfaces. Though the surface area fouled with algae often surpassed that seen on the pivoting cages, the relative absence of invertebrates on the control cages made them much lighter and easier to handle than the pivot-cages, and likely contributed to faster observed growth rates.

Fouling

Oyster Growth:

By the end of the experimental trial there were significant differences between treatments for each growth metric measured. The control cages produced the largest oysters (measured by shell length, shell width, shell depth, and total volume of oysters/cage). The barrel-cages had by far the smallest oysters, with an average length nearly a full centimeter smaller than the control. The pivot cages produced oysters that fell in-between the other treatments.

Final Shell Size

Mortality:

 There were no clear trends in mortality through the trial, with standard error exceeding the difference between treatments in most months.

Labor:

Control:

Unlike the experimental designs, control cages required manual flipping each week. The process requires farm visits on back-to-back days, and is slowed by poor weather conditions and high seas. During each monthly measurement period, these cages required manual cleaning on their top panel which primarily picked up filamentous green and red macroalgae. This fouling was easy to remove and was much lighter than the invertebrate-fouling seen on the pivot cages. The control treatment largely avoided infestations of the worst fouling pests- sea vases and tunicates.

Barrel Cages:

Barrel cages required no manual cleaning during monthly measurement periods (the only treatment where this was the case). The cage-closure was much more time consuming than the other treatments -though this was a factor to how they were built and not an inherent consequence of a rotating design.

Pivot Cages:

Pivot cages took by far the longest to clean/defoul. The fouling organisms they attracted were both heaviest and hardest to clean (sea vases and tunicates). Pest management took up to 5 times longer compared to the control (~15 min vs 3 min). The pivot cages proved very unwieldy and difficult to work with in the field.

Impacts and Contributions/Outcomes

Further exploration of the barrel cages may be warranted. Casual discussion with other growers and researchers came to the consensus that growth in the barrel cages was retarded by over-crowding (that the concave surface the oysters rested on caused “bunching”). Using a larger diameter barrel may alleviate this problem and still reap the anti-fouling benefits seen in the experiment.

Collaborators: