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Bottom racking systems, submersible rafts answer site-specific challenges

In the last Shellfish Corner (see FFN March/April 1997), I explained that some adaptation of the successful Japanese shellfish efficiency model is required to work more effectively in North American waters. The shallow nature of most lease sites here requires looking at different gear, deployment and growout methods to keep productivity levels as high as possible.

Growers must understand their site specifics for the species being raised and, as mentioned before, to think cubic with regards to the growout area in order to maximize the output of their leases - that is, to become as efficient as possible.

Bottom racks

Bottom racking systems are one way to provide this maximization. Bottom racking can be quite effective if the proper mix of current flow, predation protection, labour-saving handling design, and adaptation to husbandry practices are taken into account.

The pioneers in this method are the individuals from Digby Basin in Nova Scotia (see FFN Nov/Dec 1996), who have to work within the adverse environment caused by the extreme Bay of Fundy tides. Their fundamental research and productivity experiments in raising sea scallops have proven the value of bottom racking systems and the necessity of adaptation to site.

To help visualize how these racks are built, think of the cafeteria trolley racks that you slide dirty trays into after finishing your meal. Picture these on a little larger scale. Instead of trays, however, growers use semi rigid shellfish cages or the rectangular box-type cages that companies like ADPI Enterprises Inc. of Philadelphia, PA manufacture with its Durethene® material.

Two factors determine the size of the rack: the equipment available for deploying it (boom haulers, mini cranes, etc.); and water depth. Racks are typically made of steel tubing or something similar. An example rack may be 3' wide by 8'-10' long with levels up to 6' high. There can be multiple cages (trays), which can be accessed from either side, stacked at each level.

Since the racks are affected by the tidal action and current flow of the site, you will have to be careful with your design. If the rack becomes top heavy, there is a risk that it will fall over.

Another design consideration is the ocean bottom texture of the site. Depending on whether it is silt, mud, or firm, the rack will need legs that will allow the bottom cages to be placed as much as a few feet off bottom.

We are presently working with one company to have a rack that can be lowered or reduced by the use of controlled buoyancy. I will let you know more on this in a future column.

Submersible rafts

Surface rafting is not new to shellfish aquaculture. Stocking density and labour savings benefits make it well worth investigating.

The relatively new idea of submersible rafts has also met with a wide range of receptiveness. North America shellfish growers face three factors that are the premise or reason for considering submersible rafts:

1. The potential for damage from ice in northern areas.

2. The potential for conflict from adjacent land owners who are concerned about visual site pollution in front of these recreational properties. This is becoming more of an issue as the industry grows. The submersible raft allows an out-of-sight/out-of-mind solution.

3. Offshore siting (outside three miles) requires sub-surface deployment due to wave action. Though this use is still in the theory stage, the benefits of large-scale production are evident with regards to labour cost and less interference from close to shore regulatory agencies.

While submersing a raft may sound like a simple task, the preliminary design ideas we have been working on have proven otherwise.

There are many factors like stability, safety valves, and depth control that have to be considered. Rafts also have to be able to be shipped to sites as unassembled as possible in order to reduce shipping cost.

Rafting husbandry methods will typically use hanging cages, pre-manufactured tray type box cages, stacking tray systems, or socking in the case of mussel grow out.

Information transfer

This brings us to a very important point - the lack of information transfer and its consequences. A lot of growers do not know or understand what is available from the different types of manufacturers that are out there, and vice versa. A lot of the answers needed for aquaculture gear development already exist. However, manufacturers of the materials that could solve the challenges do not even know that the aquaculture industry exists.

A case in point is that the cage boxes used in the original bottom racking system. They were built to match the size of the rack rather than the other way around. As a result, the cost per cage box including labour was close to $30 to $35.

OBC square box cage tray that companies like ADPI produce cost around $5. If the rack maker had been familiar with the OBC size parameters he could have produced the rack to match the cage box.

That would not only have saved money, but there would also be labour savings now and in the future.

The rigid and semi rigid plastic cages and netting that are available on the market come in a wide variety of shapes, mesh sizes, and physical dimensions. Their application within the aquaculture industry is important because they fit a gear niche requirement that will allow adaptation to the specifics of the site and the species.

Companies like ADPI and a few others have made progress in transferring information about the use of their products to the aquaculture industry, there needs however to be a lot more of this pass-along by other manufactures. It will happen over time. At this point, however, we are happy to field any inquires or questions from individuals with gear supply challenges as we are in touch with a variety of manufacturing sectors.

The next Shellfish Corner will focus on semi ridged plastic netting, as well as some updates on flupsys, stocking densities, and their relationship to output productivity.

Contact Don Bishop at:
Fukui North America

110-B Bonnechere St.W.
Eganville, Ontario K0J 1T0

**NEW**Fax: 613-432-9494
Email: kate@fukuina.com or don@bishopaquatic.com

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