Multi-Species Marine Traps
Used Aquaculture & Processing Machinery
Fukui's Monthly News Letter
Shallow water siting consequences
I've talked with many individuals about my last column concerning the Japanese shellfish efficiency model (see FFN Jan/Feb 1997). The most frequently raised question I've heard is with regard to the mean water depth of that Japanese example. Most leases in North America have much less depth.
Depending on the location and the attitude of some of the regulatory bureaucracies, shellfish growout sites in North America were usually placed in areas that did not conflict with other traditional usage requirements. In other words, if the area wasn't any good for anything else, it could be used for aquaculture.
In a lot of cases that meant that the mean water depth was relatively shallow. The consequence, often, was that even proven gear was used in an experimental way, which led to less than desirable financial performance or, in many examples, failure.
This has changed a lot in some areas, as successful growout site parameters have been identified and the regulators are "in sync" with the site specifics that are desirable for aquaculture.
Most sites, however, are still in relatively shallow water when compared to Japan, where deep water growout starts at 50' and where the top biofouling area of 15' below the surface is generally unused.
We have to accept the fact that, at this point in the development of the industry in North America, we must adapt the equipment to the diversities of the lease of the growout sites that are currently in use or are scheduled for use. These include not only shallow water depths but also the presence of ice, rough seas, and a multitude of predators as well as the challenges of adjacent landowners' visual pollution concerns.
Sooner or later, depending on the time of year and specifics of your site, you are going to have to deal with biofouling, especially if you are forced to farm in relatively shallow water.
At this time, there is very little or no application on shellfish gear of the antifoulants that are being used to treat finfish cages.
The two primary reasons for this are:
1. Even though the chemical base of the antifoulant has a very low toxicity level that has been approved for finfish cages; it has not been proven acceptable for the farming of bivalves or filter feeders.
2. The plastics of polyethylene or polypropylene used to make shellfish gear do not absorb liquids and therefore the antifoulant will not stay adhered to the gear for any reasonable period of time.
The only tried and proven method of controlling biofouling is proper husbandry practices: At certain times of year, perhaps just after the mussel set and again in late summer, the cages or other gear are lifted from the water, carefully blast washed with a high pressure water sprayer to remove the fouling and then redeployed.
The farmers that use this method have adapted well to the chore and are now very efficient at it. While it might sound like more work, the payoff is quicker growout due to increase nutrient flow to the shellfish that results from the improved water flow through the gear.
There are several site parameters that could require consideration of the use of submerged gear. They include: a site that is subject to freeze over or ice pack movement; adjacent land owners who object to the sight of shellfish gear; or locations such as offshore where there are rough water conditions.
Three choices are currently available to meet the challenges of those parameters:
1. Floating longlines that have a series of floats removed so that the lesser amount of buoyancy allows the weight of the product and gear to drop to a prescribed depth.
Access for servicing is by reattachment of floats either manually from the surface or by assistance by divers.
There are a few companies that now produce inflatable buoys that can be ganged to each other by airlines. One airline with a valve tethered just below the surface.
The farmer can retrieve it when needed and, by using an air bottle to inflate the buoys attached to the tethered valve, float the long line to the surface for servicing.
2. Bottom or just off-bottom culture using stacking trays or mounting trays in specially designed racks. For example, in the Digby basin off the Nova Scotia coast, where the tidal currents are among the strongest in the world, growers have no choice but to choose a heavy bottom rack system. The current would hold a 10-level lantern net almost horizontal from its normal relaxed vertical position.
3. Submersible rafts are being developed that will probably be available some time in 1997.
Above surface rafts for shellfish growout have been successfully used for decades, but they do not satisfy the site parameters described above.
Nevertheless, they do provide a stable platform from which to work, allow higher stocking densities, requires less maintenance and anchoring than longlines. Oyster farms on the west coast that currently use the not-submersible rafts boast one of the highest returns on capital investments anywhere in the industry.
In the next Shellfish corner column, I will provide more details of the bottom racking systems, the submersible rafts, and the ridged Durethene cages.
Contact Don Bishop at:
Fukui North America
110-B Bonnechere St.W.
Eganville, Ontario K0J 1T0
Copyright © 1999-2004 Fukui North America. All rights reserved.