Multi-Species Marine Traps
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Fukui's Monthly News Letter
Submerged longlines - technology for open ocean shellfish culture
One of my primary reasons for writing this column is to share information about shellfish farming so that the industry can grow from the success of others.
This column focuses on the use of a submerged longline system for shellfish growout, a method that currently has limited application in North America but has the potential to solve site-specific problems here. The technology has been developed over the past decade by Dr. John Bonardelli, general manager of GRT Aqua-Technologies Ltd. in Gaspe, Quebec on the Eastern Gulf of the St. Lawrence.
Dr. Bonardelli, who is a producer and practical consultant, and I have known each other for a number of years and I respect his approach to aquaculture development. Combing a business attitude with biology, he sets result-oriented objectives to attain farm profitability.
Development for the submersible longline technology was driven by the limits to expansion being faced by aquaculture production in the nearshore-sheltered fjords and semi-enclosed bays. In many areas the potential for expansion is being slowed or curtailed due to user conflicts between growers and other groups such as fishermen, lobster/crab harvesters, eco-tourism, adjacent landowners, and other coastal industries.
Two-legged predators, fresh water runoff, drift ice, and, in some areas, water-borne toxins are additional challenges for the growers. To address all these concerns the most obvious solution is to move offshore from traditional lease applications.
But offshore siting has its own challenges, including: increased wave action that may cause mussels to fall off line and scallops and oysters to be stressed; biofouling; potential conflict with shipping lanes; and possible lack of access to service the farms during adverse conditions, especially weather.
In the open ocean environment, these increased risks affect each of the three traditional methods for shellfish growout - bottom culture, raft culture, and suspended culture with surface longlines.
Dr. Bonardelli decided to look at submerged longlines, a technique that had been used successfully in Japan for decades. But additional development was needed to adapt the system for the environmental conditions in the northeastern Gulf of the St. Lawrence.
The geometry of a submerged longline - how it is set in the water to maintain the necessary angels and tension or strain on the lines - depends on several factors, including: the depth of water; the depth of the line below the surface (submerged midwater depth); and the length of the longline (distance between anchors).
Dr. Bonardelli has determined that longlines of between 100 to 200 meters (328' to 656') are optimal to obtain a functional working tension. Lines longer than 200 meters will tend to buckle or sag because of loss of tension over time and the variations in current flow and depth.
In order to obtain more efficient longlines it is necessary to maintain structural tension without compromising handling flexibility. To do that depends on the geometry of the line and the quality of the equipment. For example, obtaining large enough high-pressure buoys can be the problem.
There is a significant structural difference between surface and submerged longlines, though both systems can be used for mussels, scallops, oysters, and other mollusks that growout in the water column.
The surface longline is a static structure. The tension is maintained by the buoyancy of the surface buoys and does not require any particular geometry, as long as the anchors can hold up against the currents and the boat when it attaches to it during site maintenance and harvest (see Fig. 1).
The submerged longline, in comparison, is a dynamic geometric structure (Fig. 1). It must be brought up to the surface from a midwater position so that the boat can move itself along its length without dislodging the anchors. When the mainline is set back, it should regain its original position and depth.
The tension of a longline is critical to maintaining both its working efficiency from the surface and its underwater stability, without compromising its flexibility.
The structural tension of the submerged longline and its capacity to remain flexible in order to maintain its dynamic geometry depends on three factors:
A solid anchoring system at the extremities;
A positive buoyancy that is provided by large submerged corner buoys, and;
The angle of the anchor line that is established from the relationship between the length of the anchor line and the submerged water depth (see Fig. 2). These parameters will affect the length of the unused segment or the mainline, which is the segment between the corner buoys and the extremity of the workable area.
Assembly of the longline is one of the most important steps in managing an aquaculture operation.
The longline is prepared by marking it with loops of braided twine at least every 75-100 centimeters (cm). Use of the loops, through which the mussel socks or pearl/lantern cages are attached onto the mainline, is especially important to avoid the slipping that can occur when the submerged longline is hooked with a grapnel and brought to the surface.
Cement anchor blocks are built on the site's exposure and the type of vessel being used to service the longlines.
Installation of the longlines must be done on a relatively calm day. In order to have a safe and effective operation; it's necessary to contract out the services of a large vessel for setting the gear, other than the boat used for servicing the site. It should be large enough to handle 10 anchors at once with deck space to allow lay out of the structures in preparation for deployment while underway.
Dr. Bonardelli's experience has been to develop logistics that will provide for deploying a little more than a mile of longline in about 30 minutes. He said cost works out to about $600 (US) pre 600' longline when installed 120 at a time, adding that, that is really an average estimate since the cost depends on water depth and the cost of the boat used among other variables.
The boats used for farming the site have equipping and rigging requirements that are necessary to maintain efficiency. Servicing the submerged longlines requires the use of a GPS unit to save time in locating them. Time can also be saved by using a depth sounder or sonar device to evaluate the depth of the line as the mussels grow out. Compensation buoys are added as the mussels grow and add weight.
The boast also needs an efficient hydraulic system to power the crane, which, and rollers used to bring the longline to the surface and move along its length during maintenance and harvest.
Multiple longlines are installed in parallel series to gain economics of scale on the allotted lease.
The advantages of the submerged longline system can be easily identified.
Once installed properly, upkeep is minimal and payback is possible within 5-7 years, according to Dr. Bonardelli.
The technology is simple and very efficient, but only if you get good and accurate advice. It is a large and expensive undertaking if you do not have background experience and one cannot make abstraction of the necessary security measures that are inherent in the success of the operation. Your reputation depends on them.
If anybody is interested in learning more about setting up offshore, get in touch with Dr. Bonardelli at 418-368-1652, or by contacting my office.
Contact Don Bishop at:
Fukui North America
PO Box 669
110-B Bonnechere St.W.
Eganville, Ontario K0J 1T0
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