Longline Culture Systems
Commonly used, highly productive and efficient, longline systems are flexible enough to handle a variety of shellfish species and various grow-out and harvest methods. Longlines are used worldwide to grow everything from scallops to seaweed. These systems are preferred in high exposure areas.
In a longline system a length of line is anchored at both ends, flotation is attached and various types of culture systems are hung on the line. These include lantern net, pearl net or ear-hanging of scallops, grow-out of mussels, nursery rearing of clams and oysters, and grow-out systems for oysters involving tubes, trays, bags or string cultch.
Similar to raft systems, several considerations must come into play when planning a longline system and these can help the grower determine which system best suits the farm’s mode of operation.
- What will I grow?
- What methods will I use to grow them?
- How often will I need to inspect or grade and sort the crop?
- How will I harvest from the longlines?
How dense will I grow the crop?
- How much flotation will I need?
- Where will I anchor the longlines?
- Can it withstand currents, wind, waves?
- How much does it cost to build?
- How long will it last?
Layout, Materials and Construction of Longlines
Layout of a longline system depends very much on site characteristics. The most significant feature from the point of view of security and stability, is the availability of shore that can be utilized for anchoring one end of the longline. In some locations both ends can be fixed to the shore avoiding the need for anchoring in deep water. Anchoring longlines securely in deep water at both ends is also commonly done in sites where shore anchoring is not possible or desirable.
Length of the line depends entirely on the site itself. With a large site, lines can be over 100m long which permits working along the lines with fewer time-consuming transitions from one line to the next. In the case of Great Little Oyster Company, this works perfectly well with their mechanized harvest system.
For most longline systems, the horizontal longline (or “backbone”) consists of a 1/2″ to 1″ poly rope. To this the floats and tubes or trays are attached. If this is not done carefully, the result will be lost floats, lost stock or both.
Longline systems are usually constructed either in a surface or subsurface array. Subsurface longlines can be built so that the entire system (floats and horizontal longline) is below the surface (sunken). This is commonly done in scallop farming to prevent surface agitation from affecting the nets or cages and to place scallops in deep water where temperature and salinity are relatively stable. Subsurface longlines are also constructed so that the flotation is on the surface but the horizontal longline is 0.5m or more below the surface.
Longlines are fixed to shore by means of galvanized shore pins which must be forced into holes drilled into shoreline rock.
In most cases it will not be possible to attach both ends of a longline to shore pins. Usually one- to two-ton concrete blocks, with anchor eye to attach the line, are used to secure the deep water end(s) of the line. Steel anchors similar to boat anchors (e.g. a plow type anchor) are also used to secure the line and the type will vary according to the bottom conditions on the site. Depending on the length of line and the layout, more than one line can be attached to each anchor. Great Little Oyster Company uses two-ton concrete anchors to which 3 lines are attached. Even spacing of the lines is achieved using a head line at right angles to the longlines. The length of anchor lines (“scope”) in a longline system are usually 3-4 times the depth.
To prevent excessive slack in the longlines concrete weights, boom chain or bucket of rocks is usually attached to the anchor line. Pearl Seaproducts Inc. uses special floats attached between the longline and anchor. These are similar to ones used in salmon culture operations. They are designed to submerge with high tides and hold the line tight at low tides when floating at the surface.
If site geography permits, it may be possible to construct a floating breakwater to protect the site from rough water and storm conditions. Pearl Seaproducts Inc. have built a floating tire breakwater from recycled tires. With this protection, the lines are almost always accessible to the crew on the skiff to transport the trays to and from the line. A piece of styrofoam is wedged into the inside of the tire and the tires are strapped together in staggered rows to form a breakwater which, in this case is shore anchored on one end and deep water anchored on the other.
Culture Systems on Longlines
Longline systems are versatile enough to handle a wide range of culture systems for oysters, clams, scallops and mussels. Nursery rearing as well as grow-out can be accomplished on longlines. Trays, tube modules and bags or cages can be hung in deep water for nursery rearing of clams, oysters or scallops. Seeded lines or socks (with adequate predator protection provided by modified lantern nets surrounding them) are commonly suspended from longlines. Scallops are normally grown-out on sunken longlines in suspended lantern nets or they may be ear-hung directly on to a down-line. (See the sections on Scallop Culture and Mussel Culture for more information.)
Grow-out of oysters in longline off-bottom systems is usually done in one of three ways:
- Down-lines (“gangions”) (oyster blue) of shell cultch attached to horizontal longline
- Gangions of tubes attached to horizontal longline, and
- Tray stacks attached to longline floats or horizontal longline.
1. Stringing shell cultch on “oyster blue” gangions is essentially the same process for longlines as it is for rafts. Without a platform for setting up the tables on which the shell cultch is strung, a skiff or other mobile work area is necessary. Longline rollers or starwheels attached to the sides of the skiff bring the longline up to the gunnels and the lines of cultch tied on. At harvest the skiff can be run along the longline, down-lines hooked on to a hoist and lifted on board.
2. The technology for culture systems using tubes is advancing rapidly and with it the means for handling tubes on longline systems. With limited availability of beach area for intertidal systems, Great Little Oyster Company sets tubes in floating tanks, suspends the tube modules in deep water for nursery rearing, hangs the tubes individually on the longline for grow-out, and harvests the tubes mechanically, all using lines and floats. In other operations the tube modules may be set in land-based tanks, placed intertidally for nursery rearing, then the tubes transferred directly to longlines once the oysters have grown large enough.
3. Tray culture systems that are used for growing out oysters are intended to be able to produce a high grade product that can be marketed in the halfshell. To accomplish this, however, a great deal of handling is involved because the oysters must be regularly sorted and graded throughout the growth cycle. Every three or four months trays are pulled up, stacks taken apart, oysters put through a hand or mechanical grading process, the trays restocked, stacks rebuilt and defouled and returned to the water. Pearl Seaproducts Inc. has developed a longline and tray culture system for high grade oysters featured below. (See the section on Dark SeaTM Trays.)