INVESTIGATING THE LONG-LINE NURSERY SYSTEM FOR GIANT CLAM
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND TO THE STUDY
Long-line nursery systems are versatile enough to handle a wide range of nursery systems for clams (Braley, 1999). Nursery rearing as well as grow-out can be accomplished on long-lines. Trays, tube modules and bags or cages can be hung in deep water for nursery rearing of clams. According to Braley (1999), seeded lines or socks (with adequate predator protection provided by modified lantern nets surrounding them) are commonly suspended from long-lines. Commonly used, highly productive and efficient, long-line nursery systems are flexible enough to handle a variety of shellfish species including clams and various grow-out and harvest methods. These systems are preferred in high exposure areas. Layout of a long-line nursery 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 long-line. In some locations both ends can be fixed to the shore avoiding the need for anchoring in deep water. Anchoring long-lines securely in deep water at both ends is also commonly done in sites where shore anchoring is not possible or desirable (Adams, 2003). 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. For most long-line systems, the horizontal long-line (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 (Lucas, 2001). Long-line nursery systems are usually constructed either in a surface or subsurface array. Subsurface long-lines nursery system can be built so that the entire system (floats and horizontal long-line) is below the surface (sunken). This is commonly done in clam farming to prevent surface agitation from affecting the nets or cages and to place nursed clams in deep water where temperature and salinity are relatively stable. Subsurface long-lines are also constructed so that the flotation is on the surface but the horizontal long-line is 0.5m or more below the surface (Lucas, 2001). Long-lines nursery systems for clams 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 long-line 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 (Shang, 1998). 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. To prevent excessive slack in the long-lines concrete weights, boom chain or bucket of rocks is usually attached to the anchor line. If site geography permits, it may be possible to construct a floating breakwater to protect the site from rough water and storm conditions (Adams, 2003). 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. In giant clam nursery system, aeration is important for its mixing properties rather than for the provision of oxygen, except perhaps in the embryonic and early larval stages when both good mixing and oxygen are needed. For mixing, it is wiser to use a diffuser with larger, coarser bubbles rather than one with very fine bubbles, which could cause gas supersaturation (total gases, especially nitrogen) which is lethal in the case of giant clams. Although bivalve molluscs can handle lower levels of oxygen than most other aquaculture animals it is not wise to attempt supersaturation (Braley, 1999). There are two main methods of aeration which are gravity aerators and submerged aerators. The gravity aerators are cheaper and in most cases supply the oxygen needs for both hatchery and land nursery tanks, but they do not provide mixing. An automatic mixing paddle fitted to each tank is required in combination with a gravity aerator system. Submerged aerators are probably more cost effective. This method of aeration may use air compressors or air blowers. Few hatcheries now use air compressors because there is a danger of oil leaking through rings and seals and getting into the air. Also, high pressure air is not usually needed and can cause problems such as supersaturation. Rootes type air blowers are the air blower of choice. Heslinga et al. (1990) recommend Sweetwater blowers for their reliability, corrosion resistance and quietness. Air blowers give low pressure and high volumes of air; perfect for shallow nursery or hatchery tanks.
