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.
1.2 STATEMENT OF THE PROBLEM
Giant clams have long represented a valuable resource to the people in most part of the world in the form of a readily available and nutritious food source. However, in today’s cash driven economies, giant clams have also come to represent a source of export income in many countries that involve in its farming. Current uses for farm-produced giant clams include; stock enhancement, aquarium pets, biological specimens, food and shellcraft. Several methods have been used for its hatching and nursery but this system is focused on investigations on the long-line nursery system for giant clams.
1.3 OBJECTIVES OF THE STUDY
The following are the objectives of this study:
1. To examine the approaches to giant clams nursery.
2. To investigate the long-line nursery system for giant clams.
3. To identify the advantages and limitations in the long-line nursery system for giant clams.
1.4 RESEARCH QUESTIONS
1. What are the approaches to giant clams nursery?
2. What are the procedures and processes involved in the long-line nursery system for giant clams?
3. What are the advantages and limitations in the long-line nursery system for giant clams?
1.5 SIGNIFICANCE OF THE STUDY
The following are the significance of this study:
1. Outcome of this study will educate on the easier method for the nursery of giant clams for improved method of production and better yield.
2. This research will be a contribution to the body of literature in the area of the effect of personality trait on student’s academic performance, thereby constituting the empirical literature for future research in the subject area
1.6 SCOPE/LIMITATIONS OF THE STUDY
This study will cover the various approaches and systems of giant clam nursery with special focus and experiment on the long-line nursery systems for giant clams.
LIMITATION OF STUDY
Financial constraint– Insufficient fund tends to impede the efficiency of the researcher in sourcing for the relevant materials, literature or information and in the process of data collection (internet, questionnaire and interview).
Time constraint– The researcher will simultaneously engage in this study with other academic work. This consequently will cut down on the time devoted for the research work.
Adams, T. 2003. Giant clams in Fiji. South Pacific Commission Workshop on Inshore Fishery Resources, Noumea, New Caledonia. March 2003. Background Paper 50.
Braley, R.D. 1999. Manual for the culturing of giant clams. PublIcation of James Cook University, 108 p. Heslinga. G.A., Watson, T.C. and Isarnu, T. 1990. Giant dam farming. Pacific Fisheries Development Foundation (NMFS/NOM). Honolulu,
HawaII. 179 pp. Lucas, J.S. 2001. Giant clams: description, distribution and life history. In: Copland, J.W., and Lucas, J.S., ed., Giant dams In Asia and the Pacific. Canberra, AClAR Monograph No. 9. 21-31.
Shang, V.C. 1998. Aquaculture economics: basic concepts and methods of analysis. Boulder, Colorado, Westvlew Press.