Validation of longfin eel aquaculture potential
Attractiveness: World eel production is in the order of 150,000 tonnes per year, but demand is estimated at over 200,000 tonnes and increasing. Australia’s eel production has traditionally come from wild fisheries and extensive culture by way of stocking impoundments with elvers. This production has not exceeded 500 tonnes in any year, and there is little potential for any expansion from these sources. Aquacultured eel would however have immediate market potential. Recent studies (Ford and Roberts, 1996) have confirmed that longfin eels are highly regarded by both Asian and European consumers, and that attractive prices can be achieved. The proposed research will assist in increasing the supply of this valuable product.
From a benefit/cost perspective, the continuity of the longfin growout work now underway rates very highly. It will be significantly more cost-effective to continue the existing successful program rather than terminate it after 2 years, only to re-initiate it possibly 1 year later.
Feasibility: This project seeks to develop semi-intensive pond-based aquaculture of the longfin eel. This is particularly attractive as the feasibility is enhanced by advantageous characteristics of this species, relative to shortfin eels. The longfin eel has faster growth rate, is more abundant as glass eels, is adapted to the tropical / sub-tropical climate prevailing in northern Australia, and is therefore better suited to growout in outdoor ponds which are significantly less expensive to establish and operate than indoor tank facilities.
Feasibility is further enhanced by the track record of the PI who has comprehensive research experience with the aquaculture research and development of redclaw (FRDC 92/119). QDPI has excellent facilities to support the research, located in the tropics, and will subsidise the project directly with staff and operating resources.
Outcomes:
• generate biological information regarding growth rates and survival in relation to weaning, diets, grading and density
• identify health / disease problems and establish health monitoring protocols
• develop expertise in the investigators to equip them for further and more comprehensive research
• identify priority issues for further research
• evaluate the potential for developing a commercial eel aquaculture industry
Final report
Seafood CRC: prawn Industry black spot management: problem size and appropriate research
Freshwater fish attracting structures (FAS): Evaluating a new tool to improve fishing quality and access to fisheries resources in Australian impoundments
Knowledge to improve the assessment and management of Giant Mud Crabs (Scylla serrata) in Queensland
Evaluating effective quality monitoring methods for the Australian seafood industry
Electronic cooking end point determination and the effectiveness of alternative cooking methods for Crustacea
Questions often asked that can not be answered presently by the literature include:
- When should prawns be cooked in relation to time from harvest?
- Does this have an effect on visual appearance of the cooked prawn, eg. white head etc.?
- Is there a quality and yield difference in cooking in fresh vs salt water?
- Are cooking systems which currently use boiling followed by immersion in cold water the best option. Would a lower temperature cook for a longer period improve the process?
- What is the most effective compromise between sensory acceptability and optimal recovery?
Because prawns vary substantially in size and morphology, and the cooking equipment used by industry varies in power and capacity, at present, it is impossible to determine a time based standard that will optimise yield and quality and still remain broadly applicable.
Less powerful cookers will take much longer to return to the boil than high powered units. Thus the point of returning to the boil is a somewhat arbitrary starting point as slower cookers will expose prawns to cooking temperatures for much longer periods if using a standard time.
The industry needs a simple device which will tell processors when prawns and other crustacea are cooked. This device will measure the internal temperature during the cooking of prawns and will possibly use a cumulative cooking index or formula to determine when the prawns are appropriately cooked. This device would be based on research which determines the time / temperature relationship for enzyme deactivation as the basis for cooking calculations, and would be effective regardless of the type of cooker or maximum temperature during cooking.
Final report
A prawn cooking meter and self-centring thermocouple clip has been successfully developed for monitoring the cooking of prawns. This cooking meter provides a much needed control tool for ensuring reliable and consistent quality required of modern quality assurance programs.
The design of the prawn cooking meter relies upon the fact that enzymes that discolour and soften the flesh of cooked prawns are a major cause of quality loss. Ideally, cooking should destroy these enzymes, but experience shows there is typically not enough control over the cooking step on vessels and in processing factories to bring this about. Simple methods of timing cooking do not take account of the complex factors that can influence the rate of product heating such as size and quantity of prawns, cooker efficiency etc. This results in variable quality product, which can exhibit mushiness and discoloration including black-spot (melanosis) and autolysis.
This problem has been solved by developing a meter (Objective 1) that actually monitors the heat put into a prawn in the cooker and signals the end-point of cooking when the product is cooked enough to destroy the target enzymes, without the over-cooking that might otherwise cause toughness and weight loss. The progress of the cook is tracked by fitting a prawn typical of the batch into a robust clip, also developed in this project. The clip places a temperature sensor in the thermal centre of the tail of the prawn. The cooking end-point used by the meter is calculated from the thermal destruction rates of the enzymes that are achieved at particular temperatures. These were determined in in vitro experiments using extracts from several prawn species.
The use of the meter was tested in a number of confirmatory trials (Objective 2) where the prototype of the meter was used to successfully cook prawns of several species and size, and was, as long as the cooker approached boiling temperatures, independent of the performance of various kinds of prawn cookers used.
The meter was used to monitor a number of alternative cooking techniques (Objective 3). Of these, most interest was in sub-boiling or simmering of prawns. However, after cooking trials, this practice cannot be recommended. It fails to reach the threshold temperatures necessary to denature the enzymes that cause softening, discoloration and black spot. The underlying algorithm in the meter requires the prawns to warm significantly above the temperatures reached by simmering. Perhaps a higher sub-boil temperature can be used, as the final temperature, even in boiling prawns, is typically in the order of 90-95oC. Industry interest in steam tunnels was considered in passing by the project team, but trials were not undertaken. There appears to be no reason why this technology would not be suitable.
After the meter validation trials, ten prototype meters were manufactured and tested by industry with favourable results (Objective 4). These tests used a number of species under commercial conditions with cooperation of processors of both farmed and wild prawns in Queensland and Western Australia. Important feedback was obtained on the design features needed in a commercial model of the meter.
Dissemination of the results to companies participating in the trials was rapid. In addition, two workshops were held for prawn farmers in south and north Queensland (Objective 5). A workshop manual and training video have been prepared. The successful development of the prawn cooking meter has also been widely promoted in trade magazines and general media. Negotiations are in progress with a company wishing to manufacture and market the meters.
The cooking meter was tested on four other types of crustaceans (Objective 6) namely yabbies, redclaw, western rock lobsters and sand crabs. Suitable algorithms have been developed for all four species. Its use proved relatively straightforward for freshwater crayfish, which are of similar size and morphology to prawns, and the same self-centering clip could be used. However, further work is required to develop a practical temperature sensor clip for use with rock lobsters and crabs.
Keywords: processing, cooking. prawn, shrimp, lobster, crustaceans, crayfish, redclaw, temperature; enzyme de-activation, spoilage, meter, monitor.
Population dynamics and management of spanner crabs in southern Queensland
Final report
The Queensland fleet comprises some 240 vessels specifically licenced to take spanner crabs in Managed Area A, which is subject to a Total Allowable Commercial Catch (TACC), currently set at 2600 t. Another 310 vessels are licenced to fish only in Managed Area B (north of the main fishing grounds) where the TACC does not apply. At present the TACC is competitive, but in the near future an Individual Transferrable Quota (ITQ) system is to be introduced.