Essential fatty acid content of feedstocks used in aquaculture
Final report
Aquaculture diet development subprogram: nutrient requirements of aquaculture species
Final report
Abalone Aquaculture Subprogram: formulated feeds for newly settled juvenile abalone based on natural feeds (diatoms and crustose coralline algae)
Final report
Developing and assessing techniques for enhancing tropical Australian prawn fisheries and the feasibility of enhancing the brown tiger prawn (Penaeus esculentus) fishery in Exmouth Gulf
Prawn fisheries throughout Australia are intensively fished and some have shown signs of overfishing. In some cases, the current stocks of prawns are now lower than those which would produce maximum yields. Prawn stocks can vary greatly from year to year because of environmental fluctuations and this leads to highly variable catches. Fishery managers must therefore adopt conservative harvest strategies to prevent fishers reducing stocks to dangerous levels in years when recruitment is low. However, the harvesting and processing sector tend to be on average, over-capitalised, in order to cope with years of high recruitment. Enhancement of prawn stocks through releasing juvenile prawns has the potential to reduce fluctuations in stocks. It provides a possible way of adjusting the catching and processing capacity to more stable levels of prawn stocks, which would reduce the need for over-capitalisation.
The enhancement of Australian penaeid prawn fisheries has the potential to be a useful management tool to increase fishery yields, rebuild over-exploited stocks, and reduce fluctuations in catch due to variable recruitment. It also has the ability to improve the management of fisheries by collecting more precise information about the biological characteristics of the stock (e.g. survival and growth, production in nursery grounds, migration pathways and factors affecting fluctuation in populations). For stock enhancement to be successful, the biology and ecology of the target animal must be thoroughly understood (including the production of the postlarvae/juveniles, environmental requirements, carrying capacity, and all factors that contribute to mortality), and methods must be available to monitor and assess the success of the releases. Much ecological information for stock enhancement is now available for many commercially important species of penaeid prawn in Australia, and novel approaches to tagging prawns (e.g. stable isotopes, rare alleles and reporter genes), release strategies, and assessment of carrying capacity are being developed.
Most of the preliminary assessments of the costs and benefits of prawn stock enhancement in Australia have not assessed a particular fishery or region in detail – they have to some extent developed generic models. For our knowledge on how to enhance prawn stocks in Australia to progress further, it is essential to develop, apply and refine bioeconomic models to a specific region and fishery. For the reasons outlined above (see background), the Exmouth Gulf Prawn Fishery is an ideal location to focus on applying the concepts and the simple model that have been developed from different studies around Australia. The much more intensive study outlined in this proposal will also help to evaluate enhancement projects for other prawn fisheries around Australia.
The beneficiaries of stock enhancement would be expected to contribute to the costs of research and monitoring, and ultimately pay for the enhancement. Therefore, stock enhancement must be cost-effective and a cost-benefit analysis using a bioeconomic model, is an essential part of any enhancement project. Bioeconomic models need to be developed in the early stage of the feasibility study. If the outcomes are favourable for enhancing tiger prawns in Exmouth Gulf, it will be used to optimise the design and management of the trial enhancement program proposed for Stage 2 of the full project.
Final report
Abalone Aquaculture Subprogram: optimisation of essential lipids in artificial feeds for Australian abalone
Final report
Lipids and in particular polyunsaturated fatty acids (PUFA) are important for a number of physiological functions in animals. PUFA are also major components of cellular membranes. Many marine animals cannot synthesise "essential" PUFA de novo and therefore serious deficiency signs such as low growth rates, reduced survival, low fecundity and lowered disease resistance can occur if sufficient amounts of these fatty acids are not supplied in the diet.
The main focus of this research was to determine the requirement of abalone for PUFA for optimal muscle growth and the appropriateness of alternative lipids as sources of essential PUFA. None of this work had been performed for abalone previously.
Keywords: Abalone, Diet, Formulated diets, Lipids, Nutritional requirements, Polyunsaturated fatty acids, Sterols
Increased production of juvenile Pacific oysters (Crassostrea gigas) through supplementary feeding
Final report
The standard method for growing the early stages of juvenile Pacific oysters is to hold them in systems called upwellers in land-based nurseries. Seawater is pumped through to provide the oysters with food particles. Growth rates of oysters cultured using this method were highly variable at Pipe Clay Lagoon, one of the major oyster nurseries sites in Tasmania. Growth rates during the 1996/97 production season were less than one-third of that seen in the five previous seasons, and were also significantly less than at another oyster nursery - Little Swanport.
We conducted 15 trials at Pipe Clay Lagoon to assess whether the oysters' growth rates could by improved by "supplementing" their natural diet with additional feed sources. These supplementary diets included cultured microalgae, dried or concentrated microalgae and a yeast-based artificial diet. The results were variable, depending on the diet, its concentration, and the season - though across all trials we found that supplementary feeding (on average) increased the oysters' growth rate by 60%. Diets that were most effective included the microalgae Isochrysis sp. (T.ISO), Chaetoceros calcitrans, Dunaliella tertiolecta, Rhodomonas salina and microalgal concentrates of Chaetoceros calcitrans and Skeletonema costatum. Two commercial "off-the-shelf' products - Microfeast® MB-30 and Algamac 2000 - were also effective, though not as good as microalgae. Nevertheless the cheaper cost of these commercial products (~AUS $80-100 kg1 dry weight) compared to microalgae (eg. AUS $ 375 kg-1) makes them viable alternatives to microalgae for supplementary feeding.
Supplementary feeding was most effective when natural levels of food (especially microalgae) in the inflowing seawater were low. For example, during one such 7 week period, supplementary feeding improved the growth rate of oysters by 3-fold.
Preliminary cost/benefit analysis of supplementary feeding was undertaken. Major factors include the cost of producing microalgae (dependent on scale and productions rates of the microalgae which vary from site to site, and seasonally) and the growth rates of non-supplementary fed oysters. The latter have a major influence on whether significant growth increases are possible through supplementary feeding, and were shown to vary significantly seasonally and from site-to-site. However, based on "average" microalgal production costs and the growth rates seen with supplementary feeding, we estimate that the direct additional feed costs would amount to $0.35 per thousand oysters to grow them from 0.5 to 3.0 mm. This compares to the total production cost of ~$15 per thousand oysters of 5 mm size. We believe the increased feeding costs for the nurseries would be more than offset by savings due to a reduced nursery time for the spat (less labour).
Supplementary feeding is probably restricted to use at sites like Pipe Clay Lagoon where oyster growth rates are reduced as a result of low or variable availability of food particles. There may be little benefit with supplementary feeding at Little Swanport, where the natural growth rates of oysters exceeded those of supplementary-fed oysters at Pipe Clay Lagoon. Nevertheless, we have demonstrated that supplementary feeding is an effective method for significantly enhancing growth rates of oysters at sites when natural productivity is otherwise low, providing the ability to have better control over juvenile oyster production. As a result, Shellfish Culture now plan to incorporate supplementary feeding as part of the routine production for juvenile oysters at Pipe Clay Lagoon.