Objectives:

1. To evaluate the production of mulloway juveniles using intensive and extensive techniques.
2. To stock two intermittent lagoons with some 50,000 juvenile mulloway.

Non Technical Summary:

Mulloway, Argyrosomus japonicus (formerly A. hololepidotus) is an economically important species targeted by commercial and recreational fishers in all Australian states except Tasmania. In New South Wales legal size limits have been introduced to increase protection of the fish stocks. Commercial catches in New South Wales have declined from 154 t in 1992/93 to 88 t (value $640,000) in 1997/98 (ABARE, 1995; 1998). Consequently, interest in the development of techniques for the production of mulloway to enhance wild stocks and for aquaculture of table-fish has increased.

Reseeding or enhancement of wild marine stocks is not widely practised in Australia but is common in Japan and is receiving increased consideration as a means of augmenting declining fisheries world-wide. Reseeding is usually considered when the natural population has been reduced by some event, e.g. overfishing of stock, localised depletion, or when the size of the natural stock is limited by the lack of recruitment, such as may occur when intermittently opening lagoons are closed when larvae or juveniles would otherwise enter. At least the latter is relevant for mulloway.

Increasing the numbers of any species through stock enhancement may impact on other aspects of the ecosystem and should therefore be considered before large-scale enhancement programs commence. Prior to such an evaluation, the first question is whether it is possible to produce sufficiently large numbers of juveniles for stock enhancement and if the survival once stocked is high enough to be considered as an option for fisheries management. This study addressed these questions.

NSW Fisheries has been assessing the potential for the hatchery production of mulloway since 1990. Wild-caught, mature broodstock have been domesticated and held in tanks for many years. Maturation of mulloway has been controlled with photoperiod and temperature and fish have been hormone-induced to spawn. Mulloway larvae were reared for the first time in 1993 using intensive hatchery techniques. The hatchery-reared mulloway were easily weaned from live feeds to pellet diets, and ongrown to market-size in sea cages.

This research used the intensive larval rearing method which utilised dedicated, controlled facilities, high input of labour by skilled technicians, and relied on artificial propagation of live rotifers and brine shrimp for food. Mulloway were successfully reared using this technique, but it is expensive and difficult to produce sufficient numbers for large-scale enhancement. An alternative method is extensive larval rearing. This method utilises large-scale ponds, relatively low input of labour from skilled pond managers, and relies on propagation of natural zooplankton following addition of fertilisers to the ponds. Prior to this study, mulloway had not been reared using this technique, although many other marine fish species, including the closely related American red drum, have been reared in large numbers using fertilised ponds. If this method works for mulloway, large-scale enhancement may be possible.

The potential for reseeding and aquaculture of mulloway is also supported by the success of reseeding other sciaenids such as red drum, Sciaenops ocellatus in estuaries in the USA (Rutledge, 1989). Red drum are very similar to mulloway in their life history and breeding requirements and strong similarities exist in the larval rearing of the two species.

This report describes the results of trials designed to evaluate intensive and extensive methods for production of juvenile mulloway. Eight separate larval rearing trials and three large-scale experiments in commercially operated, earthen, brackishwater ponds were completed. The report also describes results of the first attempts to reseed juvenile mulloway into three intermittently-opening coastal lagoons in New South Wales. Recommendations for the next phase of research are presented.

Objective 1: To evaluate the production of mulloway juveniles using intensive and extensive techniques.

In excess of 100,000 juvenile mulloway were successfully produced using intensive clearwater, intensive greenwater, and extensive fertilised pond larval rearing methods. Intensive hatchery production of mulloway required expensive, dedicated live food and larval fish rearing facilities and a high input of labour from skilled technicians. By contrast, extensive pond rearing was possible in multi-purpose earthen ponds using relatively low input of experienced labour. The live feeds, rotifers and brine shrimp were cultured specifically to feed to mulloway larvae reared in intensive clearwater and greenwater tanks, however a natural bloom of rotifers and copepods was available to mulloway larvae reared in extensive ponds.

Growth and survival of mulloway larvae to juvenile fish was generally lower in intensive tanks (0.3-0.5 mm/d length increment; ~2% survival to 45 dah) than in extensive ponds (1.2-1.7 mm/d length increment; >20% survival to 45 dah). Cannibalism was a problem in intensive tanks, despite regular size grading, and may have accounted for the high mortality rates experienced. Infestation of a parasite, Amyloodinium sp. also caused major mortality, particularly in the intensive greenwater trials.

The optimum salinity for growth and survival of larvae and juvenile mulloway was determined. Although both larvae and juveniles grew well over a wide range of salinities from 5-35 g/L, larvae grew fastest and survival was generally best at low salinities of 5-12.5 g/L. Trends in data for juvenile mulloway growth also suggested that low salinity of 5 g/L was better.

Approximately 100,000 juvenile mulloway were produced in extensive ponds. Survival of larvae generally increased when older larvae were stocked into ponds. The optimum number of larvae to stock into ponds was similar to other fish species and ranged from 200,000-650,000 larvae/ha. Larvae grew up to five times more quickly in extensive ponds (1.2-1.7 mm/d) than in intensive tanks (0.3-0.5 mm/d). Some variability in performance of mulloway in ponds occurred and was probably due to uncontrolled environmental effects. Predation by cormorants also caused major mortality of juvenile mulloway grown in an un-netted 1.0 ha pond.

The best strategy to maximise survival and sustainable production of juvenile mulloway, may be a combination of initial larval rearing in intensive tanks, followed by ongrowing in fertilised ponds.

Objective 2: To stock two intermittent lagoons with some 50,000 juvenile mulloway.

Three intermittently opening lagoons were each stocked with about 25,000 juvenile mulloway. All mulloway had been marked with a non-toxic chemical for identification before release. Systematic, pre-stocking and post-stocking surveys were conducted in two lagoons (Khappinghat Creek and Swan Lake) to assess their suitability for stocking (e.g. no juvenile mulloway present) and the success of the stocking exercise. A third lagoon (Smiths Lake) was also stocked even though no pre-stocking survey was conducted.

Post-stocking surveys failed to capture juvenile mulloway in two lagoons, although abundance of benthic macrophytes made the sampling unreliable in Swan Lake and both lagoons were open to the sea for several weeks after stocking. Large numbers of mulloway were recaptured from Smiths Lake. The captured mulloway were identified as being hatchery-reared fish. Growth was rapid over many months after stocking and did not slow down during winter. Mulloway reached market-size (~1.2 kg) in the lagoon at about 16 months old. Commercial fishers reported catches of mulloway (2.5-2.7 kg) in Smith Lake at the time of writing (25 months after stocking).

Our results demonstrate that large numbers of juvenile mulloway can be propagated, and stock enhancement of intermittently opening lagoons is feasible. The next phase is to determine the environmental, social, and economic impacts of stocking large numbers of predatory fish. T his would involve co-ordinated effort between researchers and managers from a number of disciplines such as aquaculture, fisheries and conservation.