The recreational fishery for barramundi in the NT is regulated by seasonal closures, a minimum size and possession limits, and the increasing practice of catch and release fishing. The effectiveness of such regulations and practices is to a great degree dependent on the rate of mortality of the released fish. Released fish may die directly from the stress of capture, or from injuries received while being landed, or they may be at a higher risk of predation as they recover. Post-release mortality (also called cryptic mortality) has been described for many species worldwide but there is no estimate available for barramundi. As recreational effort increases in the NT, in line with projected population increases, there may be a need to tighten management of barramundi by output controls. Alternatively, other arrangements could be considered such as the designation of some more popular areas for catch and release fishing only. To properly assess the effectiveness of such restrictions it will be necessary to be able to estimate cryptic mortality. In situations where large catches of sub-legal size barramundi are seasonally common (such as at Shady Camp barrage, Roper Bar and Daly River Crossing) even a low rate of post-release mortality can lead to a significant increase in actual fishing mortality. At Shady Camp in 1989 and 1990 it was estimated that catch and release of small barramundi was so high that a cryptic mortality of only 10% could have caused an increase in total deaths due to fishing of almost 70% (Griffin, 1987). A study of recreational fishing in the NT in 1995 (Coleman, 1998) found that barramundi anglers caught a total of 229000 fish but retained only 94000 (41%) of them. If 20% of the released fish had died the extra fishing induced deaths would amount to 27000, an increase of 29% on the recorded fish mortality.

A recent study of line caught striped bass (Morone saxatilis) in the southern USA (Bettoli and Osborne, 1998) estimated cryptic mortality rates to be as high as 67%. Observations during the annual Barramundi Classic fishing tournament at Daly River (White, 1998) have shown a reasonably high number of tagged fish collected after being fatally injured (ie bitten in half) by sharks. Others may well have been completely consumed by the sharks or other predators such as crocodiles or birds. In some years of the tournament around 1% of the tagged fish were recovered dead during the tournament. While this does not suggest a high rate of cryptic mortality the actual number of deaths in those instances was likely to be substantially higher given the high abundance of predators capable of disposing of the evidence.

Information on stress physiology will also be of use to the aquaculture industry in understanding and managing stress in barramundi farm stock, but is of secondary consideration in this project.

Development of a major barramundi industry in Lake Argyle in the Kimberley region of WA is proceeding, with a developer obtaining approvals for a 600 tonne production licence and production rising rapidly as the fish approach market size. In the process of seeking these approvals, and also through a strategic environmental review of the opportunities for aquaculture in this area by Department of Fisheries (Report No. R700), the issue of environmental impacts by barramundi aquaculture has been considered a key-limiting factor to expansion towards a major 10,000 tonne industry in Lake Argyle. Presently, the capacity of the Lake Argyle ecosystem to sustainably maintain certain levels of barramundi cage culture is unknown and models need to be developed that will assist in determining the carrying capacity of specific areas of the lake. By comparison to other cage aquaculture scenarios in Australia, the one at Lake Argyle is largely unique, in that large-scale aquaculture in a tropical lake system is not being conducted elsewhere in Australia. However, a sound understanding of the environmental sensitivity of the Lake Argyle ecosystem is lacking. Clearly, for long-term management of this industry, an improved understanding of the level of hydrodynamic and environmental variability and key control points for environmental management of barramundi cage culture need to be defined. Similarly, critical control points of barramundi cage culture environmental discharge (both soluble and solids) also need to be more fully explored.

This project will build on from an existing project of the principal investigator’s, presently underway at Lake Argyle, auditing the nutrient discharge characteristics associated with barramundi fed existing commercial diets.