Chapter 11 The Biology of Barramundi (Lates calcarifer) in the Fly River System

Abstract

Barramundi ( Lates calcarifer ) is a large species of Centropomidae that occurs in coastal waters, estuaries, and freshwaters from western India, around Sri Lanka to the Bay of Bengal, and through the whole of Southeast Asia to Papua New Guinea and northern Australia. In New Guinea it occurs on the coast and inland in most rivers in the south of the country, with the largest populations in the Fly River. Throughout its range it supports extensive commercial, aquacultural, and recreational fisheries, and in Papua New Guinea it is the most economically important species in the artisanal fisheries of Western Province. Recent genetic research shows that, unlike the situation in northern Australia where there are multiple small genetic stocks of Barramundi, there is just one large stock of Barramundi living in the Fly River and associated coastal waters. This stock extends into Irian Jaya (West Papua), thereby constituting a cross-border resource, but differs from a small isolated stock of Barramundi found in the far east of Papua New Guinea. It has a complex life history and is a protandrous hermaphrodite (starts life as a male and changes to female). A complete reorganization of gonad structure and function takes place after sex inversion, probably under the influence of hormones. L. calcarifer spawn as males for 3–4 years before sex inversion. The fecundity of L. calcarifer is among the highest of any teleost fish, with estimates of from 0.6 to 2.3×10 6 eggs kg −1 of body weight. Large female Barramundi move to the coastal spawning grounds near Sigabaduru (west of Daru) during the late dry season (September–October) when gonad maturation takes place in estuarine waters (salinities of 28–36‰). Spawning takes place in the sea, and between October and February, as the wet season progresses, there is a tidal-based monthly cycle of spawning, with larvae moving into shallow coastal areas and backswamps shortly after hatching at around 5.0-mm long, usually on the spring high tides. Spawning at this time ensures that the juveniles can enter sheltered coastal areas, flooded backswamps, or floodplains and remain in these areas until they dry up during the early part of the dry season. They rapidly become the dominant predators of other fish larvae and crustaceans in this habitat. Juvenile Barramundi leave these habitats and disperse eastward (and westward) along the coast at about 6 months of age and at a length of about 200 mm. Some stay in coastal waters while the majority migrate into the lower Fly River and continue to move upstream, reaching the middle Fly River, the Strickland River, and Lake Murray where they grow to become adults. These adults do not, however, follow a regular annual migration to the sea as they do in Australia, and many fish only visit the sea once during their lives, with some individuals remaining upriver for extended periods. During flood periods many Barramundi move into off-river water bodies, particularly oxbow lakes, where there is an abundance of crustaceans and small fish, such as Fly River Herring ( Nematalosa spp.), that form the bulk of their diet. Barramundi have long been subjected to heavy fishing pressure, both artisanal and commercial, in the Fly River and along the coast of Western Province. Overexploitation by commercial fisheries in the 1980s is thought to have contributed to a massive decline in numbers, which is only now being reversed with the help of enlightened management regulations based upon a thorough understanding of the life cycle and biology of the species.