Mangrove jack in Australia
  |  First Published: June 2003

IN APRIL this year the DPI released their findings from the most comprehensive study of mangrove jack (Lutjanus argentimaculatus) undertaken in Australia. The three-year project, titled ‘Biology, management and genetic stock structure of mangrove jack in Australia’, was conducted by five staff members from the Fisheries Research and Development Corporation – D J Russell, A J McDougall, A S Fletcher, J R Ovenden and R Street.

Findings from the research make fascinating reading. The report notes that mangrove jack range across the northern part of Australia, the Indo-west Pacific from Samoa and the Line Islands, to East Africa and from Australia northwards to Ryukyu Island, Japan. In northern Australia, mangrove jack generally range from northern NSW to Shark Bay in WA, although they have been caught as far south as Sydney Harbour.

The mangrove jack is the second most sought-after coastal species, behind barramundi, for northern anglers. As well as its major significance as a recreational species, mangrove jack are receiving a lot of attention for their potential as a ‘put and take’ species in impoundments and as an aquaculture species.

Mangrove jack in Australia range from the outer reef, in depths up to 100 metres, right up into the freshwater reaches of coastal streams, with juveniles being found upstream to 130km from the coast. Research indicates that all freshwater and estuary mangrove jack are juveniles and that they don't reach sexual maturity until they migrate offshore when they are between three and 11 years old. Jacks appear to spawn at sea and juveniles move up into the freshwater reaches of coastal streams during the wet season.

Mangrove jack are long-living fish, with otolith growth records indicating they live for at least 37 years and possibly as long as 40 years or more. Their growth rates slow considerably as they age. The largest confirmed length of a mangrove jack is 120cm, and the smallest captured during the project was a mere 20mm, taken at the mouth of the Russell River during the wet season.

Research indicates that juveniles move inshore during the wet season, after being spawned well offshore, and move into the freshwater sections of coastal rivers – possibly attracted by wet season run-off. Population density was heaviest around the mouths of estuaries and gradually decreased further upstream. By far the most popular habitat for mangrove jack was rock walls and bars, followed by snags. This was particularly evident with very small jacks.

Tagging research shows that juvenile jacks are territorial, with 73% of recaptures occurring within one kilometre of their previous release point. In many cases they were recaptured in exactly the same location where they were originally tagged. Fish that moved offshore travelled considerable distances, with one jack moving 335km along the north Queensland coast. Those jacks that moved offshore were generally recaptured in coral reef areas.

Nearly all mangrove jack captured by recreational fishers in estuaries are juvenile fish, with 50% of female jacks reaching sexual maturity at 512mm and males at 459mm. This has considerable significance to fishers, as the legal size of 350mm is well below the size at which they become adults. Authorities seem to feel that in spite of this the current legal size is a fair balance between allowing recreational anglers to target mangrove jack in estuaries and leaving sufficient juveniles to mature and maintain adequate stocks.

Anglers have commented that jacks seem to be bigger in southern estuaries than in the north, and this was borne out in the research. The results showed that jacks from southern streams reach maturity faster than in the north.

Jacks tend to favour streams with good clarity, so systems like the Russell/Mulgrave, Daintree and Johnstone Rivers, which drain rainforest areas and remain relatively clear, have greater populations than muddier waterways like the Fitzroy River. This seems to indicate that jacks are predominantly sight feeders rather than relying on vibrations to assist them in locating prey.

Research supports the theory that there is only one genetic strain of mangrove jack in eastern Queensland. This is great news for hatcheries rearing jacks, as they don't need to be as site specific in obtaining broodstock as they have to be with barramundi.

While research indicates that mangrove jack still maintain a healthy population in Australia and are not under threat, this position can rapidly deteriorate, with environmental changes a significant threat. The continued proliferation of weirs and dams on coastal streams (which prevent or inhibit the upstream migration of mangrove jack) and other forms of coastal and near-coastal development pose a significant threat to the continued abundance of this prized recreational species.

The full document can be downloaded from www.dpi.qld.gov.au/fishweb/7096.html.

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