Mangrove Jack Research.
Butcher P and Piddocke T (2011) Mangrove Jack Research. Poster for general display.
SummaryThe biology of mangrove jack has been extensively studied in north Queensland waters. Uncertain however, is whether those findings are reflective of its biology and ecology in sub-tropical waters more than 1500 km away in northern NSW. A substantial body of anecdotal evidence suggests the existence of some important differences between NSW mangrove jack populations and their northern counterparts. The most striking and important divergence between northern and southern populations appears likely to lie in their respective uses of inshore and offshore habitats. Tropical mangrove jack exhibit a very distinctive, two-phase life cycle where juvenile fish inhabit estuaries and rivers almost exclusively, but on attaining maturity, these fish migrate to offshore reefs, where they apparently remain for the rest of their lives.
In NSW, however, mangrove jack life history may be somewhat more variable. In particular, the capture by recreational fishers of some very large fish (over 70 cm total length and weighing over 7 kg) in NSW rivers and estuaries raises the possibility that a larger proportion of the population remain in these systems; even as adults. Furthermore, mangrove jack in NSW may also undertake seasonal migrations between estuaries and offshore reefs. This contention is supported by marked seasonality in anglers’ catches and occasional observations of large aggregations of the species near river mouths. However, the developmental and/or environmental cues driving these movements are unclear. Seasonal shifts in food availability or preferred prey types may be partly responsible, yet the dietary requirements of southern populations remain unknown.
To reduce these knowledge gaps, mangrove jack will be collected in NSW between November 2011 and 2014 by recreational anglers and spear fishers. A subset of fish will also be angled by researchers and then tagged with transmitters to track their movements in selected rivers and estuaries.
Fishers are encouraged to report any captures of mangrove jack (even if they are released). Second, where fishers do retain a mangrove jack to eat, we would like the frames (frozen or fresh filleted skeletons with head and guts intact) for research. These will be used to determine their age, growth, diet and reproductive status. Further details can be obtained from PhD student Toby Piddocke on 02 6648 3900 at the National Marine Science Centre, Coffs Harbour.