MEPS 178:55-68 (1999)  -  doi:10.3354/meps178055

ABSTRACT: A prawn larval behavioural model was coupled to a hydrodynamic model of the Gulf of Carpentaria in northern Australia to provide estimates of the size of the spawning area from which nursery ground populations are drawn, referred to here as the advection envelope. We have assumed that, during the first 8 d after the nauplii hatch, the larvae undergo a diel vertical migration in the water column, without spending any time on the bottom. After 8 d, larvae in waters shallower than a preset transition depth were assumed to switch to vertical migration cued by the tidal cycles--remaining on the bottom during outgoing tides, and swimming into the water column during flood tides. This tidal behaviour generated a net advection of postlarvae into the coastal zone and local estuaries. The model demonstrated that this mechanism is very efficient at accumulating larvae along the coastal zone. Changes in the timing and magnitude of the tides through the year generated a strong seasonal signal in the size and shape of the advection envelope, with typically a 2-fold difference in the size of the envelope between October and March. However, winds had little effect on the size of the advection envelopes, and interannual variation in the size and shape of the advection envelopes was small (<10%). The model also demonstrated that advection envelopes are very sensitive to the postlarval transition depth, which has not yet been adequately constrained by either field or laboratory studies. For example, changing the transition depth from 7 to 30 m typically resulted in a 2-fold increase in the size of the advection envelope. The results of the model may also have significant implications for the management of the prawn fishery. Comparisons of the advection envelopes with the distribution of tiger prawn catches indicate regions where fishing is most likely to have an impact on the spawning stock and subsequent recruitment to the fishery. The results also suggest that there are 3 discrete sub-stocks of Penaeus esculentus and P. semisulcatus in the Gulf of Carpentaria and, therefore, challenge the assumption that there is a single tiger prawn stock covering the entire region.

KEY WORDS: Larval advection · Penaeid · Postlarvae · Seagrass · Interannual variability · Seasonal variability