The swallowtail dart (Trachinotus botla) is a carangid fish of importance to recreational and commercial fishers in southern Queensland and northern NSW The aims of the present study were to examine: I) The reproductive biology of T. botla, 2) Age and growth of T. botla, 3) Movement pattern of T. botla and the effectiveness of cooperative tagging programmes, 4) The diet of T. botla, and 5) The commercial and recreational fishery for T. botla and review management options for the species.
The spawning season of T. botla was protracted and centred Oil the summer months (October-April) which was outside of the main period of activity in the commercial fishery, T. botla reached maturity in their third year in the 360 to 370 mm TL size range. The sex ratio was female biased with the bias influenced by fish size. Eighty-six percent of T. botla greater than 400 mm were female. Von Bertalanffy Growth Function (VBGF) parameters calculated from whole otolith analysis were L= 475mm, K=0.38, and to=-0.82 for male fish; and L= 567mm, K=0.24, and t0=-1.40 for female fish. The mean size of female fish was larger than male fish at all age classes and the difference increased with age. Length frequency analysis was also attempted as a method for estimating age and growth but proved largely unsuitable because of the protracted summer spawning period and the shoaling nature of the species.
The use of a cooperative tagging programme in this study allowed for intensive tagging over a wide geographical range. Four thousand five hundred and fifty-six T. botla were tagged and released in thirteen regions from Mourilyan southwards to northern NSW and 243 were recaptured. The majority (49.3%) of tagged fish were recaptured less than 4 km from their release site, but movements of up to 275 km were recorded. The movements of T. botla were more consistent with ranging than either station keeping or migration. The distance between release and recapture site was positively correlated with fish size; therefore, T. botla was considered polymorphic with respect to movement pattern. Tagging results verified that T. botla do not utilise inshore waters.
T. botla in the surf zone principally forage on engraulid fish, brachyuran megalopae and calanoid copepods. When brachyuran megalopae and calanoid copepods were consumed they were usually consumed in fairly large numbers, suggesting that these prey are consumed when aggregated or perhaps "superabundant" Calanoid copepods were the only major prey to show seasonality in their pattern of consumption by T. botla being consumed in autumn and winter. The diet of T. botla captured from the least exposed location (northern Moreton Island) was dominated by the infaunal bivalve Donax deltoides.
Between 1946 and 1996 the estimated commercial catch of T. botla fluctuated between 10.1 and 55.3 tonnes annually. The commercial catch was principally captured from Fraser Island southwards to the NSW border. The recreational catch of T. botla by recreational club anglers, as a proportion of the total number of fish caught, increased after 1991. The most likely reason for the increase was greater targeting of T. botla by recreational club anglers.
The three recommendations for the management of the T. botla fishery from the State Government Inquiry into Recreational Fishing were reviewed. There was found to be no justification for the proposal to declare T. botla a recreational only species. The proposed minimum legal size of 30cm was below the size at maturity (36 em) determined by this study. However, a minimum legal size of 36 cm may lead to high levels of non-compliance by recreational anglers because the chances of catching fish above this size are reasonably low. As a compromise, a minimum legal size of 30 cm was deemed suitable, as it will reduce fishing mortality from current levels and lead to better levels of compliance by recreational anglers. The proposal for a recreational bag limit was justified as a tool to reduce the total recreational harvest and distribute the catch of T. botla more evenly among recreational anglers. Closed seasons, closed areas, and a commercial TAC were also reviewed as a potential management tools but were deemed unsuitable.