GBR-prawn: modelling ecosystem impacts of changes in fisheries management of the commercial prawn (shrimp) trawl fishery in the far northern Great Barrier Reef

Abstract

GBR-prawn is a "mass-balance" trophic-based ecosystem model (ECOPATH II) of the Great Barrier Reef, which combines a model template of a generalised coral reef ecosystem with results from extensive cross-shelf surveys of the far northern Great Barrier Reef. The enhanced model includes both the trawl and line fisheries, but focuses on the effect of trawling on the penaeid prawn community in the lagoon and inter-reef habitat. Trawl and line-fishing by-catch is specified and monitored, as is the biomass of seabirds and endangered sea turtles. Network analysis of the mixed trophic impacts predicted by the model showed only a minor negative impact on prawn populations by trawling.
The balance of positive effects, such as removal of predators or competitors as by-catch, and the negative effects of direct harvest resulted in the apparent minor impact. The impact was also reduced by positive feedback from the discarded trawl by-catch, currently a 6:1 ratio by weight, which either made up a proportion of the prawn diet or was consumed by animals that made up a proportion of the diet of the prawns being harvested. Dynamic and spatial simulations of the model showed that species of prawn were effected differentially, with P. esculentus (tiger prawn) a dominant at low levels of trawling effort and M. endeavouri (endeavour prawn) a better performer at higher levels of trawling. Penaeus longistylus (reef-associated, red-spot king prawn) did poorly when trawled unless a refugia was available.
The management strategy of gradual reduction in trawl 25 effort to 50% of current levels resulted in a 59% reduction in P. esculentus and a 64% reduction in M. endeavouri catch. Catch of P. longistylus was also lower than expected due to a spatial concentration of the reduced effort in the more economically attractive inshore lagoon. The effect on by-catch species of reduced trawl effort included increases in sea turtles and small fish omnivores (the latter comprising most of the discards), but also resulted in a decrease in species that feed on discards such as seabirds, groupers, and sharks/rays. Spatial analysis of trawling effort, which included the de-facto provision of refugia areas, showed a marked buffering of the impacts of trawling on by-catch species, particularly sea turtles.