This project confirmed that the benthic dinoflagellate Gambierdiscus toxicus is an origin of putative ciguatoxin precursors called gambiertoxins. It was recently found in French Polynesia that gambiertoxins could be extracted from biodetritus containing wild G. toxicus and that gambiertoxins were structural analogs of ciguatoxin. Ciguatoxin is the toxin found in the flesh of ciguateric fishes. It is demonstrated - for the first time- production of gambiertoxins in strains of G. toxicus grown in culture.

The production of gambiertoxins appears to be limited to certain genetic strains of G. toxicus, with the majority of strains not producing these toxins. It is proposed that ciguatera occurs when blooms of G. toxicus strains genetically capable of producing ciguatoxin precursors (gambiertoxins) enter the marine food chain. The type of maitotoxin produced by G. toxicus was also shown to be a stable characteristic within culture strains, but variable between strains.

Disturbance to coral reef environments has been suggested as a factor in promotion of ciguatera outbreaks. However, the researchers have been unable to find direct evidence to support this hypothesis. Translocation of ciguateric strains of G. toxicus (gambiertoxin producers) may explain the sudden appearance of ciguatera in areas previously free of the disease. The ability of living G. toxicus cells to be transported, either in ship ballast water or epiphytically upon the hulls of ships, had yet to be demonstrated. G. toxicus can survive rafting upon drift algae. Translocation by ships may be a mechanism for introduction of the causative organism to sites where reef damage occurs (eg. harbour works, anchorages and shipwrecks).

G. toxicus is a common component of reef benthos in Queensland. The researchers show that G. toxicus populations in Queensland generally do not produce ciguatoxin or ciguatoxin precursors. In addition, it is demonstrated that maitotoxin or non-toxic precursors in wild G. toxicus populations are not bioconverted to ciguatoxin in fishes. The majority of G. toxicus populations in Queensland are therefore probably not involved in ciguatera.

Platypus Bay is a major source of ciguateric fishes in Queensland. G. toxicus populations which produce gambiertoxins have been found in Platypus Bay biodetritus. A monoclonal strain of G. toxicus which produces gambiertoxins in laboratory culture was also isolated from Platypus Bay. This indicated that Platypus Bay wass probably the source of toxin in ciguateric fishes caught in Platypus Bay. However, gambiertoxins were not detected in the majority of biodetrital samples collected from Platypus Bay. Gambiertoxins therefore appear to be only infrequently pulsed into Platypus Bay.

Benthic dinoflagellates other than G. toxicus are suggested as potentially capable of producing toxins associated with ciguatera. Ciguatoxin or gambiertoxins were not produced by cultures of the benthic dinoflagellates Ostreopsis siamensis and Goolia monotis isolated from Queensland. These species produced water soluble and lipid soluble toxins, respectively. However, there is no evidence for the bioaccumulation of these toxins in the flesh of fishes.

Based on logbook records the sand crab commercial pot fishery in Moreton Bay is worth in the vicinity of $2 million wholesale annually. Sand crabs are also taken as an incidental by-catch by otter trawlers and in addition Moreton Bay supports a substantial recreational fishery.

Sand crabs are caught throughout the year but fishing effort is greatest from October to June. Peak catches in the pot fishery occur during March and April when daily catch rates may exceed 3 legal males per pot. Gravid females are present throughout the year but the highest proportion of egg bearing females is found in the population during August and September. Peaks in mating activity occur in autumn and spring with recruitment of juvenile crabs commencing in November. The parasitic barnacle Sacculina granifera infects 3% of all sand crabs in Moreton Bay and a previously unreported microsporidian parasite is found in 0.7% of the population.

Tagging studies show that fishing effort and mortality of sand crabs is highly variable throughout the Bay. Recapture rates for different areas vary from 1% to 65% with an overall return rate of 14%. The pattern of tag returns does not indicate any ordered directional movement of crabs either into or out of the Bay.

Male and female sand crabs exhibit differences in preferred habitat. Large males are generally more abundant in deeper water, whereas females predominate in shallower water, particularly on the top of sand banks.

Management recommendations arising from the project work include a change in the method of sand crab measurement to the width at the base of the antero-lateral spines, lowering of the existing size limit, the introduction of a bag 1 limit for recreational fishermen and an abolition of the prohibition on taking females provided that suitable management conditions can be maintained.

The fishery for king prawns which occurs between 18°S and 21°S has been investigated over a three year period. The fishery takes place predominantly during winter months. Availability of alternative target stocks as well as recruitment timing is responsible for the fishery's timing.

Annual catch has increased to about 2000 tonnes in 1987, from an annual catch of about 700 tonnes in 1984. This has been brought about by increased effort in the fishery, as the average annual catch rate has remained steady.

About 70% of the catch is red spot king prawn, Penaeus longistylus. The remainder is blueleg king, P. latisulcatus. Redspot kings recruit onto the main fishing grounds in the Great Barrier Reef lagoon over an extended period, whereas blueleg kings recruit over a comparatively short period.

Blueleg king prawns follow a conventional penaeid life cycle. Juveniles use coastal flats as nursery areas and move to deeper water as they mature. Red spot kings are unique in that they use coral reef lagoons as nursey areas. The species' biology is attuned to the coral reef environment.

Tagging studies conducted on red spot king prawns showed that adults were sedentary. Estimates of growth parameters and natural mortality obtained from tagging data are sufficient to use in a basic yield model.

Of the eighteen species of Scomberomorus recognized by Collette and Russo (1979), the migratory behavior of only three species S. cavalla (king mackerel), S. maculatus (Spanish mackerel) and S. commerson (the narrow-barred Spanish mackerel) have been studied by tagging programmes. In these instances the programmes were preceded by studies of age, growth and reproduction which facilitated the analysis of tag return data.

This project was developed to specifically establish the extent of S. commerson migrations in Torres Strait and Gult of Carpentaria waters.