Unlike the majority of engineered equipment, the design of fishing gear is tested ultimately in its ability to take or capture live animals which are able to modify their escape responses. For this reason, increases in the efficiency of operation and optimization of hydrodynamic design are not always incorporated in the same objective. Fishermen have not strayed too far from the goal common to all hunting strategies - to catch as much as possible in the shortest time.

The fishing industry has followed conservative lines in gear design and reduced the chances of losing revenue through experimentation. Thus, in the absence of a better understanding of the interaction of fish and fishing gear, most design changes have been based on small modifications to proven industry standards. Unfortunately, there is rarely any scientifically-based testing of the modification. Industry-instigated gear comparisons, which are usually of catch-capability, have frequently been made between different vessels, different locations, or different seasons and the added variability has masked the true results. Because fishing gear is acted upon by water resistance, buoyancies, towing forces which may be changed by surface water conditions, and varying bottom friction, its operation must be viewed as a very dynamic system where performance should be measured only under the most stringently enforced conditions.

In addition to the dynamic nature of the gear, the designer is faced with the problem of having to satisfy conflicting requirements and therefore to arrive at compromises which offer solutions to opposing design strategies. For example, the need to maximise catch-capability, which is of paramount importance, may be in direct conflict with other objectives such as minimum hydrodynamic drag, minimum construction costs, or ease of handling on deck. Because the designer cannot assess the catch-capability of fishing gear during the design stages, final tests must be conducted under actual conditions to establish this most important parameter.

This study takes up where the fishing gear designers have left off. An attempt was made to assess the catch-capability and hydrodynamic efficiency under field conditions of each of three different otterboard designs commonly used by the Australian prawn trawling industry.

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.