Although the red-legged banana prawn has been fished in the JBG since the early 1980s, we know little of its biology or population dynamics. Apart from a 12-month study by Northern Territory (NT) Fisheries in 1990, there has been no research on this species in Australia. The NT findings suggested that growth and mortality rates - important parameters for estimating the effects of changes in fishing effort on stocks - could not be estimated precisely by length frequency analysis of data from commercial catches. There was a need to develop a preliminary model for the red-­legged banana prawn fishery in the JBG. This model would help to evaluate the impacts of changes in the pattern of fishing effort on the yield and spawning biomass of red-legged banana prawns. However, the estimates of growth and mortality used by the model would, unfortunately, be imprecise.

 

Spanner crabs (Ranina ranina) represent a valuable resource to southern Queensland and northern NSW. The fishery became established in the late 1970s, and as a result of an almost exponential increase in fishing effort between 1992 and 1995 an output-controlled limited entry management arrangement was introduced. During that period catches increased from about 800 to over 3,000 t, as the fishery expanded northwards to previously unexploited grounds, and a profitable live-export market was developed in south-east Asia.
The Queensland fleet comprises some 240 vessels specifically licenced to take spanner crabs in Managed Area A, which is subject to a Total Allowable Commercial Catch (TACC), currently set at 2600 t. Another 310 vessels are licenced to fish only in Managed Area B (north of the main fishing grounds) where the TACC does not apply. At present the TACC is competitive, but in the near future an Individual Transferrable Quota (ITQ) system is to be introduced.

Trends in commercial fisheries catch-effort statistics indicate that the spanner crab stock in southern Queensland is currently being harvested at a sustainable level. However several questions remain with respect to the application of the commercial logbook data, possibly the most important of which is how well commercial catch-per-unit-effoti represents stock abundance. The spatial distribution of spanner crabs is patchy, and the fishery operates such that patches are located, targeted and fished down. This can potentially lead to a situation of hyperstability, where the stock is actually declining despite catch rates remaining constant. This highlights the expected value of the fishery-independent monitoring programme currently being planned by QDPI with (in the case of the spanner crab fishery) a significant level of cost-recovery from industry.

Previous attempts to estimate growth rate of spanner crabs resulted in little consensus, due in part to inadequate sample sizes (length-based methods) and uncertainty surrounding the effects of tagging on growth (tag-recapture methods). Our initial objective was to determine whether the length-based methods would work if the samples were very large.

 

Variability in the size-structure of even very large samples of adult crabs was so great that we could place little confidence in growth estimates obtained from this type of data. Because of this, we negotiated a change in research direction with FRDC, focussing on two alternative approaches to the question of growth rates. The first was to investigate growth in pre-recruits. The second was to quantify the likely effect of tagging on moulting and growth, and to determine the extent of growth rate differences between NSW and Queensland.

Very small spanner crabs are not taken by baited tangle nets, regardless of mesh size, so a different sampling arrangement was required. A two-track channel dredge was successful in capturing intact megalopae and early juvenile stages, which provided length frequency data of considerable value to estimating pre-recruit growth. However because of its small size only very limited samples were able to be collected. To increase the sampling volume we developed a substantially larger, hydraulically­assisted dredge. This device has been field-tested on several occasions, but it has not yet been developed and used to full effect.

Laboratory experiments demonstrated that tagging had an adverse effect on weight increase and survival of spanner crabs, suggesting that growth rate estimates based on mark-recapture techniques may be biased. Of the several different types tested, anchor tags were superior in terms of ease of application and visibility. Recognising that the results may be biased, we released 4,804 tagged crabs at sites throughout the fishing grounds, to determine whether growth of spanner crabs in Queensland waters is significantly different from that in NSW, reported in a previous study. Fourteen of the 221 crabs recaptured in 1998 had moulted, with growth male growth increments being greater than those offemales (X = 11.86 and 7.40 mm respectively). Recapture rates were significantly higher for males than females, and were also significantly greater for larger individuals of each sex. This suggests that tag mortality was greater in the smaller size-classes. Recaptured crabs had moved distances ranging from O to 45 km since release, but showed no tendency to move in any particular direction.

Our length-based assessment model has not yet been successful in producing useful estimates of the relevant stock performance indicators for use by management. This was due to the lack of contrast in the CPUE data, the relatively short data time-series, the extreme spatial and temporal variability in population size-structure and sex-ratio as represented in commercial catches and research samples, and the absence of definitive growth data.

Mitochondrial DNA analysis indicated that the east-coast spanner crab fishery comprises a single unit stock, and there thus appears to be no biological justification for separate management arrangements in different geographic areas.

 

Analysis of reproductive chronology indicates that the timing of the existing spawning closure is appropriate for minimising mortality amongst egg-bearing female spanner crabs across the entire fishery, and we recommend that the closure be retained in legislation.

 

Exploratory surveys for spanner crabs conducted in two areas outside the current fishing grounds did not reveal any significant quantity of crabs, although small numbers were captured at two sites amongst the Swain Reefs. From the available information it seems unlikely that there are any major unexploited populations of spanner crabs remaining in Queensland waters.

We have demonstrated that limb damage to undersized discarded spanner crabs has a major effect on their survival under natural conditions. Poor handling practices in the fishery result in considerable mortality amongst discarded small crabs, highlighting the need for continuing fisher education and ongoing investigation of alternative catching apparatus.

 

The two major issues for further research into the spanner crab fishery are (i) deriving a robust estimate of the species' growth rate, (ii) investigating the source of the extreme variability in size­frequency and sex-ratios in population samples.