Northern Prawn Fishery

Non-Technical Summary of Tiger Prawn Stock Assessment 2000, Northern Prawn Fisheries Assessment Group

A new more flexible model has been developed and is used in this assessment.To test the importance of determining the correct schedule of fishing power changes, model outputs were compared when fixed rates of fishing power change and a variable fishing power schedule were used.

Brown tiger prawns are considered to be over-exploited. Both fishing power schedules show that it is likely that the stock is below 50% S_MSY. However, there is uncertainty about whether the effort levels are too high.

Grooved tiger prawn stocks are also considered to be over-exploited. With variable fishing power, grooved tiger prawns appear to be close to target levels of stock size. There remains uncertainty about whether the effort levels are too high.

Projections show that, if future effort is set at the target, then recovery may take longer than 10 years for brown tiger prawns.

During the year 2000 fishing season, there were 12,736 fishing days taken to be targeted at tiger prawns, which was marginally higher than 1999 even though the season was shorter. This increase in days was partly due to a poor, and therefore shorter, banana prawn season in the first half of 2000. In the 2000 season, using an assumption of 5% per annum increase in fishing power, effective fishing effort increased on grooved tiger prawns and brown tiger prawns with respect to 1999. In addition, the effective catch-per-unit of effort for brown tiger prawns declined to an historical low in 2000, while that for grooved tiger prawns increased (but remained below the average catch rate between 1993 and 1999). However, when a variable fishing power schedule was used, effective fishing power decreased on grooved and brown tiger prawns with respect to 1999. Effective catch-per-unit effort for brown tiger prawns declined in 2000, while that for grooved tiger prawns increased (and was above the average catch rate between 1993 and 1999).

The assessment this year differs from that of previous years in using a new, improved model. In addition to providing estimates of the usual assessment quantities (e.g. S_MSY and E_MSY), the new model provides information on how well those quantities were determined (it estimates the uncertainty associated with the analysis). The model can also be used to investigate the likely performance of the fishery under various effort strategies by making projections into the future. In this assessment, the new model is also compared to the old model (Wang & Die, 1996).

A major result of this year's modelling is that the 1999/2000 recruitment level for brown tiger prawns is the lowest on record. Recruitment for grooved tiger prawns shows a slight increase over 1998/1999 levels, but remains well below the 10-year average.

The performance of the tiger prawn stocks was investigated by considering a number of management related quantities. These included estimates of Maximum Sustainable Yield (MSY), spawning stock (S) that produces the Maximum Sustainable Yields (S_MSY), the fishing effort (E) that produces maximum sustainable yields (E_MSY), the ratio of the spawning stock biomass in 2000 (S₂₀₀₀) to the S_MSY (S₂₀₀₀/S_MSY), and the ratio of the present effective effort (E₂₀₀₀) to the E_MSY (E₂₀₀₀/E_MSY). In the new model, the estimates of Maximum Sustainable Yield (MSY), spawning stock that produces the Maximum Sustainable Yields (S_MSY) and fishing effort which produces maximum sustainable yields (E_MSY) are calculated using the age-structure of the catch and within-year effort pattern, which is an improvement on the method used in Wang and Die (1996).

Assuming an annual increase in fishing power of 5%, then on average, the fishing effort in 2000 was 165% of the E_MSY for grooved tiger prawns (Figure 1) and 114% of the E_MSY for brown tiger prawns (Figure 2). Therefore, for both tiger prawn species, the 2000 effective fishing effort was well above E_MSY. The biomass for grooved tiger prawns in 2000 (S₂₀₀₀) was in the range of 58% to 88% of S_MSY, with the most likely value being 69% (Figure 3). For brown tiger prawns the range was 24% to 53% of S_MSY, with the most likely value being 34% (Figure 4).

Alternatively, using a schedule of variable fishing power levels, then on average, the fishing effort in 2000 was 105% of the E_MSY for grooved tiger prawns (Figure 1) and 71% of the E_MSY for brown tiger prawns (Figure 2). Therefore, for grooved tiger prawn species, the 2000 effective fishing effort was at E_MSY, and that for brown tiger prawns was below or at the E_MSY. The biomass for grooved tiger prawns in 2000 (S₂₀₀₀) was in the range of 80% to 116% of S_MSY, with the most likely value being 93% (Figure 3). For brown tiger prawns the range was 33% to 65% of S_MSY, with the most likely value being 44% (Figure 4).

Some model sensitivity tests were undertaken (see Dichmont et al. (2001)). A major recommendation from this report is that further research is needed to collect species split data to confirm that there has been no shift in the species assemblages in the last few years.

Figure 1: Effective fishing effort at grooved tiger prawns using fishing power values of a) 5% constant per annum and b) variable schedule of the NPFAG relative to the effort that would, on average, produce the Maximum Sustainable Yield (EMSY). Confidence intervals in the estimate of EMSY are reflected. Points greater than 100% show years in which the effort levels are greater than the target reference point, EMSY.

Figure 2: Effective fishing effort at brown tiger prawns using fishing power values of a) 5% constant per annum and b) variable schedule of the NPFAG relative to the effort that would, on average, produce the Maximum Sustainable Yield (EMSY). Confidence intervals in the estimate of EMSY are reflected. Points greater than 100% show years in which the effort levels are greater than the target reference point, EMSY.

Figure 3: Median (with upper and lower 5%-iles) grooved tiger prawn spawning stock size in a year relative to the Spawner stock size at which the Maximum Sustainable Yield (SMSY) is achieved for a) 5% per annum fishing power and b) variable fishing power. Points less than 100% are deemed over-exploited as they are below the target reference point.

Figure 4: Median (with upper and lower 5%-iles) brown tiger prawn spawning stock size in a year relative to the Spawner stock size at which the Maximum Sustainable Yield (SMSY) is achieved for a) 5% per annum fishing power and b) variable fishing power. Points less than 100% are deemed over-exploited as they are below the target reference point.

Reference

Dichmont, C.M., Die, D., Punt, A.E., Venables, W., Bishop, J., Deng, A. and Dell, Q. 2001. Risk analysis and sustainability indicators for prawn stocks in the Northern Prawn Fishery. FRDC 96/109. 187 pp.

Wang, Y.-G. and Die, D. (1996) Stock-recruitment relationships of the tiger prawns (Penaeus esculentus and Penaeus semisulcatus) in the Australian Northern Prawn Fishery. Marine and Freshwater Research 47, 87-95.

Page last updated 24 May, 2005