2 results

BCA: Spatial and seasonal stock dynamics of northern tiger prawns using fine-scale commercial catch-effort data

Project number: 1999-100.80
Project Status:
Completed
Budget expenditure: $4,107.00
Principal Investigator: Tony Kingston
Organisation: Fisheries Economics Research and Management Specialists (FERM)
Project start/end date: 2 Oct 2004 - 16 Dec 2004
:

Need

The current assessment of the Northern tiger prawn fishery is based upon a separate analysis of the fishery for each of the two species (Wang & Die, 1996). An alternative stock modelling approach, which considered both species together (Haddon, 1997, 1998), supports the results of the first method but is somewhat more pessimistic. This alternative model provides an independent view of the status of the tiger prawn stocks and the uncertainties affecting the analysis are also different. That both approaches produce a similar conclusion (that the stocks appear to be vulnerable and fishing mortality is currently too high to be sustainable in the long term), increases confidence in the conclusion that there are serious problems with the fishery.

A weakness of the alternative model is that it is based upon commercial catch-effort data summarized over the statistical reporting areas and for each year. It is likely that this aggregation of catch and effort data is obscuring or biasing details of the stock dynamics. If the seasonal fishing behaviour of the trawl fleet, in terms of its use of the fishing grounds, has altered either in a steady manner or over shorter periods, this could have large implications for the real status of the tiger prawn stock which may have been obscured by the aggregation of the commercial catch-effort data. This may lead to the stock appearing to be more stressed than it is in reality. To test whether the alternative model is overly pessimistic, and to refine the analysis of stock dynamics, it would be necessary, using the same approaches as in the earlier model, to investigate stock and fleet dynamics at finer spatial and temporal scales.

The proposed modelling should reduce uncertainty over the present status of the tiger prawn stocks.

Objectives

1. Determine whether the spatial and temporal scales of fleet behaviour bias the interpretation of the tiger prawn stock dynamics when analyzed by a non-equilibrium stock-production model.
2. Prepare NPFAG Working Papers which will include full descriptions of the model structure, data analyses, and potential management implications.
3. Communicate to the Northern Prawn Fleet and Industry the results of the analyses in a format such that the implications become clear to everyone and that permits comments and criticisms by Industry members.

BCA: Risk analysis and sustainability indicators for prawn stocks in the Northern Prawn Fishery

Project number: 1998-109.80
Project Status:
Completed
Budget expenditure: $4,107.00
Principal Investigator: Tony Kingston
Organisation: Fisheries Economics Research and Management Specialists (FERM)
Project start/end date: 2 Oct 2004 - 16 Dec 2004
:

Need

To assess whether the ESD and MEE objectives are met there is a need to determine the status of prawns stocks in the NPF and to develop guidelines to define whether the present status of the stocks may require management actions. It is important to precisely define what population parameters should be monitored and what biological reference points should these parameters be compared in order to determine whether management action is required. The Northern Prawn Fishery Assessment Group (NPFAG) has identified that spawning stock biomass and standardised fishing effort are the two most important indicators for target stocks in the NPF. The NPFAG has also established that targets and limits for these indicators need to be set and reassessed as new information is collected. The NPFAG has also recommended that future advice provided to them by researchers should include an explicit measurement of the probabilities that each of these targets may be exceeded. Calculation of such probabilities requires formal risk analysis to be carried out as part of the stock assessment.

Additionally, there are a number of future management options that have been recently considered by NORMAC. These include reductions of pool of the licensing units used in the NPF, (A-units, representing vessel length and engine power), gear restrictions as well as further seasonal and spatial closures. Although the operational implications of adopting some of these options have been the subject of NORMAC discussions, the scientific evaluation of options is not carried out in a structured framework but rather as individual assessments as different options are proposed by NORMAC. There is a need to establish a structured framework for management strategy evaluation so that the NPFAG and NORMAC can compare different options in a consistent way. This framework for management strategy evaluation should allow for the integration of risk analysis into the evaluation of management options. The consequences of each management strategy should be quantified and evaluated against the indicator of performance established by the NPFAG. The evaluation should include the estimation of the probability that, in the future, certain undesirable or desirable states of the stock are reached.

Objectives

1. To assess the probability that current NPF prawn stocks are being fished at sustainable levels (as defined by performance indicators of stock status developed by NORMAC) by carrying out a risk analysis.
2. To predict the performance of future NPF management alternatives by comparing predicted stock parameters against NORMAC’s performance indicators of stock status.