Tactical Research Fund: Reference points for the Queensland scallop fishery
Department of Agriculture and Fisheries EcoScience Precinct
Reference points are essential for the effective management of any large fishery. The spatially complex nature of scallop fisheries makes the construction of robust reference points difficult. FRDC project 1999/120, "Reference point management and the role of catch-per-unit-effort in prawn and scallop fisheries", concluded that "new types of data are essential to improve the accuracy of stock assessments, such as spatial indices of abundance collected through fishery independent sampling and VMS", and that "more accurate and robust reference points may exist using these data". FRDC project 2006/024, "Harvest strategy evaluation to optimise the sustainability and value of the Queensland scallop fishery", made effective use of both these data types to answer questions about the optimal timing of spatial closures and other management strategies. The proposed TRF project will build on this work by completing the path to adoption of the recommendations contained in the 2006/024 report, and noted by the FRDC external reviewer (review attached). In particular this will involve using the already constructed HSE framework to devise and test robust reference points. In order to adopt the recommendations from FRDC project 2006/024 in the current review of management arrangements for the fishery, Fisheries Queensland requires this additional work on sustainability reference points. This work on the sustainabililty reference points is required to be completed by August 2010 with a final report available no later than September 2010.
1. Propose and construct a set of reference points for the scallop fishery (e.g. target and limit effort)
2. Test the reference points in the (already constructed) MSE framework, i.e. what levels for the reference points perform best in terms of the sustainability and profitability indicators
The primary aim of this research was to further develop a modelling framework originally constructed in FRDC Project 2006/024 to enable the estimation of stock status and reference points for the Queensland saucer scallop fishery. Three interrelated features of this fishery make the estimation of robust reference points challenging: 1) highly variable recruitment, both temporally and spatially, 2) a fishing fleet that is able to target the high density areas, in space and time, with great accuracy, and 3) a history of management through spatio-temporal closures. These challenges were met by: a) spatially stratifying the framework at a relatively fine scale (many of the strata are 5 nautical miles squared), b) capturing spatio-temporal variability using a Bayesian state space approach for parameter estimation and linking this with spatially explicit equilibrium simulations, c) developing a novel effort allocation mechanism based on a ‘knowledge parameter’ which quantifies the effect of fisher targeting, and d) tuning the framework with fine-scale Vessel Monitoring System (VMS) data and fishery-independent survey data.
The framework consists of an estimation component, in which posterior distributions for parameters are estimated from data, and a simulation component, in which these distributions are used to project forward under different management regimes. Both were designed in a spatially explicit fashion so that the impact of various spatio-temporal closure management strategies could be quantified in terms of their impact on reference points. Maximum sustainable yield (MSY) and the corresponding effort (Emsy) were considered, along with an indicator of the potential for taking a given amount of yield with less effort. More elaborate reference points such as maximum economic yield (MEY) can be considered modularly in future work, now that the spatially explicit equilibrium simulation framework has been developed.
Reference points for four model variants and three management scenarios were considered. The results corresponded to two hypotheses on the stock-recruitment function: a very large stock with limited productivity (model one), or a smaller stock with increasingly greater productivity (models two through four). Median results from models one and two indicated that MSY was in the range of 500 to 600 tonnes. Median MSY from models three and four ranged from 700 to 800 tonnes. EMSY was highly variable due to the possibility of two distinct hypotheses with similar fit to the data: a large, relatively unproductive stock, or a smaller, highly productive stock. For this reason we recommend working back from MSY, via target average catch rates, to arrive at sustainable effort levels.
Results relating to the management scenarios indicated that a minimum legal size (MLS) of 95mm through the winter months performed better, but only marginally, than the current 90mm year round limit in terms of MSY. A combination of the 95mm winter MLS and a modification of the current closure system from two years to three years, with closure cells closed for 33 months and open for 3, showed a 21% increase in the catch rate indicator over the current management settings for these factors.