Considered a low priority species the SEF for many years the importance of redfish research has greatly increased over the last few years because of a contentious review of the existing stock assessment.
Over the last 20 years redfish catches have varied between 900 and 4000t. In 1997 the redfish catch, including estimated discards, was around 2000t. Standardised catch rates of commercial trawlers reported in SEF1 returns have varied without trend between 115 - 240kg/h since 1986.
In 1993 the redfish stock was estimated by SEFAG on the basis of commercial catch rate trends and the changing size structure of the catch to be around 10 - 20,000t.
A Redfish Assessment Group (RAG) was established in 1998 to refine and update the redfish assessment. The RAG reviewed for the 1998 SEFAG plenary a highly contentious preliminary assessment which estimated that the redfish biomass was only 3-4,000t, and likely to decline rapidly even without further fishing. The RAG's cohort analysis also suggested that over the last five years recruitment rates have first spiked up to several times historic levels, but have then declined to virtually nothing in the last two years of the analysis.
These estimates are extremely contentious with industry in NSW who fiercely dispute them. On the basis of the size of acoustic marks they see, and the size of their own shots, they believe the biomass to be considerably higher. Fishers claim that catch rate trends reflect changing fishing patterns. That quota management has lead to them optimising the species composition of catches which has lead to decreased targeting of single species aggregations and a decline in catch rates (FRDC 97/114). They claim that changing catchability is producing a misleading stock assessment.
There was some support for this industry view amongst the scientific members of the 1998 SEFAG plenary. The plenary was also aware that cohort analyses are prone to producing unrealistic estimates in the final years of the modelled time series. SEFAG Plenary considered that the model provided unrealistic estimates of recruitment and stock biomass and decided that it could not endorse the preliminary assessment. Instead they have prescribed a series of diagnostic analyses which should be carried out on the cohort analysis during 1998/99 and placed a high prioirity on this research proposal. It is hoped that the diagnostic test, which include applying alternative assessment models to the same data, will indicate whether or not the RAG's preliminary assessment is reliable and so suggest an appropriate reaction to the new draft assessment for redfish.
However the essential problem remains that assessment will continue to be based upon questionable fishery dependent trends unless quantitative techniques are developed for surveying these stocks independently to evolving fishing practices.
The aim of this proposal is to demonstrate the feasibility of acoustically surveying an area of the SEF containing redfish aggregations during the winter of 1999.
It should be noted that the scope of this current proposal is only to prove the feasibility of the industry acoustic survey technique. It does not extend to conducting a full scale survey of redfish stocks in 1999. Should the feasibility of producing fishery independent indices of redfish abundance be demonstrated by this feasibility study it is anticipated that the proven techniques would be scaled up by future projects to provide full stock surveys for redfish, and then potentially for other amenable SEF species.
In the long term RAG and SEFAG need reliable indices of stock abundance if they are to provide meaningful assessments of the status of redfish stocks. This project aims to prove the feasibility of the industry acoustic technique for providing fishery independent estimates of trends in redfish biomass. Information flowing from this project should in the long term improve the quality of the stock assessment which is vital to ensuring effective TAC setting.
In the short term the project will also provide some subsidiary benefits that may help resolve the issues raised by the RAG's revised assessment of redfish stocks.
The project will involve Dr Jeremy Prince becoming involved in RAG meetings allowing this project to be co-ordinated with the RAG's research plan. In addition to conducting this project Dr Prince has agreed to develop an alternative synthesis assessment of redfish using an assessment framework developed by Prof. Ray Hilborn of the University of Washington. This will go some way to responding to SEFAG's call for conducting diagnostic tests during 1999 on the new cohort assessment.
In addition the data collected by this feasibility study may allow some minimum estimates of absolute redfish biomass (actual tonnage) to be made for the selected survey area (see methods). While it is acknowledged that absolute estimates of biomass will be highly imprecise because of the number of assumptions that will be required to produce the estimates; making uniformily conservative assumptions will allow some minimum level of biomass to be proved. Such minimum proven estimates may still prove useful to the RAG by allowing the stock assessment to be bounded by some scientifically proven extremes.
Finally if the industry acoustic techniques is shown to be feasible this pilot scale project should also have been able to document redfish aggregation dynamics during 1999 this will help build a documented basis upon which long term surveys can be designed.