In 2009, four stock assessment models were applied to the Indian Ocean bigeye tuna stock using an agreed list of input parameters.
Overall Assessment Results
From the range of MSY estimates, the SC chose the value of 110,000 t. This was the MSY estimated by the ASPM and it was reported ahead of the estimates from the other methods because ASPM results have been reported in previous executive summaries; and the WPTT noted that several of the other assessment approaches used in 2009 needed further exploration and development. Given that the mean annual catch for the period 2003-2008 was 123,000 t, it appears that the stock is being exploited at around its maximum level. Results from the ASPIC analysis plotting the annual catches as a function of fishing mortality illustrate the MSY and its uncertainty. Despite the broad agreement of the models in estimating MSY, they produced quite different estimates of absolute levels of virgin and current biomass, and thus in the ratios of current levels of F and SSB to MSY. This was probably due to how the variations in CPUE were interpreted by each model. While acknowledging the value of assessing the status of bigeye from a wide range of modelling perspectives, the WPTT recommended that the results of the ASPM would be used in the Bigeye Executive Summary in 2009. The ASPM results indicate that the 2008 catch is close to the MSY. Furthermore, spawning stock biomass appears to be above the level that would produce MSY, and the fishing mortality in 2008 appears to below the MSY level. Biomass trajectories indicate that the spawning stock biomass is currently just above the MSY level, but it has been declining since the late 1970’s. Similarly, the current fishing mortality is estimated be to just above the MSY level, but fishing mortality has been increasing steadily since the 1980’s.
Despite the progress made in the 2009 assessments, uncertainties in the results and projections still exist. These uncertainties relate to:
- Uncertainties concerning the available indices of abundance.
- How well the model structures used in the assessments approximate the true dynamics of the population, and about the quality of the estimation of some of the model key parameters.
- Insufficient size information for the catches of longline fisheries, especially in recent years.
- Uncertainties associated with estimating catch-at-size and catch-at-age.
- Uncertainty about the natural mortality at various life stages, including uncertainty about the functional form of its dependency with age.
Uncertainty about the changes in catchability of the different fisheries involved, especially in the purse-seine fishery. Future consideration of an increase in efficiency could result in a more pessimistic appraisal of the stock status. For example, it is possible that the fishing mortality that would result in the MSY has already beenNotes about exploitation patterns
The exploitation patterns observed in 2003 and 2004 could be considered anomalous, and heavily influenced by the high abundances of yellowfin tuna, which concentrated the activity of the surface fleets. The decrease in the fishing pressure on bigeye currently observed is likely to be temporal, as the fleets appeared to come back to their previous pattern of activity by the second half of 2006.
Two other factors could also influence the short term evolution of the fishery. Rising fuel costs appear to be having an effect on the operating procedures of the surface fleets. Distances travelled at night, and consequently the number of FADs visited, are being reduced to save on fuel costs. The effect of this change could be however reduced by the increasing use of supply vessels, tasked with visiting FADs and informing purse seiners of the abundance of fish around them. The second factor is the limitation on the activity of all fishing fleets on the coast and EEZ of Somalia, due to the increase in the activity of pirates in the area. Some purse seine fleets have received indications from their governments not to venture into those waters. An important fishery on FADs has traditionally taken place in this area on the last quarter of the year, with significant catches of juvenile bigeye. Because of piracy acts off Somalia, the fishery has shifted into the South of the Arabian Sea, north of 10°N – an area where there are relatively few bigeye and where juvenile bigeye is not taken at FADs.
Another factor to consider when analysing the possible futures trends in SSB is the increasing trend in effective fishing power observed in the fleets.Current status
The results of the stock assessments conducted in 2009 were broadly similar to previous work. The preliminary estimate of catches in 2008 (107,000 t) is below the current estimate of MSY (110,000 t), catches in the past (1997-1999) have significantly exceeded MSY. Estimated values of fishing mortality and SSB for 2008 are also close to MSY-related values, indicating a fully exploited stock.Outlook
Recent changes in the areas fished by purse seiners do not appear to have had an effect on mortality for juvenile bigeye, despite the decrease in effort in the Somali basin where fishing on FADs usually caught the majority of juvenile bigeye.
The indices of abundance from two longline fleets available for this stock present divergent trends over the last few years, the differences observed in targeting are not fully explained.
The SC recommended that catches of bigeye tuna should not exceed the estimated MSY of 110,000t.