An evaluation of Indonesia's Indian Ocean tuna longline fisheries, based on historical and newly established sources of catch and effort information

Indonesia currently has the largest longline fleet operating in the Eastern Indian Ocean. This fleet is one of the main suppliers of fresh tuna for the Japanese market and it fishes the only known spawning ground for southern bluefin tuna (Thunnus maccoyii). It is thus important to attempt to characterise the fishery and evaluate its impact. However, there is a shortage of detailed catch and effort information. Three sources of detailed catch and effort information exist for the fishery: i) a series of historical data from a single fishing company, ii) data collected by Fisheries High School students, and iii) data from a trial Observer Program. This study provides the first comprehensive. exploratory analyses of the three data sets, with a view to investigate, to the extent possible, the impact of Indonesian fishing activities on Indian Ocean tuna stocks. The results emphasize the deficiencies in the three data sets, and highlight the importance of formal data collection protocols, particularly for stock assessment. The results of each data set were integrated and compared to evaluate the consistency of the data sets and the relative merits of each in addressing different issues. The current data and other sources of information suggest that the fishery is highly variable and thus difficult to characterise, and there is some evidence that this may be more due to fleet inefficiency than the fishery operating in opportunistically. Improved data collection for the Indonesian longline fleet remains a priority and the ongoing observer program is of paramount importance. A subsequent focus was to undertake a standardisation of the catch-per-unit-effort (CPUE) data in order to develop proxy abundance indices for the main tuna species by using the Observer Program data. As a long time series of Observer Program data is yet to be established, the aim was to develop protocols for ongoing monitoring and analysis, and providing a statistical modelling framework into which future data can be incorporated to provide an updated time series. A simulation model was developed to evaluate the minimum monitoring requirements, under different conditions, for delivering data that best reflects the total catch and effort and the underlying stock abundance. For each scenario trialled, various levels of coverage were examined, to evaluate the extent of spatial effort and catch coverage, and the difference between the estimated and true tuna catches. The results suggest that the fishery can be robustly represented by sampling at least 2% of the total sets (by assuming that a truly random sample can be obtained across all vessels). The three data sets suggest that the Indonesian fishery appears to typically target yellowfin tuna (Thunnus albacares), bigeye tuna (T. obesus) and albacore (T. alalunga), but show little consistency and may not be considered fully representative of the Indonesian lungline fleet. Despite the apparently highly variable nature of the fishery, the fact remains that Indonesia has the largest longline fleet operating in the Eastern Indian Ocean. The suggestion of fleet inefficiency is of concern in this context: if latent effort does indeed exist in the fishery, there is the potential to impact stocks even more than is indicated by current effort. Given the lack of conclusive evidence that can be drawn from currently. available information, the establishment of a robust data collection strategy (as informed by the -simulation developed here), is of utmost priority, both to resolve the issue of the nature of the fishery and its level of efficiency, and to establish a robust CPUE time series.