The Regional Tuna Tagging Project (RTTP) occurred between 1989 and 1992, with the aim of updating the skipjack assessments, but with greater emphasis on yellowfin and if possible, bigeye assessments.
This was the second large scale tagging project undertaken in the Western and Central Pacific Ocean and was commenced in response to the continued expansion of skipjack and yellowfin fisheries in the Western Pacific Ocean during the 1980s primarily through the rapid growth of purse-seine fishery.
It was expressly designed to provide practical answers to the questions raised by fisheries interaction within the region. Specifically, three aspects were addressed:
- interaction between artisanal and industrial fisheries operating in coastal waters;
- interaction between fisheries of all types operating in adjacent EEZs;
- interaction between different gear types operating on the same fishing ground.
The objectives were
- to estimate interactions between tuna fisheries in areas where several different fisheries operate concurrently
- to further use the description of tuna movements to predict interactions for projected fishery developments
- to provide estimates for yellowfin tuna population parameters for selected areas of currently intense fisheries
- to provide updated estimates of skipjack tuna population parameters where fishing has increased since 1980
- to provide assessments of the potential for further expansion of tuna fishing in the region
To achieve these objectives, it was planned to release at least 20,000 fish in each of two years, with emphasis on releases of yellowfin tuna.
The operational phase commenced in late 1989 using the chartered vessel MFV Te Tautai.
A similar approach to the SSAP was adopted, with a Japanese-built pole-and-line vessel chartered from Tuvalu, and supported by funding from the 6th European Development Fund (€3.5 million). The use of the primary vessel was augmented by local pole-and-line vessels in Solomon Islands, Kiribati and Fiji, and as an extension to the project, troll/pole vessels for albacore tagging in the southern part of the WCPO. The main focus remained on tropical tunas, with tagging activity concentrated on the main fishing area for the international fleet (10N – 10S, 140E – 180), with extensions for the first time to Philippines, Indonesia and northern Australia. Separate in-country projects were carried out in Kiribati, Solomon Islands, Fiji and Philippines.
During RTTP, tagging activities were undertaken in Solomon Islands, Papua New Guinea, Federated States of Micronesia (FSM), Palau, Philippines, Kiribati, Tuvalu, Indonesia, Marshall Islands, Australia, New Caledonia, Fiji, Wallis and Futuna, Nauru and the international waters enclosed within these zones.
A total of 98,401 skipjack, 40,075 yellowfin and 8,074 bigeye were tagged, with good success in targeting juvenile yellowfin and juvenile/medium sized bigeye. Approximately 18,500 recaptures (12.5%) were received, with extensive publicity to cover all possible sources of recoveries; sources of tag loss (slippage, non-reporting) were comprehensively estimated for the first time in such experiments.
Data generated by this highly successful work were applied to stock assessments for skipjack and yellowfin. Both species showed moderate levels of exploitation at that time (around 20%) with information obtained on size-specific natural mortality (Hampton 2001), mobility (Sibert and Hampton 2003), and growth. Valuable information on the poorly understood bigeye was obtained for the first time in the region from the experiments. These outcomes were concurrent with the development and evolution of powerful length-based age structured models (MULTIFAN-CL) which depend to a large extent on parameter estimation from tagging data.
The separate albacore tagging work was less successful, given the greater difficulty of tagging large numbers of this non-schooling species in good condition. However sufficient data were gathered from all sources to generate preliminary estimates of basic stock parameters.
Movements of conventionally tagged South Pacific albacore
The published MULTIFAN-CL analysis of South Pacific albacore (Fournier et al. 1998) analysed catch, effort and length frequency data up to 1993.
An update of this analysis included fishery data up to and including the 1997 year and has also, for the first time, included available tagging data in the analysis. Also, the definition of fisheries has been re-structured to recognise the recent development of important fisheries, such as the Samoan longline fishery.
Distance traveled by Bigeye tuna between the tagging time and the recapture. (Restriction to fish which distance > 1000 NMi)
Bigeye stock assessments to date have been largely restricted to the analysis of longline catch per unit effort time series.
The OFP is developing a standardized CPUE series for bigeye to account for the changes in targeting by the longline fishery and temporal and spatial variation in bigeye habitat (similar to the yellowfin analysis). Archival tagging of bigeye is be an important source of detailed information on bigeye tuna vertical movements, which to a large extent determine their vulnerability to longline gear. Archival tagging of bigeye began in September 1999 in the Coral Sea.
Work commenced, in collaboration with Japanese and IATTC scientists, on the development of a MULTIFAN-CL analysis of bigeye tuna on a Pacific-wide basis. A planning workshop for this analysis was held in association with SCTB12 in Papeete, and preliminary results were reported to SCTB13 in June 2000. SCTB scientists generally concur that a detailed age-structured model such as MULTIFAN-CL is required for long-term bigeye tuna assessment.
Given that the development of a MULTIFAN-CL bigeye analysis takes considerable time, the OFP investigates the application of simpler, "biomass dynamics" models in order to provide preliminary advice on the status of the bigeye tuna stock. This work utilizes the standardized longline CPUE referred to above.
Distance traveled by Skipjack tuna between the tagging time and the recapture. (Restriction to fish which distance > 1000 NMi)
Two large-scale skipjack tagging experiments, the last in 1989-92 (RTTP), have provided considerable insight into skipjack population dynamics and fishery impacts. Both tagging experiments demonstrated the high productivity of the resource and low to moderate impact of the fisheries at the times when the experiments were carried out.
However, it would be desirable to extend the assessment from the tagging "snapshots" to a continuous record of the stock and fisheries. Therefore, the OFP discussed the application of MULTIFAN-CL to skipjack with scientific colleagues from Japan. The Japanese scientists have contributed a large amount of tagging data and well as size composition data for fisheries in the North Pacific.
Distance traveled by Yellowfin tuna between the tagging time and the recapture. (Restriction to fish which distance > 1000 NMi)
Analyses of RTTP tagging data suggested that the exploitation rate in the early 1990s was low to moderate (0.2).
The application of the MULTIFAN-CL model to yellowfin stock assessment is well advanced. The analysis covers the period 1970-1997, with data aggregated by quarter, and sixteen fisheries (various gears in seven statistical areas) represented. The results of the model provide good fits to the various data sets and are generally consistent with independent biological understanding of yellowfin tuna.
Development of the yellowfin analysis will continue, providing information on recent and historical stock levels and fishery impacts. It will also be possible to address the question of fishery interaction using the model. In particular it will be possible to provide explicit estimates of the impacts of surface fishery catches on longline catch rates of yellowfin.
Longline catch and effort are important input data to the MULTIFAN-CL yellowfin analysis. However, changes in the longline fishery over the years (particularly with respect to species targetting) need to be taken into account for the fishery data to be consistently interpreted. Therefore, the OFP is putting considerable effort into understanding how longline gear sample the yellowfin and bigeye stocks, and how changes in operational strategy and in various environmental parameters that determine their optimum habitat affect the "effectiveness" of longline effort. Estimates of effective effort have been developed for the distant-water longline fishery and these are being used in the MULTIFAN-CL analysis.
The late 1990s has seen the development of electronic (archival and pop-up satellite) tags, which are now providing much new information at the individual level useful for better understanding catchability and the interpretataion of catch/effort data; there has also been the development of environment-based models, which are increasingly leading to better understanding environmental effects on tuna abundance and availability, and movements.