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Pacific-Wide Analysis of Bigeye Tuna (Thunnus obesus) using a Length-Based, Age Structured Modeling Framework (MULTIFAN-CL)MULTIFAN-CL web site - (this URL posted July 29, 2003)
Reports (PDF): FY
Workshop Report, Feb. 2000, Honolulu.
Since the 1970s the catch-per-unit-effort (CPUE) by longliners has declined steadily, particularly in the area east of 160° W where the largest longline catches of bigeye are taken. Surplus production model analysis based mainly on the longline data have suggested that the maximum sustainable yield (MSY) may be somewhat less than the maximum observed longline catch (Miyabe, 1995)2, leading to the conclusion that the stock of large bigeye exploited by longliners is at least fully exploited, and possibly over-exploited. It is probable, however, that the surplus production model is not an appropriate model for bigeye stock assessment, making the results of this analysis inconclusive.
Since about 1994, a rapid increase in purse seine catches of juvenile bigeye, first in the eastern Pacific and, in the past two years in the western and central Pacific, has created further uncertainty regarding the sustainability of the current levels of exploitation. The Pacific-wide purse seine catch of mostly juvenile bigeye has increased from less than 20,000 tons per year prior to 1992 to almost 80,000 tons in 1997. There are concerns that purse seine catches, often processed as low-priced canning products, may have an adverse impact on the longline catches of high-priced, sashimi-quality bigeye. This may reduce the profitability of the longline fisheries, especially for the smaller Pacific Island countries.
Problems also exist with estimating bigeye catch by purse seiners. It is often difficult to identify juvenile bigeye from yellowfin (which are of similar appearance) so bigeye catch is estimated from species composition samples. Also, the coverage of purse seine trips in the western and central Pacific is relatively low as compared to the eastern Pacific. Lack of understanding of various aspects of bigeye tuna (stock structure, east-west population mixture, mortality rates) also contribute to inadequate catch estimates.
Project researchers plan to address these problems by analyzing available bigeye tuna catch, effort, size composition and possibly tagging data, using a length-based, age-structured modeling framework, MULTIFAN-CL2. MULTIFAN-CL is a statistical age-structured model that uses size composition samples to derive catch age composition as an integrated part of the model. It has been successfully applied to South Pacific albacore (Thunnus alalunga). MULTIFAN-CL has advantages over traditional tuna stock assessment methods such as Virtual Population Analysis, including:
With data compiled from all available sources project researchers will assemble a bigeye tuna research and stock assessment database to apply the MULTIFAN-CL modeling framework. Anticipated outputs of the model would include estimates (and confidence intervals) of recruitment, population biomass, adult biomass, and fishing mortality time-series for each sub-region defined in the model, growth parameter, natural mortality rates, and movement parameters. The capability of comparing current and future adult biomass to a predetermined reference point is also intended. It will also be possible to use the model to derive various measures of interaction among the different fisheries and to indicate areas where additional data (including biological data) would reduce the uncertainty in future assessments. This type of information is typically required for fisheries management purposes.
Funding for this project awarded in October 1999.
Secretariat of the Pacific Community
Noumea Cedex, NEW CALEDONIA
Inter-American Tropical Tuna Commission
c/o Scripps Institute of Oceanography
8604 La Jolla Shores Drive
La Jolla, CA 92037-1508
Ph. (619) 546-7020
FAX (619) 546-7133
This page updated August 22, 2006