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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)

Progress Reports (PDF): FY 2000, MULTIFAN-CL Workshop Report, Feb. 2000, Honolulu.

Project Overview
Bigeye tuna (Thunnus obesus) are an important component of tuna fisheries throughout the Pacific Ocean. They are the principal target species of the large "distant-water" longliners from Japan, Korea, the longline fleet based in Hawaii, and the smaller "fresh sashimi" longliners based in several Pacific Island countries. In Hawaii, bigeye tuna are the highest-value component of the longline catch - $17.6 million in landed value in 1997 (Anon., 1998a)1 - a trend that is likely to continue as more longline trips in Hawaii target tuna instead of swordfish. Prices paid for both frozen and fresh bigeye on the Japanese sashimi market and in Hawaii are the highest of all the tropical tunas. Bigeye tuna are therefore fundamental to the economic well being of the longline fishery in the Pacific Ocean, the catch of which has a landed value in 1996 of approximately $1.5 billion.

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:

  • the incorporation of explicit observation error assumptions in catch estimates (thus directly addressing the problem of variable quality of purse seine catch estimates);
  • a flexible formulation of spatial structure for analysis of different assumptions regarding population mixing;
  • simultaneous estimation of catch age structure from length frequency samples and the parameters of the age-structured model;
  • the ability to estimate time series changes in catchability;
  • the ability to incorporate other model structure such as density-dependent growth; and,
  • Bayesian methods to derive approximate confidence intervals for parameters or other dependent quantities of interest and to assist in determining optimal model complexity for the available data.

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.

(1) Anon. 1998a. Pelagic fisheries of the western Pacific region: 1997 annual report. Western Pacific Regional Fishery Management Council, Honolulu, Hawaii.
(2) Fournier, D.A., J. Hampton, and J.R. Sibert, 1998. MULTIFAN-CL: A length-based, age-structured model for fisheries stock assessment, with application to South Pacific albacore, Thunnus alalunga. Can. J. Fish. Aquat.Sci., 55:1-12.


Principal Investigators:
Dr. John Hampton
Secretariat of the Pacific Community
B.P. D5
Phone 687-26-01-47
FAX 687-26-38-18
email: JohnH@spc.org.nc
Dr. Richard Deriso
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
email: rderiso@iattc.ucsd.edu

Dr. Naozumi Miyabe
National Research Institute of Far Seas Fisheries (NRIFSF)
Tropical Tuna Section
Division of Pelagic Fish Resources
5-7-1 Orido
Shimizu-shi, Shizuoka-ken 424
Ph. 81-54-3366000
FAX 81-54-3359642
email: miyabe@enyo.affrc.go.jp


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This page updated August 22, 2006