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Causes of Rapid Declines in World Billfish Catch Rates Project
Reports (PDF): FY 2004,
FY 2003 During the development of pelagic longline fisheries, billfish catch rates often show a pattern of decline that is both rapid and massive. After the period of decline, catch rates tend to remain at much lower levels, seemingly indefinitely. This is the case for many billfish stocks in the Pacific as well as in the Atlantic and Indian Ocean. Standardized catch rates are often less than half those when longlining began. Nominal catch rates for blue marlin in Hawaii's troll fisheries, for example, are now less than 50% of the level in the 1960s; those of striped marlin are 33% of the 1960s level. Even the relatively recent American Samoa longline fishery shows a depletion pattern (Dalzell and Boggs 2001). It is not particularly surprising for catch rates to decline by 50%; at the maximum sustainable yield, catch rates can be expected to be about half of the level when the fishery first began to exploit the stock. The present project is prompted, however, by the precipitous nature of the declines and situations where standardized catch rates or abundance estimates are well below 50% of initial levels. The catch rate declines observed in longline fisheries present a paradox: do they reflect real declines in billfish abundance or are longline catch rates simply a poor indicator of abundance? The answer is critical to understanding billfish population dynamics and effectively managing the stocks. By expanding on meta-analytical techniques developed by the Dalhousie team of Myers and Mertz (1998), project researchers hope to provide fishery managers with a historical perspective on trends in billfish populations by documenting evidence of the depletion pattern and investigating hypotheses that might explain it. It will help to explain why billfish population size now seems to be a small fraction of what it was 50 years ago. The main objectives of this project are to:
Project researchers will concentrate on the following hypotheses with regard to the decline in billfish catch rates:
Year 1 funding for this 2-year project to be awarded January 2003. Literature
cited: |
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Principal Investigators: |
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Dr. Ransom
A. Myers |
Mr. Peter
Ward |
This page updated August 16, 2006 |