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Causes of Rapid Declines in World Billfish Catch Rates

Project Reports (PDF): FY 2004, FY 2003

Project Overview
Bycatch of non-target species, such as billfish, is emerging as a driving force in the management of pelagic fisheries around the world. There is evidence that billfish population size is now a small fraction of historical levels. Billfish are of considerable social and economic importance to U.S. states, territories and possessions in the Western Pacific. Fishery managers there, and worldwide, need to know whether the depletion pattern is real and whether billfish should be managed differently from other species.

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:

  1. Determine whether the pattern of rapid decline in billfish catch rates is real and, if the pattern is found to be real:
  2. Identify the most likely cause(s) of the depletion pattern.
  3. Highlight implications of the depletion pattern for stock assessment of billfish and management of fisheries taking the species.

Project researchers will concentrate on the following hypotheses with regard to the decline in billfish catch rates:

  1. Competition among fishing gears (as fleets expanded, competition for the most productive areas increased, resulting in the displacement of longliners to less productive waters.
  2. Stock decline linked to density-dependent habitat selection (catch rates from marginal habitats will provide misleading abundance indices if density is maintained in the ideal habitat but fluctuates in marginal habitats).
  3. Depletion of sub-populations (the stock actually consists of genetically discrete units that are slow to recover from fishing).

Year 1 funding for this 2-year project to be awarded January 2003.

Literature cited:
•Dalzell, P. and C. Boggs (2001). Pelagic fisheries catching marlins [in] the U.S. Western Pacific islands (draft). Proceedings of the Third International Billfish Symposium, Cairns, Australia, Australian Journal of Marine and Freshwater Research.
•Myers, R.A. and G. Mertz (1998). Reducing uncertainty in the biological basis of fisheries management by meta-analysis of data from many populations: A synthesis. Fisheries Research Amsterdam, 37(1-3), pp. 51-60.

Principal Investigators:

Dr. Ransom A. Myers
Killam Chair of Ocean Studies
Department of Biology
Dalhousie University
Halifax, Nova Scotia
Phone (902) 494-1755
FAX (902) 494-3736
email: Ransom.Myers@dal.ca

Mr. Peter Ward
Department of Biology
Dalhousie University
Halifax, Nova Scotia
Phone (902) 494-3910
FAX (902) 494-3736
email: ward@mathstat.dal.ca

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This page updated August 16, 2006