Calculations Indicate Striped Marlin Stock Harvested in the North Pacific Occurs Mostly North of 20° N Latitude

The Northern Committee of the Western and Central Pacific Fisheries Commission (WCPFC) is responsible for recommending management measures to the WCPFC concerning "northern stocks", defined by the WCPFC as fish stocks occurring mostly in waters north of 20° N. With respect to striped marlin caught in North Pacific fisheries, the Northern Committee is awaiting a determination by the WCPFC that the stock is indeed a northern stock. The issue is important because the Marlin Working Group of the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC) concluded in 2007 that the fishing mortality on the striped marlin stock was excessive. Accordingly, the WCPFC has advised that fishing mortality be decreased from the level prevailing in 2003, the last year for which data were available in the stock assessment.

To help resolve the matter of the stock's geographical distribution, the ISC Marlin Working Group used results of its 2007 stock assessment to estimate the percentage of striped marlin stock biomass (fish aged 1 yr or older) occurring north of 20°N latitude and west of 125°W longitude. The group prepared a paper describing its analysis and concluding that 65-70% of striped marlin biomass in the western and central North Pacific Ocean occurs north of 20°N latitude.

Decade Model 1
(h = 0.7)
Model 2
(h = 1.0)
1952–1959 N/A N/A 0.96
1964–1969 0.71 0.66 0.82
1970–1979 0.73 0.71 0.66
1980–1989 0.65 0.65 0.67
1990–1999 0.64 0.64 0.77
2000–2004 0.64 0.64 0.73
Estimates of the average decadal proportion of striped marlin in the central and western Pacific north of 20°N latitude (weighted by area) under two models with different assumptions about stock recruitment steepness (h).  The proportion of striped marlin catch north of 20°N is also shown.

The paper was presented and discussed at the May 2008 ISC Plenary meeting in Takamatsu, Japan. Assumptions in the analysis about catchability of striped marlin in longline gear were critiqued, in particular whether gear selectivity in more southern tropical waters is lower than in other areas, and whether this difference could result in biased estimates of the spatial distribution of stock biomass. It was pointed out that the catchability parameters in the analysis were based on all information available, and are consistent with what is known about hook depth deployment in the northern and southern portions of the fishery. Comments were also raised in the Plenary meeting about the adequacy of growth and maturity parameters used in the analysis. Nevertheless, the ISC agreed that the paper should be put forward to the WCPFC Scientific Committee (SC) at its August 2008 meeting in Papua New Guinea, as this is the scientific body authorized to advise the WCPFC on designation of northern stocks. After reviewing the paper, the SC concluded it contained insufficient information to reach a conclusion about whether the striped marlin stock is a northern stock and advised additional analysis. The paper is available at: [striped marlin biomass est].pdf.

For more information contact: Gerard DiNardo