Preliminary Stock Assessment Confirms Overexploitation of Striped Marlin Stock

Since its formation in 1995, the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC) has provided scientific advice for management of fisheries exploiting tuna and billfish in the North Pacific. The advice stems from research and quantitative stock assessments conducted by ISC working groups. During 6-16 December, 2011, the ISC Billfish Working Group (WG) held a workshop in Honolulu to assess the status of striped marlin (Kajikia audax) in the Western and Central North Pacific (the WCPO stock). Scientists from Chinese Taipei, Japan, the Secretariat of the Pacific Community, and the United States of America participated in the workshop. The meeting was chaired by PIFSC scientist Jon Brodziak.

This was the group's first assessment based on a new stock definition that splits the former North Pacific striped marlin stock into two stocks: the WCPO stock and the Eastern North Pacific stock. The new stock assessment used the most recent updates of fishery data and new life history information on the growth, maturity, and stock-recruitment resilience of the WCPO striped marlin stock. In an integrated analysis, the WG applied the stock synthesis modeling framework to fit fishery observations and estimate population parameters of alternative assessment models. During the course of the workshop, the WG reviewed six scientific working papers and conducted numerous additional analyses in order to determine a "base case" assessment model, the foundation for further analysis.

Figure 1.  Trends in total biomass (from the base case assessment model) and catch of the Western and Central North Pacific 
               striped marlin stock during 1975-2010.
Figure 1. Trends in total biomass (from the base case assessment model) and catch of the Western and Central North Pacific striped marlin stock during 1975-2010.

Results from the base case assessment model were used to determine trends in population biomass, female spawning biomass, and harvest rate of the WCPO striped marlin stock during 1975-2010. Estimates of overall population biomass (all age classes of fish) showed a long-term decline, as depicted in the accompanying Figure 1. Total population biomass averaged roughly 18,200 metric tons (mt) during 1975-1979 (42% of the expected unfished biomass - the expected biomass in the absence of fishing) and declined to an average of 6,500 mt during 2008-2010 (15% of unfished biomass). Reported catches of WCPO striped marlin also declined from an average of 8,100 mt during 1975-1979 to an average of 2,900 mt during 2008-2010 (Figure 1). Adult female biomass also exhibited a declining trend during 1975-2010 (Figure 2). Estimates of female spawning biomass averaged roughly 3,500 mt during 1975-1979 (127% of BMSY, the biomass to produce maximum sustainable yield [MSY]) and declined to an average of roughly 1,200 mt during 2008-2010 (45% of BMSY). Fishing mortality rates fluctuated at or above FMSY, the fishing mortality to produce MSY, during 1975-2010 (Figure 3). Estimates of annual fishing mortality averaged roughly F1975-1979 = 0.90 during 1975-1979 (47% above FMSY) and declined by about 10% to average roughly F2008-2010 = 0.82 during 2008-2010 (33% above FMSY).

If the status of the WCPO striped marlin stock was evaluated relative to MSY-based reference points, using the average estimates during 2008-2010 to measure current status and with the minimum stock size threshold set to be 50% of BMSY, then we would judge the stock to be currently depleted and currently experiencing overfishing.

Figure 2.  Trend in estimated female spawning stock biomass of the Western and Central North Pacific striped marlin stock 
               during 1975-2010, from the base case assessment model.   Vertical bars indicate 80% confidence intervals for biomass. Since 
               1995, the spawning stock biomass has been significantly less that the biomass associated with maximum sustainable yield 
               (green line).
Figure 2. Trend in estimated female spawning stock biomass of the Western and Central North Pacific striped marlin stock during 1975-2010, from the base case assessment model. Vertical bars indicate 80% confidence intervals for biomass. Since 1995, the spawning stock biomass has been significantly less that the biomass associated with maximum sustainable yield (green line).
Figure 3.  Trend in estimated fishing mortality (per year) for the Western and Central North Pacific striped marlin stock 
               during 1975-2010, from the base case assessment model.  Vertical bars indicate 80% confidence intervals for fishing 
               mortality.  Overfishing (fishing mortality above the red line) has been occurring for most of the past 2 decades, including 
               recent years.
Figure 3. Trend in estimated fishing mortality (per year) for the Western and Central North Pacific striped marlin stock during 1975-2010, from the base case assessment model. Vertical bars indicate 80% confidence intervals for fishing mortality. Overfishing (fishing mortality above the red line) has been occurring for most of the past 2 decades, including recent years.