North Pacific Swordfish Stocks Assessed

In February 2014, PIFSC Stock Assessment Program scientists completed assessments for the Western and Central North Pacific Ocean (WCNPO) and Eastern Pacific Ocean (EPO) swordfish stocks. The geographic stock boundaries, delineated in Fig. 1A, encompass swordfish exploited by U.S. and other fishing fleets in the North Pacific Ocean. Assessments of both stocks were conducted through international collaboration by members of the Billfish Working Group of the International Scientific Committee for Tuna and Tuna-Like Species in the North Pacific Ocean (ISC). For both stocks, total catch and standardized catch-per-unit- effort (CPUE) were applied in a state-space Bayesian surplus production model based on data from 1951 to 2012 and included stock projections for 2013-2017. Annual swordfish catches from both stocks have fluctuated around 10,000 metric tons in recent years for a combined yield of approximately 20,000 metric tons (Fig. 1B). Stock status was assessed relative to maximum sustained yield (MSY) reference points with an overfished definition of B<½BMSY and an overfishing definition of H>HMSY. Here, B is stock biomass and BMSY is the stock biomass supporting maximum sustainable yield; H and HMSY are equivalent harvest rates. The stock status results were summarized in Kobe plots (Fig. 2). Trends in exploitable biomass and harvest rate were also examined (Fig. 3).

Swordfish stock status in the Western and Central North Pacific: Overall the WCNPO swordfish stock is not overfished or experiencing overfishing (Fig. 2A). The WCNPO estimated biomass in 2012 was 72,500 metric tons, or 19% above the estimated BMSY of 60,720 metric tons. The WCNPO estimated harvest rate in 2012 of 0.14 was 44% below the estimated HMSY of 0.25 (Fig. 3A). Future annual WCNPO catches could increase to 16,000 metric tons (50% above the recent average yield) before reaching either a 50% probability of overfishing or becoming overfished by 2017.

Swordfish stock status in the Eastern Pacific: In contrast, the EPO swordfish stock was experiencing overfishing from 2010-2012 with a 57 out of 100 chance that overfishing was occurring in 2012 (Fig. 2B). The EPO assessment model exhibits a retrospective pattern that causes the model to overestimate biomass and underestimate harvest rate, and any management decisions should consider this fact. The EPO estimated biomass in 2012 was 59,300 metric tons, or roughly 90% above the estimated BMSY of 31,300 metric tons. The EPO estimated harvest rate in 2012 was 0.19, or about 5% higher than the estimated HMSY of 0.18 (Fig. 3B). Recent annual yields of 10,000 metric tons are roughly twice the MSY level and if these high catches continue, the probability of overfishing will increase in the future. If future annual EPO yields were reduced to 7,800 metric tons (80% of recent catch), there would still be a 50% probability of overfishing by 2017.

(A) Two-stock structure for swordfish (Xiphias gladius) in the North Pacific Ocean, indicating separate stocks 
        in the Western and Central North Pacific Ocean (WCNPO) and in the Eastern Pacific Ocean (EPO). (B) Total catch (metric tons) from 
        1951 to 2012 for each of the two swordfish stocks in the North Pacific Ocean.
Figure 1. (A) Two-stock structure for swordfish (Xiphias gladius) in the North Pacific Ocean, indicating separate stocks in the Western and Central North Pacific Ocean (WCNPO) and in the Eastern Pacific Ocean (EPO). (B) Total catch (metric tons) from 1951 to 2012 for each of the two swordfish stocks in the North Pacific Ocean.
Figure 2. (A) Kobe plot indicating the status of swordfish (Xiphias gladius) stocks in the  
        Western and Central North Pacific Ocean. 
        Green quadrant indicates biomass is above BMSY (stock is not overfished) and harvest rate is below 
        HMSY (overfishing is not occurring). Red quadrant indicates stock is overfished and overfishing is occurring. Figure 2. (B) Kobe plot indicating the status of swordfish (Xiphias gladius) stocks in the  
        Eastern Pacific Ocean.
        Green quadrant indicates biomass is above BMSY (stock is not overfished) and harvest rate is below 
        HMSY (overfishing is not occurring). Red quadrant indicates stock is overfished and overfishing is occurring.
Figure 2. Kobe plots indicating the status of two swordfish (Xiphias gladius) stocks in the North Pacific Ocean. (A) Kobe plot for the Western and Central North Pacific Ocean stock of swordfish. (B) Kobe plot for the Eastern Pacific Ocean swordfish stock. Green quadrant indicates biomass is above BMSY (stock is not overfished) and harvest rate is below HMSY (overfishing is not occurring). Red quadrant indicates stock is overfished and overfishing is occurring.
[A] WCNPO Swordfish Stock
Trends in exploitable biomass (units of 1000 metric tons) of swordfish 
        (Xiphias gladius) for the Western and Central North Pacific Ocean stock.
        Estimated mean values (black dots and solid line), 95% confidence intervals (black dotted line), and estimated biological reference 
        points (BMSY, gray solid line) are presented. Trends in harvest rate of swordfish 
        (Xiphias gladius) for the Western and Central North Pacific Ocean stock.
        Estimated mean values (black dots and solid line), 95% confidence intervals (black dotted line), and estimated biological reference 
        points (HMSY, gray solid line) are presented.
[B] EPO Swordfish Stock
Trends in exploitable biomass (units of 1000 metric tons) of swordfish 
        (Xiphias gladius) for the Eastern Pacific Ocean stock. 
        Estimated mean values (black dots and solid line), 95% confidence intervals (black dotted line), and estimated biological reference 
        points (BMSY, gray solid line) are presented. Trends in harvest rate of swordfish 
        (Xiphias gladius) for the Eastern Pacific Ocean stock. 
        Estimated mean values (black dots and solid line), 95% confidence intervals (black dotted line), and estimated biological reference 
        points (HMSY, gray solid line) are presented.
Figure 3. Trends in exploitable biomass (left, units of 1000 metric tons) and harvest rate (right) of swordfish (Xiphias gladius) for (A) the Western and Central North Pacific Ocean stock and for (B) the Eastern Pacific Ocean stock. Estimated mean values (black dots and solid line), 95% confidence intervals (black dotted line), and estimated biological reference points (BMSY and HMSY, gray solid line) are presented.