Assessment of North Pacific Swordfish Indicates Healthy Resource and Sustainable Harvest

A stock assessment of swordfish Xiphias gladius in the North Pacific Ocean was completed by FBSAD scientists in collaboration with the Billfish Working Group (BWG) of the International Scientific Committee for Tuna and Tuna-Like Species in the North Pacific Ocean (ISC). The assessment was conducted using a Bayesian surplus production (BSP) model assuming two hypotheses for stock structure: A single stock in the North Pacific Ocean, entirely above the equator, and two stocks separated into sub-areas by an irregular boundary extending southwestward from Mexico towards the equator and including part of the eastern South Pacific north of 20°S latitude.

Boundaries dividing North Pacific swordfish into distinct sub-areas under the two-stock hypothesis. The 
             Western-Central Pacific Ocean (WCPO) stock occurs in Sub-Area 1 and the Eastern Pacific Ocean (EPO) 
            stock in Sub-Area 2.
Boundaries dividing North Pacific swordfish into distinct sub-areas under the two-stock hypothesis. The Western-Central Pacific Ocean (WCPO) stock occurs in Sub-Area 1 and the Eastern Pacific Ocean (EPO) stock in Sub-Area 2.

In the two-stock scenario, the sub-area stocks are denoted the Western-Central Pacific Ocean (WCPO) stock and the Eastern Pacific Ocean (EPO) stock. The patterns of catch-per-unit effort in Japan's longline fishery, differences in life history characteristics between the two sub-areas, and genetics research all suggest that the two-stock scenario is more plausible than the single-stock hypothesis.

Stock assessments were conducted under both stock structure hypotheses using data for the period 1951-2006. Under the more plausible two-stock scenario, the maximum sustainable yield (MSY) for the WCPO stock was estimated as 14,400 t ± 2,000 t (metric tons). The analysis indicated that the exploitable biomass of the WCPO swordfish stock has fluctuated, but for most years remained above the biomass level producing MSY (BMSY), estimated at 57,300 t ± 11,800 t. It fell below BMSY for a couple of years in the 1990s but was above BMSY in the most recent 5 years analyzed (2002-2006). The exploitation rate (H) for the WCPO stock fluctuated during the period 1951-2006, but remained below the level required for maximum sustainable yield (HMSY), estimated as 26.2% ± 6.2%. The probability that the exploitation rate in 2006 exceeded HMSY is very low (1%). Assuming a constant exploitation rate equal to the latest 3-year average exploitation rate (for 2004-2006) and that fishing operations remain unchanged, exploitable biomass levels were projected to exceed BMSY through 2010, sufficient to sustain recent levels of catch.

Under the two-stock hypothesis, MSY for the EPO stock was estimated at 3,100 t ± 1,400 t and BMSY at 24,800 t ± 6,900 t. As in the WCPO, biomass for the EPO stock fluctuated above the BMSY level in most years. Biomass was below the BMSY for some years in the 1950s, but for the most recent 5 years it was well above BMSY. The exploitation rate, H, during the period from 1951 to 2006 has remained well below the level required for HMSY (estimated at 12.7% ± 4.9%) with a very low (1%) probability that H exceeded HMSY in 2006.

History of exploitable biomass of North Pacific swordfish, 1951-2006, in 1000s of metric tons, estimated 
             from the Bayesian surplus production model in Sub-Area 1 (WCPO) under the two-stock scenario, and 
             projected biomass through 2010. Exploitable biomass has exceeded the level required to support maximum 
             sustainable yield (BMSY) except for a brief period in the 1990s.
History of exploitable biomass of North Pacific swordfish, 1951-2006, in 1000s of metric tons, estimated from the Bayesian surplus production model in Sub-Area 1 (WCPO) under the two-stock scenario, and projected biomass through 2010. Exploitable biomass has exceeded the level required to support maximum sustainable yield (BMSY) except for a brief period in the 1990s.

The North Pacific swordfish assessment under the two-stock scenario (WCPO and EPO stocks) was reviewed at the annual meeting of the ISC Plenary Session in Kaohsiung, Taiwan, 15-20 July 2009. After reviewing the assessment, the ISC offered the following conservation advice: The WCPO and EPO stocks of swordfish are healthy and well above the levels required to sustain recent catches.

In collaboration with the ISC BWG, PIFSC is developing an integrated statistical catch at age model (i.e., incorporating size structure information) for North Pacific swordfish assessment under the two-stock scenario. However, given the limited size structure data currently available for North Pacific swordfish, an integrated model is not currently recommended as a basis for providing management advice. Preliminary comparisons of the integrated model and the BSP model, conducted by PIFSC in collaboration with the ISC BWG, showed consistent results from the two approaches and suggested that conservation advice for North Pacific swordfish would not change based on the model used.