Sheng-Ping Wang

High Value and Long Life—Double Jeopardy for Tunas and Billfishes

The first standardized, global assessment of these fishes, using Red List criteria, reveals threatened species needing protection. There is growing concern that in spite of the healthy status of several epipelagic (living near the surface) fish stocks (1), some scombrid (tunas, bonitos, mackerels, and Spanish mackerels) and billfish (swordfish and marlins) species are heavily overfished and that there is a lack of resolve to protect against overexploitation driven by high prices (2–5). Many populations are exploited by multinational fisheries whose regulation, from a political perspective, is exceedingly difficult. Thus, assessment and management is complicated and sometimes ineffective (4). Regional Fisheries Management Organizations (RFMOs) were created to manage and conserve scombrids and billfishes because of their transnational distributions and widespread economic importance (6). However, species-specific catch data for many scombrids and billfishes are not collected or are aggregated with other species. Even for the larger tunas, for which relatively rich data exist, population assessments and data are complex (1) and are difficult to combine across RFMOs, which prompts a need for alternative means of assessment.

Application of the sex-specific age-structured assessment method for swordfish, Xiphias gladius, in the North Pacific Ocean

Swordfish are known to be sexually dimorphic. However, previous assessments of the status of swordfish in the North Pacific Ocean have ignored his. A sex-specific age-structured assessment model was therefore constructed and fitted to catch, catch-rate and length–frequency data for the wordfish fisheries that operate in the North Pacific Ocean. Except if natural mortality is lower than its “best” estimate, the results indicate that the pawning stock biomass in 2002 was at a high fraction of its unfished level and that the fishing intensity in 2002 was less than FMSY. Therefore, the wordfish stock in the North Pacific Ocean appears to be relatively stable at the current level of exploitation. However, the results of the assessment odel are sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship. Forward projections based on samples rom a Bayesian posterior distribution indicate that there is negligible risk of the stock dropping below 40% of the unfished spawning stock biomass f fleet-aggregated fishing intensity remains at the current level. However, the risk of population depletion is higher if natural mortality is lower han the “best” estimate. 2006 Elsevier B.V. All rights reserved.

Evaluating the impacts of uncertainty on the estimation of biological reference points for the shortfin mako shark, Isurus oxyrinchus, in the north-western Pacific Ocean

Biological reference points (BRPs) are commonly used to assess the harvest level and stock status of marine fish populations. However, BRP estimates may be influenced by uncertainties about life-history parameters and fishing practices. The shortfin mako shark, Isurus oxyrinchus, is one of the most important by-catch species for tuna longline fisheries; however, its stock status remains poorly understood. To understand its population dynamics, the present study examined fishery statistics data and biological measurements of the shortfin mako in the north-western Pacific, and evaluated the impact of uncertainties associated with various sources of uncertainty, by using a simulation approach. The simulations indicated that BRPs are especially sensitive to changes in weight–length relationship and the growth parameters. Failure to take this into account may lead to inaccurate estimates and have serious consequences for management decision-making. To improve the accuracy of BRP estimates, biological measurements and life-history parameters should be updated regularly to account for changes in size composition and population structure. Further research could focus on increasing the sex-ratio sample size, updating the weight–length relationship and growth equations, incorporating ageing error into BRP calculations and determining BRPs by using appropriate levels of spawning-potential ratio.