An Age-, Sex-And Spatially-Structured Stock Assessment of the Indian Ocean Swordfish Fishery 1950-2012, Using Stock Synthesis (original) (raw)
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Stock Structure of Swordfish in the Pacific
At this time, the best scientific evidence indicates that there are four stocks of swordfish in the Pacific with centers in the northwest, northeast, southwest and southeast. Analyses of previously hypothesized stock structures using genetic data presented herein indicate that those hypotheses are not to be preferred to the four-stock structure of Alvarado Bremer et al. (2006). The IATTC is continuing investigations of stock structure in of swordfish in the Pacific, and stock assessments for eastern Pacific swordfish stocks. 2. STOCK STRUCTURE 2.1. Analyses of fisheries data Information used by fisheries scientists to define the boundaries of stocks of fish include data on relative abundance, size and spawning condition of individual fish, distribution of larval fish, genetics, and movements as determined from tagging. The boundaries determined from this basic research (Brown et al. 1985) may then be employed in studies that estimate the status of a stock with respect to conservation and management objectives (Hinton and Deriso 1998). Hinton and Deriso (1998) reviewed and discussed various definitions and applications of the term stock in fisheries research and management: "The least useful definition of stock, from a management or conservation point of view, is "the part of a fish population which is under consideration from the viewpoint of actual or potential utilization" (Ricker 1975). For example, strictly interpreted this definition may exclude those portions of populations which contribute to the presence and level of the stock, i.e. the stock may consist only of juveniles whose existence derives from the reproductive success of those surviving utilization, but the survivors are not considered part of the stock. The environment and fishing in any part of the range of a fish population affects the subsequent abundance and distribution of the species throughout its range, giving this narrowly constricted definition little value within our current framework of knowledge in oceanography and fisheries science. Application to management or conservation questions requires that a stock be a "self-sustaining component of a particular species" (Sinclair 1988). So defined, a stock has biological and genetic significance (Sinclair 1988), which provides the basis of measures (Brown et al. 1985) commonly used to differentiate among stocks. However, "in all cases some indications of significant degree of physical separation at spawning is required to support biological bases for separate stocks" (Brown et al. 1985). To the extent that relative abundance data accurately depicts the spatial and temporal distribution of a stock, the stock is a function of nature and not an abstraction (Sinclair 1988). A number of stock structure hypotheses have been used or advanced for analysis of status of swordfish stocks in the Pacific Ocean. Sakagawa and Bell (1980), and more recently Bartoo and Coan (1988) using data through 1980, each included an EPO stock in their three-stock hypotheses (Figures 1 and 2). These analyses bounded the distribution of swordfish stocks using estimates of relative abundance based on annual nominal catch-per-unit-of-effort (CPUNE). The ability of such annualized data to accurately
Stock assessment of albacore tuna in the Indian Ocean for 2014 using Stock Synthesis
A stock assessment for albacore tuna in the Indian Ocean was developed using Stock Synthesis version 3. The model included catch data from 1952 to 2012. A Stock Synthesis assessment was run in 2012, and this assessment makes a number of changes and documents their effects on the results. Size data were analyzed and the spatial structure of the fisheries was changed to improve the consistency of sizes within the fisheries. Sensitivity runs were carried out with alternative parameters for natural mortality, growth, selectivity, steepness, and spatial structure. Alternative values of biological parameters were explored, given that the different tuna-RFMOs use different assumptions in their stock assessments, in some cases with little evidence, and there are substantial data gaps for Indian Ocean albacore. We examined conflicts among different sources of data and assumptions by down-weighting the different data sources. The sensitivity of management advice to the above explorations was used to identify priority areas for further research. The inferred structural uncertainty, including interactions, was included in the management advice. The assessment incorporates projections for 10 years and provides a Kobe II Strategy Matrix decision table. The preliminary stock status using a reference base case assessment contradicts results obtained in 2012, indicating that the stocks is in a healthy status and is not experiencing overfishing or is in an overfished status. The only scenario that contradicts this conclusion is a low steepness value, with a very low natural mortality rate that is highly unlikely given the life history of Albacore.
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Swordfish are known to be sexually dimorphic. However, previous assessments of the status of swordfish in the North Pacific Ocean have ignored this. A sex-specific age-structured assessment model was therefore constructed and fitted to catch, catch-rate and length-frequency data for the swordfish fisheries that operate in the North Pacific Ocean. Except if natural mortality is lower than its "best" estimate, the results indicate that the spawning stock biomass in 2002 was at a high fraction of its unfished level and that the fishing intensity in 2002 was less than F MSY. Therefore, the swordfish stock in the North Pacific Ocean appears to be relatively stable at the current level of exploitation. However, the results of the assessment model are sensitive to the values for natural mortality and the steepness of the stock-recruitment relationship. Forward projections based on samples from a Bayesian posterior distribution indicate that there is negligible risk of the stock dropping below 40% of the unfished spawning stock biomass if fleet-aggregated fishing intensity remains at the current level. However, the risk of population depletion is higher if natural mortality is lower than the "best" estimate.
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