POPULATION ANALYSIS OF NORTH PACIFIC ALBACORE BASED ON AN AGE-

Population models for North Pacific albacore (Thunnus alalunga) have been developed and reviewed within the North Pacific Albacore Workshop (NPALBW) forum since 2000. Currently, the NPALBW relies on a Virtual Population Analysis (VPA) model for purposes of formulating an international-based consensus regarding the ‘status ’ of this fish stock. Recently, an equally important research directive from the Workshop has been to develop alternative, more detailed statistical-based models, in efforts to evaluate more fully the relationship between this species’ population dynamics and associated fishery operations (i.e., areas of uncertainty in an overall stock assessment). We have developed one candidate model based on the Age-structured Assessment Program (ASAP), which generally represents a maximum likelihood-based numerical approach for conducting relatively straightforward, forward-simulation catch-at-age analyses. In general, the ASAP model indicated that the stock remained at relatively low levels through the late 1980s, then began to increase in size through the 1990s, and now shows signs of a ‘leveling off, ’ with abundance remaining at roughly 450,000 to 500,000 mt. Although highly variable, estimates of recruitment (age-1 fish) could be broadly classified into two ‘productivity ’ regimes; an early, lower productivity period that existed into the late 1980s and a later, higher productivity period throughout the 1990s. Details regarding input data, model structure and parameterization, and results are presented. Finally, we present a brief discussion concerning development of other alternative stock assessment models, particularly length-based/age-structured platforms (e.g., MULTIFAN-CL and Stock Synthesis 2). RÉSUM