Assessment of Pacific Skipjack Tuna (<i>Katsuwonus pelamis</i>) Resources by Estimating Standing Stock and Components of Population Turnover from Tagging Data

Abstract

More than 140 000 tagged skipjack tuna (Katsuwonus pelamis) were released during 3 yr over a large portion of the central and western Pacific. Tag returns exceeded 6000. We developed a set of tag attrition models to analyze tag release and return data and catch and effort statistics for the study area. We used these models to assess the status of the skipjack resource in the whole study area and within subdivisions thereof. Total standing stock was estimated at 3 million metric tons (Mt), (95% confidence range 2.5–3.7 Mt). Overall attrition rate (including losses to natural mortality, fishing mortality and emigration) was 0.17 mo −1 (0.15–0.20∙mo −1 ). Total throughput was estimated at 6.2 Mt∙yr −1 (5.5–7.1 Mt∙yr −1 ) compared with catch of &lt; 0.3 Mt∙yr −1 . Overall harvest ratio was 0.04. Harvest ratios for seven subareas for which detailed catch and effort statistics were available ranged from 0.02 to 0.46; only one exceeded 0.17. Low harvest ratios over most of the study area during the period tags were at large imply a potential for increased skipjack catches in many subareas and in the whole study area.