A holistic stock analysis of Albacore tuna (Thunnus alalunga) from the North Atlantic Ocean and Mediterranean Sea

Abstract

Albacore tuna (Thunnus alalunga, Bonnaterre, 1788) is a highly migratory scombrid with a cosmopolitan distribution inhabiting all the worlds’ ocean, in both tropical and temperate regions. Albacore juveniles and adults are highly exploited, with the main commercial fisheries in the North East Atlantic Ocean and the Mediterranean Sea targeting juvenile feeding aggregations. The life history of this species is poorly known and very little is understood regarding the stock origin of the exploited fish. Current management assumes a single panmictic stock in the North Atlantic with the Mediterranean as a separate management unit; previous studies have identified potential heterogeneity within the North Atlantic stock. Uncertainties are evident in age estimations and the interpretation of growth increments of this species; this presents problems for stock assessment which is based on an age-structured model. This study evaluates age estimation and stock discrimination techniques (otolith chemistry, otolith shape and genetics). It addresses the potential evidence of stock heterogeneity. This study provides new insight into stock structure in the North Atlantic Ocean and Mediterranean Sea. A comparative exercise was undertaken to assess bias and precision in interand intra-reader age estimations. Dorsal spines are the preferred aging structure in albacore tuna. The level of precision obtained using spines and alternative structures (scales and otoliths) for age estimation was assessed. Inconsistencies in interpretation between structures and between age-readers may introduce error into age data used for stock assessment. Linear bias was detected among readers for all structures, with readers systematically over- or underestimating age relative to each other. Agreement of age estimates derived using different structures from the same fish was low, and scales appeared to provide lower estimates of age than spines. The vascularisation of the core region of the spine can affect accurate age assessments in older fish. The readability of the first annual increment in the dorsal spine was tested. The first annulus was consistently identified in albacore aged two to five. Inter-cohort differences in growth were identified. The study highlighted the need for further inter-calibration of albacore tuna ageing methodologies. Additional study is required on age-specific growth rates within different albacore cohorts.