Otolith chemistry as an indicator of movements of albacore (Thunnus alalunga) in the North Atlantic Ocean

Abstract

Albacore (Thunnus alalunga) in the North Atlantic Ocean is currently managed as a single well-mixed stock, although this assumption remains contentious. We measured stable isotopes (delta C-13 and delta O-18) and trace elements (Mg, Mn, Sr, Ba) in otoliths of albacore collected from two feeding grounds, namely the Bay of Biscay and Atlantic offshore waters, and compared them among sampling locations and life history stages. Measurements in otolith core, post-core and edge were used to determine whether albacore from these two regions have the same nursery origin and migratory patterns. We found no clear evidence of distinct nursery grounds based on otolith core chemistry, but Sr : Ca and Mg: Ca were different in the post-core portions of albacore from the two locations, suggesting residency in different regions during the early juvenile stage. Otolith edge chemistry, particularly stable isotopes and Sr : Ca, proved to be a valuable tool for classifying individuals to their capture locations. Annual cycles of Sr : Ca ratios were visible along life history transects, likely reflecting migratory patterns between water masses of differing salinity, but the timing of Sr : Ca cycles differed between the two groups. Differentiation in trace element concentrations in the otolith post-core and the timing of Sr : Ca cycles suggest the occurrence of two migratory contingents of albacore in the north-east Atlantic Ocean.