Regional variation in the otolith chemistry of blue marlin (Makaira nigricans) and white marlin (Tetrapturus albidus) from the western North Atlantic Ocean

Stable carbon (δ13C) and oxygen (δ18O) isotopes in the whole otoliths of blue marlin (Makaira nigricans) and white marlin (Tetrapturus albidus) were quantified, and regional variation in otolith composition was used to examine the population structure of both species in the western North Atlantic Ocean from collections taken over three decades (1981–2007). Otolith δ13C and δ18O of blue marlin and white marlin varied significantly among the regions investigated (Gulf of Mexico, Straits of Florida, Caribbean Sea, and U.S. Atlantic). Overall cross-validated classification success was 62% for blue marlin and 46% for white marlin (collected in three of four regions), with highest classification success for blue marlin in the Gulf of Mexico (85%) and for white marlin along the U.S. Atlantic (58%). Variability in otolith δ18O of blue marlin and white marlin was higher in regions where individuals displayed a greater degree of movement based on previous tagging studies in the same regions. Reduced variability in otolith δ18O of blue marlin in the Gulf of Mexico combined with high classification success of individuals from this region suggests that movement out of this basin may be more limited than in other regions investigated. Conversely, higher variability in otolith δ18O and lower classification success for white marlin signifies that mixing among regions may be more common for this species. These results suggest that the concept of migratory contingents may have some application to istiophorids in the western North Atlantic Ocean (i.e. blue marlin), but continue to support the concept of single Atlantic wide stocks for both species.