Abstract

Increased knowledge of stock mixing and migration patterns of Atlantic bluefin tuna (Thunnus thynnus) is required to appropriately manage and conserve this diminishing specie. Here we present a novel approach to identify the nursery origin of T. thynnus present in the Gulf of St. Lawrence using stable ?13C and ?18O isotopes in otoliths. Isotope signatures representative of eastern (Mediterranean Sea/Eastern Atlantic) and western (Gulf of Mexico/Western Atlantic) origin were first determined by measuring ?13C and ?18O in the whole otoliths of yearling T. thynnus. Due to cooler, more saline waters, T. thynnus otoliths from the eastern stock were enriched in otolith ?18O, causing a distinct difference from the western stock; no distinct difference in ?13C was detected between regions. Different yearling signatures allowed us to determine the contribution rates of eastern and western stocks to the Gulf of St. Lawrence fishery by examining the isotopic compostion of otolith cores (corresponding to the first year of life) from giant T. thynnus. In addition, ?13C and ?18O values across the entire otolith (core to margin) were quantified in an attempt to predict the timing of movement to and from spawning grounds. Results from this study will aid future management by providing contribution estimates from eastern and western spawning grounds to the Gulf of St. Lawrence fishery.

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