Abstract
Uncertainty regarding the movement and population exchange of Atlantic bluefin tuna (Thunnus thynnus) from the two primary spawning areas (Gulf of Mexico, Mediterranean Sea) is increasingly implicated as a major impediment for the conservation of this species. Here, two mixture methods were applied to natural chemical markers (18O and 13C) in otoliths (ear stones) to comprehensively investigate the nature and degree of transoceanic movement and mixing of eastern and western populations in several areas of the North Atlantic Ocean that potentially represent mixing hotspots. Areas investigated occurred on both sides of the 45W management boundary as well as waters off the coast of Africa (Morocco, Canary Islands) where both populations are known to occur. Projections of population composition (i.e., natal or nursery origin) from a multinomial logistic regression (MLR) classification method with different probability thresholds were generally in agreement with maximum likelihood estimates from the commonly used mixed-population program HISEA; however, predicted contributions for the less abundant population were occasionally higher for MLR estimates. Both MLR and HISEA clearly showed that mixing of Atlantic bluefin tuna in the Central North Atlantic Ocean was highly variable from year to year with expatriates of eastern or western origin commonly crossing into the other management area. Pronounced transoceanic movement and mixing of western migrants was also present off the coast of Africa, with the occurrence of western migrants in the Canary Islands and Morocco ranging from zero to the majority of the individuals assayed for the years examined. Results indicate highly variable rates of movement and population exchange for Atlantic bluefin tuna, highlighting the need for temporally resolved estimates of natal origin in mixing hotspots to improve population models used to evaluate the status of this threatened species.
Highlights
Migratory pelagic species such as billfishes and tunas are capable of transoceanic crossings that often traverse international borders or jurisdictions (Ortiz et al, 2003; Madigan et al, 2014; Rooker et al, 2016, 2019)
Otolith core δ18O and δ13C values of adult Atlantic bluefin tuna run in HISEA and multinomial logistic regression (MLR) modeling frameworks indicated that individuals collected in the Central North Atlantic Ocean (CNAO) readily crossed the 45◦W management boundary with expatriates being relatively common both east and west of the boundary (Figure 2)
For HISEA, Atlantic bluefin tuna of western origin accounted for 13% to 96% of the samples collected west of 45◦W over the four years investigated (Table 1)
Summary
Migratory pelagic species such as billfishes and tunas are capable of transoceanic crossings that often traverse international borders or jurisdictions (Ortiz et al, 2003; Madigan et al, 2014; Rooker et al, 2016, 2019) Many of these apex predators require multiple ecosystems and/or ocean basins to complete their life cycle (Rooker et al, 2007), with individuals commonly venturing outside of their respective management area (Block et al, 2011; Wilson et al, 2015; Harrison et al, 2018). Areas in the Eastern North Atlantic Ocean (ENAO) off the coast of Africa have been identified as putative mixing areas of eastern and western populations using otolith δ18O and δ13C values as indicators of natal origin (Rooker et al, 2014; Fraile et al, 2015); the prevalence and regularity of western-migrants in this region is unresolved
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