Wide-Ranging Temporal Variation in Transoceanic Movement and Population Mixing of Bluefin Tuna in the North Atlantic Ocean

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 (δ 18 O and δ 13 C) 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 45 • W 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.

Natural geochemical markers reveal environmental history and population connectivity of common cuttlefish in the Atlantic Ocean and Mediterranean Sea

“…Cuttlebone δ 13 C and δ 18 O were quantified for core and edge material to characterize geochemical signatures associated with early ( juvenile) and recent (sub-adult/adult) life-history periods, respectively. Regional shifts in cuttlebone δ 13 C and δ 18 O values were detected across the 12 sites investigated. Individuals collected from sites in the NEAO displayed more enriched δ 13 C and δ 18 O values relative to sites in the Mediterranean Sea, with the latter also showing salient differences in both markers among western, central and eastern collection areas.…”

“…Regional shifts in cuttlebone δ 13 C and δ 18 O values were detected across the 12 sites investigated. Individuals collected from sites in the NEAO displayed more enriched δ 13 C and δ 18 O values relative to sites in the Mediterranean Sea, with the latter also showing salient differences in both markers among western, central and eastern collection areas. Classification success based on cuttlebone δ 13 C and δ 18 O values to four geographical regions (NEAO, western, central and eastern Mediterranean Sea) was relatively high, suggesting that environmental conditions in each region were distinct and produced area-specific geochemical signatures on the cuttlebones of S. officinalis.…”

Fish and Sclerochronology Research in the Mediterranean: Challenges and Opportunities for Reconstructing Environmental Changes

“…Chemical analysis of otoliths has been conducted on over 41 fish species from the Mediterranean, and the most studied species are from the family Sparidae (Table 1). Published studies include data for the entire otolith (e.g., Iacumin et al, 1992;Gillanders et al, 2001;Marini et al, 2006;Arechavala-Lopez et al, 2016), data for a specific area of the otolith (e.g., Tanner et al, 2012;Mirasole et al, 2017;Rooker et al, 2019), and time series data (e.g., Correia et al, 2012;Mercier et al, 2012;Bouchoucha et al, 2018). In most reports, only a single species was analyzed, while Papadopoulou and Moraitopoulou-Kassimati (1977), Iacumin et al (1992), Marini et al (2006), Swan et al (2006), Khemiri et al (2014), Arechavala-Lopez et al (2016), Mirasole et al (2017), Bouchoucha et al (2018), and Demirci et al (2018) presented data for 2 to 24 different species.…”

“…The majority of research on otolith chemical composition has been conducted on commercially important species. Atlantic Bluefin tuna (Thunnus thynnus) was the target of several studies on the element (Secor and Zdanowicz, 1998;Rooker et al, 2003) and isotope composition (Secor et al, 2002;Rooker et al, ,b, 2014Rooker et al, , 2019, aiming to reconstruct movement and population exchange. Fraile et al (2016) observed the depletion in δ 13 C in T. thynnus otoliths over time, associating this with the oceanic uptake of anthropogenically derived CO 2 from the Mediterranean Sea over the past two decades.…”

“…Instrumental restrictions, related to the quantity of material required for the analysis of stable isotopes in otoliths, has been the main limitation for the development of isotope related research in otoliths (Sreemany et al, 2017). Due to the small size of the otolith, this resulted in time averaging of data, and analyses were limited to whole otoliths (e.g., , or certain parts of otoliths, e.g., the core (Siskey et al, 2016;Rooker et al, 2019) or edge (Hidalgo et al, 2008;Tanner et al, 2012), without the possibility of obtaining time series data. Development of instruments and methods, including highresolution laser ablation systems (e.g., Sreemany et al, 2017) and continuous flow isotope ratio mass spectrometry system for ultra-microvolume carbonate samples (Kitagawa et al, 2013;Sakamoto et al, 2017), have opened new opportunities for obtaining time series data from otoliths.…”