Study area in the North Atlantic Ocean Sample Clauses

Study area in the North Atlantic Ocean. Otoliths collected in the western North Atlantic (west of 45ºW, blue), central North Atlantic (east of 45ºW, light green), Bay of Biscay (dark green), Moroccan traps (orange), Portuguese traps (yellow) and Canary Islands (pink). Xxxxxxx handling followed the protocols previously described in Xxxxxx et al. (2008). Briefly, following extraction by GBYP participants, sagittal otoliths of bluefin tuna were cleaned of excess tissue with nitric acid (1%) and deionized water. One sagittal otolith from each bluefin tuna specimen was embedded in Struers epoxy resin (EpoFix) and sectioned using a low speed ISOMET saw to obtain 1.5 mm transverse sections that included the core. Following attachment to a sample plate, the portion of the otolith core corresponding to approximately the yearling periods of bluefin tuna was milled from the otolith section using a New Wave Research MicroMill system. A two-vector drill path based upon otolith measurements of several yearling bluefin tuna was created and used as the standard template to isolate core material following Xxxxxx et al. (2008). The pre-programmed drill path was made using a 500 µm diameter drill bit and 15 passes each at a depth of 50 µm was used to obtain core material from the otolith. Powdered core material was transferred to silver capsules and later analyzed for δ13C and δ18O on an automated carbonate preparation device (KIEL-III) coupled to a gas- ratio mass spectrometer (Xxxxxxxx MAT 252). Stable δ13C and δ18O isotopes are reported relative to the PeeDee belemnite (PDB) scale after comparison to an in-house laboratory standard calibrated to PDB. Stable isotope signals of mixed stocks were compared with yearling samples from Mediterranean and Gulf of Mexico nurseries revised in GBYP-Phase 3 and presented in Xxxxxx et al. (2014). HISEA software (Xxxxxx 1990) was used to generate maximum likelihood estimates (MLE) of mixed-stock proportions in each of the mixing zones. Results

Study area in the North Atlantic Ocean. Otoliths collected in the central Atlantic (west of 45ºW, blue), central Atlantic (east of 45ºW, green), Moroccan coast (orange) and around Canary islands (pink). Region-specific estimates of nursery origin of bluefin tuna were based on comparing otolith ‘cores’ (corresponds otolith material deposited during the first year of life or yearling period) of adult bluefin tuna to the baseline or reference samples of yearling bluefin tuna. Estimate of origin for adult (medium and large) bluefin tuna were obtained using the maximum likelihood estimate (MLE) from the mixed-stock program HISEA developed by Xxxxxx (1990). Baseline data for mixed-stock analysis was otolith 13C and 18O of yearling samples collected in the east and west from 2000-2012, with recent samples (e.g. 2009-2011) supplied through GBYP. Otolith cores of adult bluefin tuna collected in the central and eastern Atlantic Ocean were then used to estimate the origin of these recruits in the bootstrap mode of HISEA, which provided non-parametric estimates of the reliability of predicted contributions from eastern (Mediterranean) and western (Gulf of Mexico) spawning grounds. 13C and 18O were measured in the otolith cores of large (>100 kg) bluefin tuna from different locations in the Atlantic Ocean between October 2011 and May 2013: 1) central North Atlantic (east of 45ºW; year=2011, n=28), 2) central North Atlantic (west of 45ºW; year=2012,n=18), 3) central North Atlantic (east of 45ºW; year=2012, n=150), 4) Morocco (year=2012, n=49), 5) Canary Islands (year=2013, n=20) and 6) Morocco (year=2013, n=59) (for detailed information see Table 4.1). Mixed-stock analysis based on maximum-likelihood estimates (MLE) indicated that bluefin tuna collected south of Iceland and in Moroccan waters (during October- November 2011 and May 2012 respectively) were determined to be entirely comprised by individuals from the ‘eastern’ or Mediterranean nursery (Fig. 4.2 and 4.3, Table 4.2). Standard deviation around estimated percentages was + 0%, indicating the degree of confidence in our predicted assignment was high. In September 2012, the majority of bluefin tuna captured by Japanese longliners in the central North Atlantic (west of 45ºW) were from the western nursery (94%), whereas catches during the next months in the central North Atlantic (east of 45ºW) were dominated by the eastern population (83%) (Fig. 4.3, Table 4.2). During 2013 isotopic analyses revealed mixing in of the two populations in the eastern No...