Do western Atlantic bluefin tuna spawn outside of the Gulf of Mexico? Results from a larval survey in the Atlantic Ocean in 2013

In 2013, a larval survey was conducted north and east of the Bahamas aboard the NOAA Ship NANCY FOSTER. Sampling areas were selected based on larval habitat model predictions, and daily satellite analysis of surface temperature and ocean color. Samples were collected at 97 stations, and 18 larval BFT (Thunnus thynnus) were found at 9 stations. Six of these stations came from oceanographically complex regions characterized by cyclonic and anticyclonic gyres. Larvae ranged in size from 3.22mm to 7.58 mm, corresponding to approximately 5-12 days in age. Analysis of satellite derived surface currents and CTD data suggest that these larvae were spawned and retained in this area. Larval habitat models show areas of high predicted abundance extending east to 650 W, but the actual extent of spawning in this area remains unknown.En prens

Trophic Ecology of Atlantic Bluefin Tuna (Thunnus thynnus) Larvae from the Gulf of Mexico and NW Mediterranean Spawning Grounds: A Comparative Stable Isotope Study

The present study uses stable isotopes of nitrogen and carbon (δ15Nandδ13C) as trophic indicators for Atlantic bluefin tuna larvae (BFT) (6–10mm standard length) in the highly contrasting environmental conditions of the Gulf of Mexico (GOM) and the Balearic Sea (MED). These regions are differentiated by their temperature regime and relative productivity, with the GOM being significantly warmer and more productive. MED BFT larvae showed the highest δ15N signatures, implying an elevated trophic position above the underlyingmicrozooplankton baseline. Ontogenetic dietary shifts were observed in the BFT larvae from the GOM and MED which indicates early life trophodynamics differences between these spawning habitats. Significant trophic differences between the GOM and MED larvae were observed in relation to δ15N signatures in favour of the MED larvae, which may have important implications in their growth during their early life stages. These low δ15N levels in the zooplankton from the GOM may be an indication of a shifting isotopic baseline in pelagic food webs due to diatrophic inputs by cyanobacteria. Lack of enrichment for δ15N in BFT larvae compared to zooplankton implies an alternative grazing pathway from the traditional food chain of phytoplankton— zooplankton—larval fish. Results provide insight for a comparative characterization of the trophic pathways variability of the two main spawning grounds for BFT larvaeVersión del editor4,411

Mean nitrogen and carbon isotopic signature in MICRO, MESO and BFTL.

<p>δ¹⁵N (A) and δ¹³C (B) isotopic signature (Mean±SE) for different planktonic compartments analyzed (microzooplankton [MICRO,55−200 μm], mesozooplankton [MESO, >200 μm] and bluefin tuna larvae [BFTL]) in Eastern Gulf of Mexico (E-GOM) (●), Western Gulf of Mexico (W-GOM) (●) and Balearic Sea (MED) (○) regions. Regions (E-GOM, W-GOM and MED) and plankton size fraction (MICRO, MESO and BFTL) were the main factors for the 2-way ANOVA analysis. Post-hoc comparisons were made using a Tukey’s test. Different letters indicate significant differences (<i>p</i>< 0.05) among plankton size fractions within same region. * denotes a significant difference (<i>p</i>< 0.05) between regions for the same plankton size fraction.</p

Biplot of mean nitrogen and carbon stable isotopes in bluefin tuna larvae.

<p>Mean (±SE) δ¹³C versus δ¹⁵N (‰) values mesozooplankton (MESO-triangles) and bluefin tuna larvae (BFTL-circles) in Eastern Gulf of Mexico (E-GOM) (●), Western Gulf of Mexico (W-GOM) (●) and Balearic Sea (MED) (○) regions. Microzooplankton as primary consumers has been used as baseline. The ellipses represent the isotopic variance for each region, encircling the isotopic signature for the different plankton size fractions (MICRO, MESO and BFTL).</p

Nitrogen and carbon stable isotopes <i>vs</i> bluefin tuna larvae size relationship.

<p>Different δ¹⁵N (A) and δ¹³C (B) isotopic signature <i>vs</i> standard length (SL) relationships of bluefin tuna larvae in Eastern Gulf of Mexico (E-GOM) (●), Western Gulf of Mexico (W-GOM) (●) and Balearic Sea (MED) (○) regions.</p

Bluefin tuna larval size frequency distributions.

<p>Eastern Gulf of Mexico (E-GOM), Western Gulf of Mexico (W-GOM) and Balearic Sea (MED) bluefin tuna larval size frequency distributions analyzed for trophic ecology studies.</p

Location of the study areas.

<p>(A) Gulf of Mexico (GOM) and Balearic Sea (MED). (B) Eastern (E-GOM) and Western (W-GOM) and (C) MED bluefin tuna study area showing the oceanographic-planktonic stations sampling distribution. Bathymetric image generated from ETOPO1 database [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133406#pone.0133406.ref051" target="_blank">51</a>].</p

Biomass of zooplankton size fractions.

<p>One-way ANOVA analyses of both (A) microzooplankton [MICRO,55−200 μm]- and (B) mesozooplankton [MESO, >200 μm] size fraction biomass (mg·m^-3) in Eastern Gulf of Mexico (E-GOM) (●), Western Gulf of Mexico (W-GOM) (●) and Balearic Sea (MED) (○) regions. Post-hoc comparisons were made using a Tukey’s test. Different letters indicate a significant difference between ecosystems. Box represents std error and whisker Std dev respectively.</p