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

Abstract

The present study uses stable isotopes of nitrogen and carbon (δ15Nandδ13C) as trophic indicators for Atlantic bluefin tuna larvae (BFT) (6–10 mm 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 underlying microzooplankton 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 larvae.

Phytoplankton Community Structure Is Driven by Stratification in the Oligotrophic Mediterranean Sea

“…During this season, the water column stratification results in a two-layered euphotic zone (Coale and Bruland, 1987), with the upper layer being nutrient limited and the lower layer being light limited (Mignot et al, 2014). Regardless of the general oligotrophic condition in this region, many studies have described a surprising diversity and biomass of higher trophic level communities (Alemany et al, 2006; Rodriguez et al, 2013; Fernández de Puelles et al, 2014; Hidalgo et al, 2014; Laiz-Carrión et al, 2015; Reglero et al, 2017), often utilizing chlorophyll- a (Chl- a ) or fluorescence estimates as a proxy for primary production biomass to assess food web linkages of many marine species. However, the relation between carbon biomass (C) and Chl- a concentration (C:Chl- a ) in phytoplankton cells is largely variable and is influenced by light, nutrients and temperature (Jakobsen and Markager, 2016).…”

Trade-offs between risks of predation and starvation in larvae make the shelf break an optimal spawning location for Atlantic Bluefin tuna

“…Although the IBM does not simulate preflexion, flexion, and postflexion stages explicitly, we utilize the observed larval weight at the transition from flexion to postflexion as a reference for comparing simulated larvae to field-collected larvae. Based on larvae collected in the GoM, postflexion occurs at ~6 mm (Shiroza et al, this issue) which corresponds to ~10 days post hatch (dph) and 0.54 mg DW based on relationship presented in Malca et al (2017) and Laiz-Carrión et al (2015). Thus, in our model simulated larvae weighing <0.54 mg DW are considered to be obligate planktivores.…”

“…Once exogenous feeding begins, larvae are assumed to have utilized all egg yolk (i.e., there is no overlap between endogenous and exogenous feeding). Post yolk-sac larval weights are initialized based on a weight-to-age relationship determined from larvae collected in the GoM (Malca et al , 2017; Laiz-Carrión et al , 2015).…”

“…Females produce >10 million eggs (Aranda et al , 2013) and individuals hatch in 1–2 days (Tanaka et al , 2014). Within 2–4 days, larvae begin exogenous feeding at a size of ~3 mm length (Malca et al , 2017) and ~0.1 mg dry weight (DW)(Laiz-Carrión et al , 2015). The pelagic larval duration lasts 3–4 weeks (Fukuda et al , 2014), during which individuals grow quickly (~0.4–0.7 mm d −1 ) (Malca et al , 2017; Muhling et al , 2017).…”

Plankton food webs of the Gulf of Mexico spawning grounds of Atlantic Bluefin tuna

“…Gut pigment contents were analyzed as in Decima et al (2016) to estimate grazing rates (Landry et al, in prep.). More frequent net tows were conducted to a depth of 25 m to collect larval ABT for abundance, size, dry weight, gut content, otolith-based age, and isotopic measurements (Laiz-Carrion et al, 2015, Malca et al, 2017.…”