Trophic position increases with thermocline depth in yellowfin and bigeye tuna across the Western and Central Pacific Ocean. (May 2017)

• Record Type: Journal Article
• Authors: Houssard, Patrick; Lorrain, Anne; Tremblay-Boyer, Laura; Allain, Valérie; Graham, Brittany S.; Menkes, Christophe E.; Pethybridge, Heidi; Couturier, Lydie I.E.; Point, David; Leroy, Bruno; Receveur, Aurore; Hunt, Brian P.V.; Vourey, Elodie; Bonnet, Sophie; Rodier, Martine; Raimbault, Patrick; Feunteun, Eric; Kuhnert, Petra M.; Munaron, Jean-Marie; Lebreton, Benoit; Otake, Tsuguo; Letourneur, Yves
• Is Part Of: Progress in oceanography. Volume 154(2017:May)
• Page Start: 49
• Page End: 63
• Publication Date: 2017-05
• Subjects: Nitrogen isotopes -- POM -- Isoscapes -- Bigeye tuna -- Yellowfin tuna -- Depth of the 20°C isotherm -- Amino acid -- Compound-specific isotope analysis -- Biochemical markers; Oceanography -- Periodicals; 551.4605
• DOI: 10.1016/j.pocean.2017.04.008
• Languages: English
• ISSNs: 0079-6611

Abstract

Highlights: A unique data-driven baseline δ 15 N isoscape was provided. Large spatial variations in baseline and tuna bulk δ 15 N values were observed. Higher trophic positions for tuna, particularly bigeye, in the southern latitudes. Higher trophic positions were observed as the depth of the 20°C isotherm increased. Vertical habitat use was shown as key to our understanding of foraging ecology. Abstract: Estimates of trophic position are used to validate ecosystem models and understand food web structure. A consumer's trophic position can be estimated by the stable nitrogen isotope values (δ 15 N) of its tissue, once the baseline isotopic variability has been accounted for. Our study established the first data-driven baseline δ 15 N isoscape for the Western and Central Pacific Ocean using particulate organic matter. Bulk δ 15 N analysis on 1039 muscle tissue of bigeye and yellowfin tuna were conducted together with amino acid compound-specific δ 15 N analysis (AA-CSIA) on a subset of 21 samples. Both particulate organic matter and tuna bulk δ 15 N values varied by more than 10‰ across the study area. Fine-scaled trophic position maps were constructed and revealed higher tuna trophic position (by ∼1) in the southern latitudes compared to the equator. AA-CSIA confirmed these spatial patterns for bigeye and, to a lesser extent, yellowfin tuna. Using generalized additive models, spatial variations of tuna trophic positions were mainly related to the depth of the 20°C isotherm, a proxy for the thermocline behavior, with higher tuna trophic position estimates at greater thermocline depths. We hypothesized that a deeper thermocline would increase tuna vertical habitat and access to mesopelagic prey of higher trophic position. Archival tagging data further suggested that the vertical habitat of bigeye tuna was deeper in the southern latitudes than at the equator. These results suggest the importance of thermocline depth in influencing tropical tuna diet, which affects their vulnerability to fisheries, and may be altered by climate change.