Pre-spawning habitat use of Atlantic bluefin tuna (Thunnus thynnus) inferred from stable isotope analysis

Abstract

Atlantic bluefin tuna (ABFT; Thunnus thynnus) spawn primarily in the Gulf of Mexico and Mediterranean Sea, but migrate to foraging habitats throughout the North Atlantic where they are the target of commercial and recreational fisheries. Their natal origin has been characterized through otolith oxygen isotope analysis to link fish on both spawning grounds and foraging habitats to their spawning ground origins, but connectivity on a shorter, seasonal timescale is still not completely understood. Nitrogen isoscapes in the North Atlantic include a distinct separation of productive, nearshore and more oligotrophic open ocean foraging habitats. We used linear discriminant analysis of bulk nitrogen isotope data to estimate the percent of ABFT that occupied shelf or open ocean foraging habitats prior to capture on eastern and western Atlantic spawning grounds. ABFT in the Gulf of Mexico were mainly classified as previous shelf foragers (91%), while ABFT associated with eastern Atlantic spawning grounds primarily had an open ocean/Mediterranean Sea classification (96% Morocco, 79% Strait of Gibraltar, 91% Balearic Sea, 100% Adriatic Sea). Amino acid nitrogen isotope data of ABFT from the Gulf of Mexico confirmed that observed bulk nitrogen isotope differences were due to baseline rather than trophic variability and source amino acid values generally aligned most closely with literature values from shelf and slope waters rather than open ocean habitats. These data provide insight into the foraging habitats that support eastern and western Atlantic spawning assemblages.

Full text

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Marine Biology (2023) 170:67
https://doi.org/10.1007/s00227-023-04210-7
ORIGINAL PAPER
Pre‑spawning habitat use ofAtlantic bluefin tuna (Thunnus thynnus)
inferred fromstable isotope analysis
JohnM.Logan1 · AndrewS.Wozniak2· JoséLuisVarela3· AlisonRobertson4,5
Received: 30 August 2022 / Accepted: 6 April 2023 / Published online: 29 April 2023
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023
Abstract
Atlantic bluefin tuna (ABFT; Thunnus thynnus) spawn primarily in the Gulf of Mexico and Mediterranean Sea, but migrate
to foraging habitats throughout the North Atlantic where they are the target of commercial and recreational fisheries. Their
natal origin has been characterized through otolith oxygen isotope analysis to link fish on both spawning grounds and foraging
habitats to their spawning ground origins, but connectivity on a shorter, seasonal timescale is still not completely understood.
Nitrogen isoscapes in the North Atlantic include a distinct separation of productive, nearshore and more oligotrophic open
ocean foraging habitats. We used linear discriminant analysis of bulk nitrogen isotope data to estimate the percent of ABFT
that occupied shelf or open ocean foraging habitats prior to capture on eastern and western Atlantic spawning grounds. ABFT
in the Gulf of Mexico were mainly classified as previous shelf foragers (91%), while ABFT associated with eastern Atlantic
spawning grounds primarily had an open ocean/Mediterranean Sea classification (96% Morocco, 79% Strait of Gibraltar,
91% Balearic Sea, 100% Adriatic Sea). Amino acid nitrogen isotope data of ABFT from the Gulf of Mexico confirmed that
observed bulk nitrogen isotope differences were due to baseline rather than trophic variability and source amino acid values
generally aligned most closely with literature values from shelf and slope waters rather than open ocean habitats. These data
provide insight into the foraging habitats that support eastern and western Atlantic spawning assemblages.
Keywords Chemical tracers· Isoscapes· Movement· Nitrogen· North Atlantic
Introduction
Atlantic bluefin tuna (ABFT; Thunnus thynnus) are highly
migratory top predators that support fisheries throughout the
North Atlantic and Mediterranean Sea (Mather etal. 1995;
Fromentin and Powers 2005). ABFT are currently managed
by the International Commission for the Conservation of
Atlantic Tunas (ICCAT) as two separate (i.e., eastern and
western) stocks divided at the 45ºW meridian. Stock inde-
pendence has been supported by genetic (Carlsson etal.
2007) and tagging (Block etal. 2005) data. However, shelf
and open ocean waters throughout the North Atlantic act as
both foraging and fishing grounds for the species; further-
more, there is mounting evidence from electronic tagging
and chemical tracer studies of ABFT migrations and mixing
between eastern and western management areas (Walli etal.
2009; Rooker etal. 2014; Kerr etal. 2020). Electronic tag-
ging data have revealed complex and varied migratory pat-
terns, including movement across the stock boundary (Block
etal. 2001; Walli etal. 2009; Galuardi etal. 2010; Aarestrup
etal. 2022). The extent to which eastern and western stock
fish occupy the foraging grounds of the other stock compli-
cates management of this highly valuable species. Making
this problem more contentious is the large difference in stock
sizes with the eastern stock being estimated at an order of
magnitude larger than the western stock (Anonymous 2017).
Like other tunas, bluefin tuna are “energy speculators”
(Brill 1996; Korsmeyer etal. 1996) with high metabolic
Responsible Editor: S. Hamilton.
* John M. Logan
john.logan@mass.gov
1 Massachusetts Division ofMarine Fisheries, NewBedford,
MA02744, USA
2 School ofMarine Science andPolicy, University
ofDelaware, Lewes, DE19958, USA
3 Departamento de Biología, Facultad de Ciencias del Mar y
Ambientales, Universidad de Cádiz, República Saharui s/n,
11510PuertoReal, Spain
4 School ofMarine andEnvironmental Sciences, University
ofSouth Alabama, Mobile, AL36688, USA
5 Dauphin Island Sea Lab, DauphinIsland, AL36528, USA
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