Tracking Atlantic Bluefin tuna (Thunnus thynnus) movements in Nordic waters

Abstract

The commercially important Atlantic bluefin tuna (Thunnus thynnus, ABFT) has recently returned to Nordic waters after being absent for half a century due to intense, unregulated overfishing and eventual stock collapse. The present study processed and explored big data obtained from Pop-off Satellite Archival Transmitters deployed on large (CFL 248 +/- 16 cm) ABFT (n =75) in the Skagerrak from 2017-2023 to investigate their return to the area and identify key research questions on their behaviour for future study. Daily tag locations were mapped in the Atlantic by month. ABFT followed similar migration patterns reported in previous studies. The majority of individuals (n= 48) followed a West-East migration, crossing the Mid-Atlantic Ridge (MAR) to forage along the North Atlantic Labrador Current convergence Zone, while the others remained east of the MAR. All ABFT entered the Mediterranean for putative spawning except two who either skipped spawning or spawned elsewhere. They showed high site fidelity to Nordic waters (94% return rate) usually returning in June-July along warm North Atlantic currents to feed on seasonally abundant prey. Tagged individuals spent 50.08% of their time in the region from June to December. In previous studies, ABFT tagged at lower latitudes were smaller and less wide-ranging compared to the individuals studied here, which is evidence that ABFT exhibit size-dependent migration patterns. The Nordic ABFT spatial density hotspot (99% Kernel Density Estimate) spanned 26,357 km2 in Skagerrak north of the Jutland peninsula. Archival temperature and depth data at 5-second sampling intervals were processed and explored from retrieved tags (n = 33) and summarised into habitat envelopes and dive profiles. Surface net primary production data were sourced from Copernicus for the study area, for the same time period that ABFT frequented the area, and compared inside and outside the ABFT hotspot. In the Nordic region, experienced temperatures ranged 0.9 – 23.7 oC, swimming depth ranged 0 – 757.5m and net primary production ranged 2.5 – 62.4 mg m-3day-1. Tagged ABFT predominantly occupied shallow, warm waters and undertook frequent deep dives whilst in the Nordic waters as evidenced by the habitat envelopes, dive profiles and temperature and depth values. ABFTs experienced different swimming temperatures and depths depending on month and location, related to seasonal warming and stratification. Dive profiles showed the development of a strong thermocline at 20 - 30m during summer months and its dissipation in winter. Individuals were more surface-oriented in summer in Norwegian Trough and Skagerrak-Kattegat subregions compared to waters west of Norway in October-December, which could be explained by differences in oceanography, seasonality and prey switching from surface-oriented prey to demersal or mesopelagic prey. ABFTs were consistently surface-oriented in post-tagging ‘exit’ dive profiles which could also be explained by the tagging effects on fish welfare. It could not be concluded that there were differences in habitat use inside and outside the hotspot due to the problematic statistical approach. Based on the findings and previous research, the overarching hypothesis is that seasonal warming, oceanography (circulation, eddies, fronts, stratification, topography) and prey abundance are key drivers of ABFT migration. Several key questions were identified for further investigation: (1) What drives movement of ABFT tagged in Nordic waters?; (2) Do ABFT exhibit size-specific migration patterns and if so, what are they?; (3) How does ABFT movement relate to distribution and abundance of known prey species?; (4) Can ABFT dive profiles be used to identify different behavioural states and associated regions?; (5) Are ABFT spawning in areas other than the Mediterranean and can we use dive profiles to identify spawning timing and location?. Further study could use habitat suitability modelling approaches with size as a fixed effect, overlay and correlate ABFT locations with geolocated fisheries data on prey populations, and use behavioural state modelling to classify behaviour types based on the dive profiles. These research avenues should be prioritised to inform improved management and conservation of the species given its socio-economic and ecological importance.

Popular Abstract

Tracking Atlantic Bluefin tuna movements in Nordic waters


Atlantic Bluefin tuna (Thunnus thynnus) are some of the largest predatory fishes existing in our oceans, and they migrate long distances across the Atlantic Ocean in search of food and to find a mate. They can grow over 400cm long and weigh up to 680kg, which makes them valuable especially to the Japanese sushi market. In the early 20th century, they were widespread not only in the open ocean but also in and around Scandinavia. In the 1960s, however, they suddenly disappeared because they were fished almost to extinction by humans. In response, an organization was established who were tasked with managing the species so it could rebound. Recently, Atlantic Bluefin tuna have returned to Nordic waters which presents a unique opportunity to study their movements to make sure they are not overfished again in the future. My project involved processing, exploring and mapping a large dataset gathered by electronic satellite tags attached to 75 Atlantic Bluefin tuna in the Skagerrak from 2017 to 2023. I did this with the goal of understanding their three-dimensional movements to, from and in the region and to identify important questions for future research.

The tuna’s movements were recorded daily after being tagged and showed that most followed a known migration route in a southwest direction across the Atlantic to feed in the open ocean, while others stayed closer to Europe in the eastern Atlantic. Almost all tunas entered the Mediterranean Sea through the Strait of Gibraltar, probably to reproduce. Two individuals did not do this, which may be because they skipped spawning, or they spawned somewhere else like the Bay of Biscay. The tunas were loyal to Nordic waters, with 94% returning every year in June and July to feed on seasonally abundant fish such as Garfish and Mackerel. They do this by travelling along warm ocean currents entering the area at this time of year. Overall, they spent half of their time in the area from June to December. Compared to smaller tunas studied elsewhere, the larger Nordic tunas covered greater distances, suggesting that migration patterns depend on size. A key hotspot of tuna activity in the Nordic region was in the Skagerrak and covered an area of 26,357 km2.

Data from the 33 retrieved tags included more detailed measurements and revealed tuna’s temperature and depth preferences: they mostly preferred warmer shallower waters but also dove to great depths. During the summer months tunas spent more time near the surface in specific areas (Norwegian Trough, Skagerrak and Kattegat). In autumn and winter, they moved to deeper waters west of Norway likely because of changing ocean conditions and availability of different prey species. These tunas are known to choose prey based on what is most available in the surrounding area. I also investigated whether there was a difference in behaviour inside the hotspot compared to surrounding areas, but the results were inconclusive.

The study suggests that seasonal warming, ocean currents and prey availability are likely to drive Atlantic Bluefin tuna migration, but this is something that needs to be explored in more detail with more advanced methods. It also raises questions for future research, such as:

1. What specific aspects influence movements of these tunas in Nordic waters and to what degree?
2. Do larger tunas migrate differently compared to smaller ones? If so, how?
3. How do their movements relate to the location and abundance of prey?
4. Can diving patterns reveal different behaviour types?
5. Are tunas spawning elsewhere, and can diving data help pinpoint the timing and location of spawning?
6.
Studying tuna movements and behaviour is critical for managing and protecting this species moving forward, given that it is important to marine ecosystems and to society.

Master’s Degree Project in Biology 45 credits, 2025
Department of Aquatic Ecology, Lund University
Advisor: Kaj Hulthén
Department of Aquatic Ecology

• author: Azzopardi, Freja
• supervisor: Henrik Baktoft; Kaj Hulthén
• organization: Degree Projects in Biology
• course: BION02 20241
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9184983
• date added to LUP: 2025-02-12 14:27:41
• date last changed: 2025-02-12 14:27:41