PhD students

Background of the project
Marine habitats in Denmark have undergone degradation during past decades due to factors as stone fishing and global warming. Suitable marine habitats are important for the commercially and recreationally valuable species Atlantic cod (Gadus morhua) and anadromous brown trout (Salmo trutta). Brown trout smolts are particularly dependent on suitable coastal habitats as smolts are vulnerable when they enter the marine environment and often experience severe predation. Atlantic cod populations in the Baltic Sea are currently under pressure and most cod stocks in this region have been depleted and are unable to reproduce in a stable manner. Despite various efforts to increase populations of cod and trout, only little attention has until now been put into understanding the dynamics and beneficial effects of coastal habitat improvements and marine protected areas (MPAs) for these species in the Baltic Sea. 

About the project
The aim of this PhD project is to examine and document the effects of coastal habitat improvements and MPAs on the presence of anadromous brown trout and Atlantic cod. Acoustic telemetry will be applied to track and examine the presence of juvenile and adult trout as well as cod at different coastal sites in Denmark. The presence of juvenile trout will be studied in association with the establishment of a coastal boulder reef. The reef is expected to provide appropriate habitats for the juvenile trout when they migrate into the marine environment. The presence of adult trout and cod will be investigated in two coastal MPAs and examined in relation to temperature. 

Perspectives
The results of this PhD project will strengthen our understanding of the beneficial effects that coastal habitat improvements and MPAs may have on trout and Atlantic cod. The movement patterns and presence of the tagged fish within the study areas will provide crucial information for future coastal habitat improvements and MPAs and how to use such management tools in the years to come.

Supervisors
Jon C. Svendsen, Henrik Baktoft & Elliot Brown, DTU Aqua

Background of the project
Fish must cope with numerous stressors and threats, including pollution, habitat destruction, overfishing, fisheries interactions, and climate changes. Understanding when and why fish are vulnerable is crucial for conservation, especially for migratory species, facing energy-intensive migrations and relying on specific environmental conditions. Although previous studies have assessed the vulnerability of salmonids and tunas under some circumstances, many knowledge gaps on the threats they face remain. Understanding the behaviour and threats of brown trouts (Salmo trutta) in direct sea systems (without fjords) and the impact of increasing temperatures on their behaviour, activities and performance remains unclear. Additionally, crucial information is lacking regarding how catch-and-release interactions affect the behaviour and survival of Atlantic bluefin tuna (Thunnus thynnus).

About the project
The main goal of this PhD is to assess the underlying mechanisms that determine when and why migratory fish are vulnerable to stressors in the marine and freshwater regions of the Skagerrak-Kattegat-Øresund region, using brown trout and Atlantic bluefin tuna as model species. Biotelemetry and physiological data help assess vulnerability by tracking movements and analyzing fitness, offering insights into critical instances and factors influencing vulnerability.

Perspectives
This PhD project will strengthen our understanding of the critical instances or circumstances under which fish are most vulnerable, as well as the factors that may modulate this vulnerability. Those findings should enable adaptative handling practices and the development of conservation measures for brown trout and Atlantic bluefin tuna, but the findings are expected to be transferable to similar species.

Supervisors
Kim Aarestrup & Kim Birnie-Gauvin, DTU Aqua

Background of the project
Freshwater fish represent one of the most threatened taxonomic groups in Europe, highlighting the urgent need for improved conservation and monitoring strategies. Despite being a significant driver of fish population declines, the extent of cormorant predation on threatened fish species remains underexplored, posing a challenge for informed conservation decision-making. Another main problem is the lack of reliability of monitoring data. EU member states are mandated to report on species listed in the Habitats Directive, yet monitoring data for riverine fish is often incomplete or absent.

About the project
Through my PhD, I aim to address these challenges by investigating the relationship between cormorant predation and fish population dynamics in European rivers. Using a combination of historical data analysis, field studies, and experimental approaches, I seek to quantify the extent of predation pressure and identify key environmental factors that influence its severity. For example, I am exploring how habitat complexity may mitigate predation and assessing the effectiveness of exclusion measures in protecting fish populations. This PhD is part of the EU-wide ProtectFish initiative and uses the European grayling (Thymallus thymallus) as a case study, to highlight the predator-prey dynamic of cormorants and riverine fish, and propose solutions to enhance protection measures.

Perspectives 
We aim to produce actionable insights that can guide policy changes, improve biodiversity monitoring systems, and enhance the conservation status of freshwater fish. Ultimately, this research aspires to contribute to a more sustainable coexistence between predator populations, and the aquatic ecosystems they share. The findings will contribute to policy updates at the EU level, fostering improved biodiversity monitoring frameworks.

Supervisors
Niels Jepsen & Kim Birnie-Gauvin, DTU Aqua

Background of the project
Atlantic Bluefin Tuna (Thunnus thynnus) is one of the largest predatory teleosts alive today in our oceans and is both ecologically and economically important. Because of its high economic value to fisheries, the eastern Atlantic Bluefin Tuna population underwent intense, unregulated fishing resulting in a stock collapse in the 1950s and, in turn, their disappearance from northern latitudes, including Scandinavia. Atlantic Bluefin Tuna have since returned to these waters only recently as a result of a successful stock recovery plan. Researchers have hypothesized that climate change and changing ocean currents could also be at play with regards to their resurgence, namely by influencing shifting distributions and abundance of prey resources. Due to the absence of Atlantic Bluefin Tuna in Scandinavia, very little is known about their movement ecology in the region, despite being a relatively well-studied oceanic species.

About the project
This PhD project revolves around processing and analysing a large dataset obtained from Pop-off Satellite Archival Transmitters (PSATs) deployed on Atlantic Bluefin Tuna over the last several years in Danish waters. The aim is to investigate the movement and behaviour of Atlantic Bluefin Tuna that visit the Skagerrak and Øresund seas each year, how this relates to the wider Atlantic population and the implications of this in the context of commercial fishing and climate change.

Perspectives
Highly migratory marine species such as Atlantic Bluefin Tuna are particularly vulnerable to overexploitation and are also challenging to study and manage, given they regularly cross several jurisdictions and ecosystems. Ascertaining biological information of the population is therefore essential to prevent stock collapse in the future, as well to safeguard the ecosystem services provided by the species.

Supervisors
Henrik Baktoft & Kim Aarestrup, DTU Aqua