PhD students

Lene K. Sortland

Title of the PhD project
Increasing adult salmon recruitment via optimizing management decisions during outmigration

Supervisors
Kim Aarestrup, Niels Jepsen and Kim Birnie-Gauvin

Background of the project
Salmon and sea trout are iconic salmonids that migrate between the freshwater and marine environments. Salmonids reproduce in rivers, where they spend their juvenile phase before migrating to sea as “smolts” for feeding and growth. During their seaward migration smolts can experience high mortality rates, both natural (e.g., predators) and human induced (e.g., hydropower regulations). Smolt survival is generally considered to be density-independent, meaning there should be a correlation between smolts leaving the river and adults returning to spawn. Thus, increasing the number of smolts leaving a river can increase the number of adult returns.

About the project
The aim of my PhD is to use telemetry to identify bottlenecks that limit the survival of seaward migrating smolts. Telemetry involves attaching animals with electronic transmitters and tracking their movements through listening stations in the river, estuary, and fjord or with manual tracking along the river. Using telemetry and other sources of information (i.e., physiology, environmental conditions), I will investigate how migration and survival of smolts are influenced by predators, surrounding temperature, energetic status of individuals, and also look into the impacts of trapping, handling and tagging smolts with electronic transmitters.

Perspectives
Salmonids are facing multiple threats in their marine and freshwater environments, with humans often being the source. Despite conservation efforts, the number of wild Atlantic salmon has declined during the last couple of decades. Identifying and reducing bottlenecks for smolt survival can aid management actions to optimize adult returns and aid population recoveries.

Marie Hartlev Frausing

Title of the PhD project
Documenting the effects of coastal habitat improvements and marine protected areas on the presence of anadromous brown trout (Salmo trutta) and Atlantic cod (Gadus morhua)

Supervisors
Jon C. Svendsen, Henrik Baktoft and Elliot Brown

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. 

Benedikt Merk

Title of the PhD project
Lake restauration and its effects on fish behaviour 

Supervisors
Christian Skov, Henrik Baktoft, Martin Hage Larsen & Anders Koed

Background of the project
The eutrophication of lakes can alter aquatic ecosystems, by creating algal blooms and anoxic conditions, often resulting in reduced water quality and a loss of biodiversity. Zooplanktivorous fish like roach (Rutilus rutilus) reduce the predation of phytoplanktonic algae increasing their abundance. Thus, water turbidity increases and growth of benthic macrophytes that provide valuable micro-habitats for zooplankton and piscivores, e.g., pike (Esox lucius) is hindered. Alteration of the fish composition, in combination with reduction of nutrient influx, can minimize the effect of eutrophication on the lake, and lakes can even be restored to their mesotrophic origin. However, alterations in fish biomass (e.g., by removing roach from lakes) and environmental changes (e.g., water clarity and lake stratification) likely have various effects on the ecology, behaviour, inter- and intraspecific interactions of the remaining fish community.

About the project
This PhD project is centred around a eutrophicated lake undergoing restora-tion efforts through first biomanipulation and next sediment-removal with the aim to improve water quality and biodiversity. The project utilizes high-resolution acoustic telemetry allowing for fine scale 3D-postioning of tagged fish in combination with continuous measurements of several biotic (e.g., growth, fecundity, density) and abiotic parameters (e.g., water temperature, oxygen). Thus, detailed behavioural and ecological analyses of roach and pike before, during and after lake restauration measures can be conducted. 

Perspectives
The project contributes to expanding our knowledge regarding fish behaviour in a changing environment by exploring novel aspects of fish behaviour relating to inter- as well as intraspecific interactions which can be of relevance for future management of other eutrophic lakes. Additionally, biomanipulation and fish observation methods are assessed, aiming to increase their effectiveness. All in all, a wider understanding of the impacts lake restoration methods have on fish behaviour and lake ecology can be generated.

Marie Pedaccini

Title of the PhD project
When and why are fish vulnerable?

Supervisors
Kim Aarestrup and Kim Birnie-Gauvin

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.

Franziska Bockelmann

Title of the PhD project
Cormorant predation and river fish conservation

Supervisors
Niels Jepsen and 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.

Previous PhD students within the research area Freshwater Fisheries and Ecology

Bruno Ibanez-Erquiaga
The importance of oil and gas platform foundations for fish communities in the North Sea (link awaits publication of thesis)

Kristi Källo
Do all roads lead home? Straying of anadromous brown trout (Salmo trutta) in a fjord system

Casper Gundelund Jørgensen
Citizen science as a method to collect recreational fisheries data; strengths and limitations

Hugo de Moura Flávio
Bottlenecks for Atlantic salmon smolt migration in freshwater and estuarine ecosystems

Kim Birnie-Gauvin
The unspoken truth about impacted rivers: Consequences and implications of barriers for freshwater fish

Martin Lykke Kristensen
The marine life of sea trout (Salmo trutta L.)

Michael Schwinn
Effects of artificial lakes on migrating juvenile brown trout (Salmo trutta)

Joan Holst Hansen
Spatial ecology of lacustrine fishes: ecological effects and applied aspects

Martin Hage Larsen
Effects of hatchery rearing practices and cortisol manipulation on growth, survival and seaward migration success of stocked and wild Atlantic salmon and brown trout smolts

Diego del Villar-Guerra
The marine life of sea trout (Salmo trutta). Aspects of their migratory behaviour and survival

Henrik Baktoft
Aspects of lentic fish behaviour studied with high resolution positional telemetry