PhD students

PhD students and projects at DTU Aqua within the research area Marine Populations and Ecosystem Dynamics.

Amalia Papapostolou

Title of the PhD project
Trophic efficiency of the pelagic food chain

Supervisors
Ken Haste Andersen, André Visser and Camila Serra-Pompei, DTU Aqua

Background of the project
The structure of the marine food web plays a crucial role for fisheries and ocean biogeochemistry. Food webs consist of interconnected food chains and in the ocean a food chain typically follows the sequence: phytoplankton, herbivorous zooplankton, carnivorous zooplankton, upper trophic levels (i.e. forage fish). It can take a varying number of steps within a food chain for energy to transfer from phytoplankton to fish across different oceanic regions, depending on the planktonic community composition. The length of the food chain is tightly linked to the concept of “trophic efficiency”, namely the efficiency with which energy flows from one trophic level to the next through predation.

About the project
The aim of my PhD is to make global estimates of the ‘microbial’ trophic efficiency from phytoplankton to small pelagic fish; basically how does energy flow across the food chain. This is crucial to improve our estimates and predictions for fisheries yields and carbon export. To do so, I will explore the mechanisms that govern marine food web dynamics through trait-based modeling, by implementing and further developing the “NUM” model framework, created at the Center for Ocean Life. NUM is a mechanistic size- and trait-based model along the Nutrient-Unicellular-Multicellular axis, based on individual-level processes. In NUM, the multicellular component encompasses ontogeny and describes the population dynamics of key copepod groups, characterized by their adult size and feeding mode. The composition of the plankton community is an emergent property of the model, resulting from predation and competition.

Perspectives
With this PhD, we expect to identify the main mechanisms linking higher trophic levels, such as fish, to primary producers, and see how trophic efficiency correlates to fisheries yields and carbon export.

Paco Rodriguez-Tress

Title of the PhD project
Optimizing important small pelagic fish resources by using data from scientific & commercial vessels

Supervisors
Stefan Neuenfeldt, DTU Aqua, and Claus Reedtz Sparrevohn and Lise Laustsen, Danish Pelagic Producers Organisation

Background of the project
Schooling and swarming are dominant features in marine ecosystems and are observed in over 25% of bony fish species throughout their life. Despite being present through most of the commercial fish species, the fundamentals of schooling mechanisms, as for example triggers, are still not well understood to date. Thus, the distribution and the spatial structure of pelagic schools remains elusive, potentially controlled by a range of factors, that still remains to be elucidated. These small- and large-scale aggregation dynamics can lead to a mismatch between traditional surveys and fish distribution, and to predictable variability in occurrence and density of schools, which are important elements of the commercial cruise planning, fuel optimization and target decision.

About the project
Using direct commercial fishing data and underwater acoustic observations, this industrial PhD project will provide insight into the spatial distribution and schooling mechanisms of pelagic fishes. Going further in-depth with understanding causes of uncertainty in biomass estimates, it investigates the behavioural mechanisms of schooling fish during school-formation and -deformation at dusk and dawn. This way, spatial structure is understood as an emerging and predictable property of individual behavior and can be separated from measurement uncertainty.

Perspectives
Small pelagic schooling fish are a sustainable and healthy protein source, which plays a key role in food security. They also have a significant potential for the green transition, as they are a CO2-friendly protein source. Conversely, the challenge when mobilizing fish in the green transition is to ensure sustainability and to maintain a low CO2 emission in the catching process. Thus, this project will provide mechanistic knowledge about spatial distribution of fish, so that stock assessments can be improved, and fishing optimized.

Costanza Cappelli

Title of the PhD project
Physical processes affecting stock dynamics of blue whiting in the northeast Atlantic Ocean

Supervisors
Brian MacKenzie and Andre W. Visser, DTU Aqua, and Hjálmar Hátún, Faroe Marine Research Institute

Background of the project
Blue whiting (Micromesistius poutassou) is a mesopelagic gadoid species widely distributed in the northeast Atlantic Ocean, and it is commercially exploited throughout much of the region. It is both an important prey for many higher trophic level species and a zooplanktivore exerting significant top-down pressures. In the last 15 years, blue whiting has experienced considerable swings in abundance and recruitment. Despite its commercial and ecological importance, little is known about blue whiting stock dynamics in relation to atmosphere-ocean variability, constituting a major source of uncertainty for the management of this species.

About the project
In this PhD project, I will examine how large-scale changes in the ocean-climate conditions affect blue whiting recruitment and distribution in the North Atlantic Ridge area. Using statistical tools and agent-based particle tracking modelling approaches, I will provide a quantitative analysis of the relationship between large-scale oceanographic features in the North East Atlantic (e.g., subpolar gyre dynamics, wind stress curl) and the early life history of blue whiting (larval drift patterns, growth, and survival rates).

Perspectives
This project will assess factors shaping temporal and spatial dynamics of blue whiting populations in relation to variable marine climate conditions in the North East Atlantic Ocean. The purpose is to develop models to understand the reproductive variability of blue whiting, as well as to improve stock assessments and fishery-related forecasts for this species. This knowledge will contribute to new ecosystem-based approaches to the management of blue whiting, UN Sustainability Development Goals related to Life Below Water, and an increased understanding of how climate change might impact productivity and biomass of this species.

Thøger Engelund Knudsen

Title of the PhD project
Fish migration and ecosystem processes

Supervisors
Brian MacKenzie and Patrizio Mariani, DTU Aqua, and Uffe H. Thygesen, DTU Compute

Background of the project
Seasonal migration can serve as a strong advantage for many species across large scales of the animal kingdom. The Atlantic bluefin tuna is one such species, and travels vast distances, presumably in search for abundant prey resources. This tuna is of special interest in Denmark, as parts of its population have recently started consistently making the journey from the Mediterranean all the way to Skagerrak and Øresund.

About the project
The general objective of this PhD project is to create one or more models that are able to mathematically describe the existence and evolution of migratory routes in pelagic fish populations. It is possible to mathematically show how migration and its benefits can manifest in the evolution of a species due to natural selection. An individual will to an extent remember successful journeys it has undergone during its lifetime. However, it is poorly understood how this knowledge accumulates and persist, transcending through generations, especially for species without means of explicitly communicating this information. I seek to develop a novel framework that explains the existence and creation of migratory routes in social memory so that we can understand the spatio-temporal dynamics of populations of fish like the Atlantic bluefin tuna.

Perspectives
This project will help determine the key aspects for local populations of Atlantic bluefin tuna, hopefully ensuring that they have come to stay. Furthermore, the existence of a framework that describes the creation of migratory routes through collective behaviour will advance our ability to understand and predict changes in migrational patterns in a changing world.

Yixin Zhao

Title of the PhD project
The role of fish in carbon cycling and the impact of climate change

Supervisors
Ken H. Andersen and P. Daniël van Denderen, DTU Aqua

Background of the project
The Earth’s oceans play a crucial role in regulating the Earth’s climate system, absorbing approximately 30% of anthropogenic carbon emissions through the physical and biological carbon pump. In the biological pump, carbon dioxide (CO^₂) is initially fixed by phototrophic organisms, forming the foundation of the marine food webs. This fixed carbon can be transported to the deep ocean through particle sinking (e.g., aggregates, carcasses, and fecal pellets), organism vertical migration, and water movement where it can be stored for long periods, ranging from months to millennia. So far, most studies have focused on plankton’s role in the biological carbon pump, while our comprehension of the contributions of fish to the biological carbon pump remains largely uncertain.

About the project
This project aims to bridge this knowledge gap by applying the FishErIes Size and functional TYpe model (FEISTY) alongside earth system models to assess and quantify the role of fish in global ocean carbon cycles, as well as the effects of fishery and climate change. To accomplish the goals, the FEISTY model will be two-way, online coupled with biogeochemical models through the Framework for Aquatic Biogeochemical Models (FABM). We will utilize reconstructed global fishing data with historical fishing patterns and different emission scenarios to force the coupled models, to investigate how fish-mediated carbon fluxes and sequestration vary under various climate change scenarios and fishing scenarios from past to future.

Perspectives
We expect the project to significantly improve our comprehension of fish-mediated carbon flux and sequestration in marine carbon dynamics. We will offer a novel approach to investigating and quantifying fish-mediated carbon fluxes and sequestration through coupling with earth system models. Furthermore, this research will underscore the importance of sustainable fisheries management in maintaining carbon sequestration from fish, thus contributing to climate change mitigation strategies and corresponding policy decisions.