4FatQs

The importance of dietary omega-3 fatty acids for development of cognitive sk ills in wild fishes through the lens of Tinbergens four essential questions (4FatQs) The physiological dependence of animals on a diversity of vital dietary biomolecules such as vitamins, amino acids andfatty acids is ubiquitous. Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are vital biomolecules and their dietary intake is necessary for survival and reproduction many vertebrates from fishes to humans. However, how dietary n-3 LC-PUFA influence brain biochemical and cellular composition and what is their impact on cognition and ecologically important behaviour remains largely unknown.In fact, we have only begun to understand how strong differences in the availability of vital dietary biomolecules across food webs can force animal consumers to adopt a range of strategies including behavioural, physiological, neural, and life-history traits to satisfy their needs for these biomolecules. Anthropogenic pressures such as climate change, eutrophication, or biological invasions are predicted to cause rapid changes the amount of available essential fatty acids in aquatic ecosystems. Theses change will likely have particularly serious consequences for top consumers in these ecosystems such as stream dwelling salmonid fishes. Therefore, there is an urgent need to understand how do wild fish cope with changes in availability of essential fatty acids in their diet. The aim of 4FatQs project is to use the framework of four essential equations proposed by a Nobel Price winner Nikolass Tinbergen to understand how availability of n- 3 LC-PUFA in diet affects development and evolution of brain in an experimental system of European stream-dwelling salmonids. Specifically, we will look into how diet quality affect changes density of neurons in the fish brain and how are these changes in the brain quality related to cognitive skills and behaviour of individuals. We will also explore how individuals from different populations response to the changes in availability of these vital dietary biomolecules, which will help us to understand how local fine scale evolutionary adaptations influence sensitivity of individuals to quality of their diet. This project combines field and laboratory experiments, while utilising aspects of biochemistry, behavioural ecology, neurobiology, and evolutionary biology. The results of this research will substantially advance the field of cognitive and ecological research as it is the first study that will link, at the individual level, the availability of vital dietary biomolecules with brain biochemical and cellular composition, cognitive skills, personality traits, ecological niche of wild animals. While answering fundamental ecological and evolutionary questions, the knowledge that will be acquired during this project has practical applications with regards to the management of wild fish populations as well as for welfare of fish in aquaculture.

All animals-from fish to humans-depend on essential nutrients in their diet, including vitamins, amino acids, and certain fatty acids. Among these, omega3 fatty acids play a particularly important role in brain development and function. However, not all food sources provide the same levels of these nutrients. In natural ecosystems, some diets are rich in omega3s, while others are relatively poor, meaning animals must adjust their feeding behaviour and physiology to meet their needs. The 4FatQs project investigated how differences in dietary omega3 fatty acids affect the brains and behaviour of freshwater fish, with a focus on brown trout. The central idea is that what fish eat may influence how their brains develop, how they think and learn, and how they behave in the wild. These behaviours-including boldness, activity, and explaratory behaviour-can ultimately affect survival and reproductive success. To address these questions, the project combined laboratory experiments, controlled outdoor studies, and fieldwork in natural streams. Researchers tracked fish diets using chemical markers, measured brain characteristics, and assessed behaviour and performance. The project demonstrated that dietary omega3 fatty acids have a profound impact on brain development and cognition in salmonid fishes. Laboratory experiments showed that trout fed diets low in omega3 fatty acids developed smaller brains, which likely reduced their cognitive abilities. This effect was consistently observed across multiple experiments. For example, trout with low omega3 intake performed worse in simple tasks such as navigating a maze. These fish were also less effective at learning from social interactions with other individuals, which led them to spend more time and energy fighting with others. As a result, they tended to occupy lower positions in social hierarchies and experiencing slower growth. These findings suggest that fish with insufficient omega3 intake would struggle to survive and reproduce in natural environments, highlighting the importance of these nutrients for fitness in brown trout. The project also examined these effects in the wild. In a study conducted in Scotland, we found that trout migrating to lakes and the ocean have a lower capacity to synthesise omega3 fatty acids internally and therefore depend more on dietary sources than trout that remain in headwater streams. This pattern is consistent with the fact that prey in lakes and oceans are richer in omega3 fatty acids than prey available in streams. We also investigated the importance of omega3 fatty acids for reproduction in different brown trout lineages in Austrian streams. While introduced Atlantic and domestic Danubian lineages did not differ in their consumption of omega3rich diets prior to spawning, females of both lineages consumed more omega3rich food than males. This is likely because females require these critical nutrients to support the healthy development of their eggs.