SalmoPUFA

You are what you eat is a long-standing assumption in ecology that has been validated across diverse ecosystems, ranging from terrestrial to aquatic food webs. Nutritious quality of diet is indeed a key factor that affects characteristics of individuals, but its effects in interaction with other stressors like water temperature are not well understood. Climate change is predicted to cause an increase of water temperatures and decrease the amount of available essential nutrients in aquatic ecosystems during the next century. Therefore, there is an important knowledge gap that needs to be addressed: How do wild fish cope with changes in diet quality under conditions of rising water temperatures? Vertebrates including fishes and humans cannot synthetize long-chain polyunsaturated fatty acids LC-PUFA and thus they need to be supplied by diet, which makes LC-PUFA essential dietary nutrients of high ecological importance. The aim of SalmoPUFA project is to explore how diet quality and water temperature affect brain and eye tissue development, whole-animal metabolism, learning capacity, and behavioural traits in salmonid fishes. We use brown trout Salmo trutta as a model species because it is a typical species living in small streams and its individuals expresses permanent differences life history and feeding strategies. We will combine feeding and water temperature treatments in a long-term laboratory experiment, which will consist of two parts: first, individuals exposed to the abovementioned treatment will be challenged by a set of standardized behavioural trials to evaluate their behaviour and capacity to learn new tasks. Second, we will measure how much oxygen individuals consume to sustain their minimal resting metabolism and how much they consume immediately after exhaustive exercise. This information will help us understand the energy budget of their metabolism. In addition, in both part of the project we will evaluate differences among treatments in somatic growth, size, and condition factor, in lipid composition of liver, muscle, and neural tissue using spectrometry, and differences in brain morphology using a picture analysis. Results from this project will provide fundamental knowledge about how diet and temperature changes fish behaviour and physiology. This new knowledge will be helpful for the management of salmonid fishes, which are used as an important human food source (e.g. industrial fisheries and aquaculture) and for recreational fishing. Developing our understanding about the role of dietary LC-PUFA and temperature on performance of salmonids can help us improve welfare of salmonid fishes kept in hatcheries and fish farms and increase fish survival to individuals stocked to our lakes and rivers. Project results can also help find a better balance between ecological costs of fish meat production and high quality food for human consumers.

Early brain development in most vertebrates including fishes depends on the supply of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), and particularly on docosahexaenoic acid (DHA). These nutrients can be supplied to the brain through diet or energetically costly internal synthesis. Although the current research on evolution of animal behaviour and cognition increasingly focuses on ecological factors that promote and maintain variation in these traits, the role of ubiquitous trade-off between diet quality and energetic costs of synthesis of important nutrients have not been investigated. Overlooked within this context is the fact that the availability of dietary DHA will decrease in freshwater ecosystems due to ongoing climate change. This new combination of selection pressures on freshwater fishes could produce a major whole-life lasting shift in brain development and their behaviour and cognition that is yet to be considered, but which could directly determine population resource requirements, resilience, and responses to the climate change. The results of SalmoPUFA project have shown that both temperature and diet quality have substantial impact on brain development and behaviour of wild freshwater fishes. It has shown that the high-water temperature can lead to changes in brain morphology of a common minnow and result in reduced cognitive skills. SalmoPUFA project have further demonstrated that the lower quality of diet leads to development of smaller brain in wild brown trout and Atlantic salmon. Finally in a study comparing the effects of diet quality and water temperature, we have found that diet quality impacts the brain development and cognitive skills of rainbow trout more than the increase of water temperature. Importantly, we have found that the changes in cognitive skill measured under the laboratory conditions might affect body growth of wild fish in their natural environment. Therefore, the SalmoPUFA project provides for the first-time evidence on how diet quality, brain development, behaviour and fitness are interconnected in wild animals. We still known little about the role of biosynthesis of DHA and its link to foraging strategies and brain development of wild animals. Therefore, this project is the first step towards bridging the gap between purely laboratory and field ecological research. While answering fundamental ecological and evolutionary questions, the knowledge that have been acquired during this project can have practical applications with regards to the management of salmonid populations, as well as for sustainable aquaculture production, and cognitive training of hatchery-reared fish to increase their survival in the wild.