What Fish decrease in size with global warming, yet the reason why is not understood. In theory, it could be caused by a limitation in oxygen uptake over the gill surface area, which decreases relative to their body size as the they grow larger, limiting physiological performance and, in turn, limiting growth. However, the gill oxygen limitation theory has never been experimentally validated and has recently been debated heavily by scientists. Indeed, surprisingly little effort has been given to directly investigate physiological and behavioral mechanisms behind why fish are getting smaller with increasing temperature, knowledge gaps which my project seeks to remedy. Why My project will provide novel insight to why fish are becoming smaller with increasing temperatures, and will furthermore be the first direct experimental testing of gill oxygen limitation theory. The project is interdisciplinary in nature, and will investigate the subject on multiple organismal level, and innovative as it uses state-of-the art methods for studying fish physiology. Fish are the most diverse and abundant group of vertebrates on the planet, with enormous ecological and socio-economic importance. The project will thus be of interest to a broad range of both fundamental and applied fields, including, but not limited to, animal physiology, behaviour, ecology, fisheries, and aquaculture. How I will experimentally test if the ability to take up oxygen is limited more in larger fish compared to smaller fish at high temperatures by measuring the oxygen partial pressure in the dorsal aorta of rainbow trout of different sizes at intermediate and high temperatures. I will also determine the potential effect of size on optimal metabolic temperature in the species, to investigate the potential effects of a gill oxygen limitation on physiological performance. In addition, I will determine the temperature preference of rainbow trout of different size to see if the gill oxygen limitation translate into behavioural thermoregulation, which can thus be a reason for why fish are smaller at higher temperature by larger fish preferring lower temperatures due to a limitation in oxygen uptake. SSR The livelihood of 57 million people is supported by fisheries and aquaculture, and 400 million people depend critically on fish as protein source. Knowledge on why fish are becoming smaller at high temperatures is therefore key if we are to successfully predict life-history shifts, conserve species biodiversity, manage populations, and maintain food security in the face of continued global climate change. The topic of fish body-size responses to temperature therefore presents an ideal platform for illustrating the effects of environmental change to a broader audience because it is a concept that people can relate to if being told that the fish on their dinner plate will be as much as 24% smaller within the next 30 years.