Regulation of appetite and growth of Atlantic salmon (Salmo salar L.) and effect of water oxygen, temperature and dietary energy

High water temperature combined with low dissolved oxygen (LO or hypoxia) is one of the most challenging environmental conditions farmed fish experience. The oxygen requirement of fish increases in parallel to this, which limits the aerobic energy metabolism and consequently reduces feed intake and growth of fish. The global ocean warming followed by reduced oxygen availability, is expected to exacerbate associated physiological stress on fish in several areas where Atlantic salmon are currently farmed. Understanding the impact of temperature and limited oxygen on growth regulatory mechanisms and the energy metabolism, will be of significant relevance to both cultured and wild fish populations. Conditions of high temperature and hypoxia are related with reduced feed intake and growth in fish. It is unclear whether the low oxygen availability directly affects growth regulatory mechanisms, and if low feed intake is the primary cause of depressed growth under LO conditions. Studies of appetite and growth regulation in salmon under such conditions are few, and considerations of the fluctuating character of endocrine signals and nutrient absorption are scarce. Limitation of the aerobic energy metabolism under reduced oxygen availability is further restricted by a thermal increase. It is therefore interesting to find out how high energy diets can potentially impact appetite and growth regulation under LO conditions. This thesis therefore investigated mechanisms by which LO and high temperature conditions impact appetite and growth regulation in seawater adapted Atlantic salmon. Free amino acid (FAA) and endocrine dynamics in relation to meal time were also studied. Four fish trials were conducted, including the following variables; dissolved oxygen (DO; LO and high, HO), temperature and digestible energy (low and high, LE and HE). Endocrine appetite and growth signalling was investigated through analyses of plasma ghrelin and IGF-1 concentration, and mRNA levels of the growth hormone receptor (ghr1) and insulin like growth factor-1 (igf1) in liver and muscle tissue. LO conditions demonstrated direct depressed effects on appetite and growth in salmon across temperatures. Reduced growth in salmon under LO was not caused only by a reduced feed intake, but appeared to be a combined effect of impairment of growth regulation and increased metabolic costs, as demonstrated by a pair-feeding technique. Increased metabolic costs by LO were indicated by responses in oxyregulating mechanisms, such as increased haemoglobin, reduced blood pH and imbalanced osmoregulation. Reduced specific growth rate (SGR) and feed intake were also found for salmon under LO compared to HO groups, at both optimal and high temperatures. High temperature demonstrated a diminished growth potential in salmon compared to an optimal temperature. This was reflected in a faster 24 hour postprandial catabolism of absorbed FAA, and generally lower and faster declines of IGF-1 (plasma and mRNA) at 19°C compared to 13°C. Ghrelin was found to signal feed anticipation in salmon, consistent with mammalian findings, and reflected by clear preprandial plasma ghrelin peaks at 12°C. Ghrelin and GHIGF factors responded to LO at a high temperature, but further studies should focus on a postprandial perspective to confirm the preprandial peaks at 12°C. Results from feeding HE diets to salmon, indicate that it is possible to stimulate growth, feed utilisation and the energy metabolism under LO conditions through DE level, regardless of temperature. To summarise, the thesis shows that growth regulation in seawater adapted salmon is negatively affected by LO at optimal and high temperatures. Positive effects from feeding HE diets under LO, demonstrate possibilities to support energy metabolism through dietary means under challenging environmental conditions. Diet effects and environmental impact on growth regulation are of great relevance to salmon farming, as further knowledge can improve growth, welfare, health and future farming possibilities.