| dc.description.abstract | Aquatic foods are crucial to food security and nutrition for billions of people, their production-related industries support livelihoods, economies and cultures all around the world. Understanding the physiological costs of feeding in fish is key to optimise aquaculture production and sustainability. Most aquaculture production occurs in tropical areas, where high stocking densities and limited water aeration results in low oxygen conditions (hypoxia). As a result, the farming of air-breathing fishes has significantly increased. Air-breathing fishes are evolutionarily important to understand as they were the first vertebrates to evolve the ability to breathe air.
Chapter 1 of this thesis outlines the research concepts covered throughout this work, focusing on physiological differences in water- and air-breathing fishes and exploring how air-breathing in these fish may influence the processing of the postprandial alkalosis - ‘the alkaline tide’ - a blood acid-base disturbance generated by gastric acid secretion after feeding. I focused on the physiology of the striped catfish (Pangasianodon hypophthalmus), a facultative air-breathing fish.
Chapter 2 presents my research on the acid-base disturbance associated with feeding on different meal sizes (rations) in striped catfish, investigating the acid-base fluxes into the external water, which overall resulted in a net excretion of acid after every ration. The acid-base parameters in blood and gut fluid three hours after feeding showed no significant increase in blood pH but an increase in [HCO3-] in the intestinal fluid up to ~30 mM. No difference in air-breathing frequency or an increase in blood CO2 after feeding indicated that striped catfish do not use respiratory mechanisms (i.e. the air-breathing organ) to cope with the alkaline tide.
Chapter 3 explored the postprandial acid-base fluxes into the external water in a model water-breathing fish, the rainbow trout, and the effect feed ration had on these rates. Feeding resulted in a change from a negligible acid-base flux prior to feeding to a net base flux after every ration. In this study rainbow trout showed a linear relationship for cumulative HCO3- excretion (over the whole digestion period) with ration size. I found a non-linear relationship between NH4+ excretion and ration, indicating that at rations greater than 2 % of body mass the dietary protein intake is in excess of protein synthesis, resulting in proportionally more of the absorbed amino acids being catabolised for energy and nitrogen being excreted as ammonia.
Chapter 4 discusses the broader implications of my thesis, including general limitations, future directions and applications to sustainable aquaculture. I compared the acid-base fluxes between air- and water-breathing fish after feeding; the water-breathing fish excreted a comparatively higher amount of excess HCO3- after a meal. I reviewed the literature which highlighted that catfish become less efficient at processing protein as the protein content of the diet increases. My research supports this finding, as catfish fed a high protein diet exhibited a high cumulative ammonia excretion after a meal. With the requirement for the aquaculture industry to move away from fish meal and oil, striped catfish provide a more sustainable source of protein and nutrition to people, growing well with a low protein diet. Further research is required to understand the digestive physiology, optimise diets and reduce use of wild fish stocks for aquafeed, to promote sustainable growth of these species. | en_GB |
