Using physiology to improve the sustainability of fish production in aquaculture

Abstract

Globally, aquaculture development is leading the way in the ‘Blue Revolution’ with it rapidly outstripping wild-capture fisheries. However, in the face of climate change and greenhouse gas emissions, it is vital that going forward, aquaculture develops in a sustainable and ecologically aware manner. One aspect of this is ensuring farms are operating at maximum efficiency, particularly from a biological perspective. Here we present an assessment of the impact that elevated environmental CO2 often found in fish farms can have on the digestive physiology of salmonids and the development of new tools for the field of fish physiology. Chapter 2 presents an experimental investigation into how exposure to elevated CO2 interacts with the metabolic costs of digestion. Utilising unsealed intermittent flow respirometry it was observed that rainbow trout (Oncorhynchus mykiss) are metabolically robust to elevations in environmental CO2 across the scope of one large meal and 6 small meals. This indicates that the reduction in growth observed in salmonids exposed to elevated CO2 is not due to an increased metabolic cost of digestion and therefore may be due to other aspects of a fish’s energy budget. The contents of Chapter 3 validate a novel method of fish phlebotomy that opens up a range of possibilities for assessing physiological responses that have thus far been unattainable with current methods. By comparing the novel method to grab ‘n’ stab and cannulation it was found that the novel method was able to avoid the acute stress responses associated with grab ‘n’ stab and generate data comparable with cannulation. This method particularly allows assessment of accurate acid-base responses to stimuli in a more ‘natural’ state and in fish smaller than cannulation can typically achieve. Notably, this allows for fish to be voluntarily feeding before blood samples are taken, opening up routes for novel methods of assessing physiological responses to feeding. Utilising the method developed in Chapter 3, Chapter 4 assesses the internal physiological processes associated with feeding and how these processes are impacted by exposure to elevated CO2. Utilising a combination of post-prandial acid-base whole animal flux measurements, and characterisation of post-prandial blood acid-base state, haematology, and osmotic components, a full characterisation of the physiological processes of digestion were assessed. This chapter found significant alterations to typical transport processes associated with feeding and indicates that elevated CO2 may cause switching of the routes of gastric bicarbonate excretion. There was also evidence of elevated CO2 impacting protein utilisation and assimilation supporting the evidence presented in Chapter 2.