The 2019 QUEX PhD candidates are working on the following projects. Please note, these positions have been filled.

8. Using integrative physiology to optimise diets for sustainable aquaculture and reassess optimal foraging theory in wild fish

UQ academic lead

Professor Craig Franklin, Deputy Head of School, School of Biological Sciences

Exeter academic lead

Dr Rod Wilson, Professor of Integrative Animal Physiology, College of Life and Environmental Sciences

Project description

Optimal foraging theory (food selection based on calorie/nutrient value) has not previously considered dietary acid-buffering capacity (which is not linked to calorific/nutrient content). Carnivorous fish commonly ingest whole prey, and hard skeletal parts (calcium phosphate Ca3(PO4)2 in fish bone, calcium carbonate CaCO3 in invertebrate shells) are often entirely dissolved by gastric acid secretion. This process has an energetic cost, and in turn, it induces an alkaline tide (rise in blood pH and HCO3during digestion) which we were the first to discover in teleost fish (Cooper & Wilson, 2008).

Recovering post-feeding acid-base balance can take up to 3 days in fish, and has energy costs (gill, intestinal and kidney ion transport), as well as physiological consequences for O2 transport by haemoglobin.

Preliminary experiments used rainbow trout fed on isocaloric pellet diets that differed only in the calcium salt added (CaCO3, Ca3(PO4)2, or CaCl2 [as non-buffering control]) in isomolar quantities that mimic the skeletal content of crustacean/mollusc or fish prey (Fig. 1). This demonstrated that acid-buffering minerals can have major energetic consequences during digestion. This raises hypotheses that dietary buffer capacity should influence: a) prey selection and hence optimal foraging theory in nature, and b) efficiency of growth in aquaculture. Specifically, for prey/diets with otherwise identical calorific and proximal composition, the cost of digestion would be ranked as follows: calcium carbonate (CaCO3) shelled invertebrates will be ~30% higher than vertebrates with a bony skeleton of calcium phosphate (Ca3(PO4)2) which will be ~ 40% higher than soft-bodied invertebrates (Fig. 1).

Several species are amenable to test these hypotheses. For aquaculture this includes trout, salmon, sea bass and barramundi (providing both UK and Australian relevance). In nature, freshwater tench and sticklebacks consume whole snails (with CaCO3 shell) as well as fish and soft-bodied invertebrates. A study of invasive lionfish (of marine conservation relevance) found they ate 78% teleosts and 14% crustaceans by volume (Morris and Akins, 2009). This integrative project will apply automated respirometry to characterise energetic costs, and assess physiological consequences (ion/acid- base balance, blood O2 transport) of consuming different artificial and natural diets across a realistic range of acid-buffering capacities. The supervisory team provides ideal physiological (RW/CF), behavioural (RW/SS/AH) and ecological/conservation expertise (CF/AH/SS), as well as current aquaculture collaborations within UK (RW) and Australia (CF). We can also use our existing collaborations with field stations (e.g. Lizard & Heron Island, Australia; CEI, Bahamas) to run mesocosm- and field-based studies of prey selection to further evaluate this novel idea regarding our understanding of optimal foraging theory.

Related reading

Cooper CA, Wilson RW. 2008. Post-prandial alkaline tide in freshwater rainbow trout: effects of meal anticipation on recovery from acid- base and ion regulatory disturbancesJournal of Experimental Biology211: 2542–2550

Morris JA, Akins JL. 2009. Feeding ecology of invasive lionfish (Pterois volitans) in the Bahamian archipelagoEnvironmental Biology of Fishes 86: 389–398