Method of meal delivery influences estimates of postprandial metabolism and alkaline tide in a benthic fish

Intragastric gavage is widely used in studies of digestive physiology to deliver standardised meals, yet it remains unclear whether it elicits postprandial metabolic and acid-base responses comparable to voluntary feeding. Here, we tested whether the method of meal delivery affects the magnitude and timing of the postprandial rise in metabolic rate known as specific dynamic action (SDA) and the associated alkaline tide (transient rise in blood pH and bicarbonate following feeding) in sand flathead (Platycephalus bassensis) using a standardised 1.5% body-weight ratio either as intact pellets (voluntary feeding) or as a homogenised slurry via gavage, alongside fasted and sham-gavage controls. SDA magnitude was ~3.6-fold greater and duration was ~37 h longer in voluntarily fed fish. Removing the post-handling period reduced this difference, and the early post-entry period accounted for a greater proportion of metabolic expenditure in voluntarily fed fish, suggesting apparent differences partly reflect procedural effects of feeding method, including the mandated use of anaesthesia during the gavage procedure. Voluntarily fed fish showed a clear alkaline tide, to our knowledge the first report in this species, whereas gavage-fed fish showed no tide at 6 h, with lower blood pH and a delayed bicarbonate rise. Gut content distribution at 6, 12 and 24 h post feeding indicated reduced temporal separation between gastric acid secretion and intestinal neutralisation in gavage-fed fish. These results show that method choice can modify digestive physiology in ways that should be considered when interpreting and comparing studies of postprandial responses in fish.