Preliminary information on temporal changes in the blood chemistry of farmed southern bluefin tuna, 'Tunnus maccoyii' (Castelnau) after feeding and repeated sampling disturbance

Abstract

Southern bluefin tuna, 'Thunnus maccoyii' (Castelnau), are highly active pelagic carnivores that inhabit the coastal waters of Australia as juveniles before being recruited into an offshore population. Since 1991, juvenile tuna have been caught and transferred to Port Lincoln, South Australia, where they are kept in large net pens and grown for several months before being harvested, (van Barneveld. Smart, Clarke, Carter, Davis, Tivey & Brooker 1997). The large expense involved in working with a high-value fish means that investigating nutrition, growth and flesh quality requires strategies and sampling procedures that allow valid biochemical measurements to be taken (van Barneveld et al. 1997: Carter, Seeto, Smart, Clarke & van Barneveld 1998: Carter, Bransden, van Barneveld & Clarke 1999). Severe stress can result in fish death, and sublethal stress, such as that which may result from sampling and handling of fish, can lead to suppressed disease resistance and growth rate. Examining blood samples can give a good indication of the stress level of the fish (Thomas, Pankhurst & Bremner 1999). Plasma cortisol level is a recognized indicator of the magnitude of the stress response in fish (Pickering 1998). In addition, physiological disturbance in fish can be measured as changes in the level of plasma lactate and pH: changes in the concentration of these metabolites indicate a shift in the balance between aerobic and anaerobic metabolism. In the current work, plasma cortisol, lactate and pH were measured in order to detect a stress response in tuna. In addition, insulin-like growth factor I (ICF-I) was measured to establish whether there are changes related to feeding events and/or stressors, as has been shown by Pickering, Pottinger, Sumpter, Carragher & Le Bail (1991) for growth hormone. The present report represents the preliminary results of an ongoing investigation into the stress response of farmed southern bluefin tuna. The aim of the current experiment was to examine whether, following feeding, the blood chemistry parameters of tuna would be affected by repeated sampling of fish from the same pen.