strated a number of adverse effects associated with areca nut chewing.3'5 These include oral submucous fibrosis (OSMF)5'6 and oral cancer.!'3'4'7'8 Apart from ...
152
South African Journal of Science
RESEARCH LETTERS
'Breath holding' in tilapia Oreochromis mossambicus During experiments on the use of anaesthetics in fishes, it was observed that Oreochromis mossambicus was able to cease opercular movementfor up to 15 minutes. It could be shown that this was not an immediate anaesthetic effect. The cessation of ventilation corresponded to a reduction of oxygen consumption. The pathway of detection of foreign substances appears to be olfactory. We discuss possible reasons for this specific, previously unobserved behaviour, as well as its consequences for culturing this and other fish species. This paper reports on a specific respiratory behaviour of the Mozambique tilapia, Oreochromis mossambicus, first observed during experiments on the use of the anaesthetic 2-phenoxyethanol,1 a commonly recommended and investigated anaesthetic for fish.2 The original aim of the study was to gain an insight into the mode of action of 2-phenoxyethanol, to quantify the required dosages to achieve anaesthesia, and to record the recovery time after anaesthesia.1 During an initial experiment to quantify the time required to achieve anaesthesia, it was observed that O. mossambicus exhibited avoidance behaviour to 2-phenoxyethanol by ceasing gill ventilation. Irregular gill ventilation has been found by other authors. Roberts and Rowell3 reported intermittent ventilation in Ictalurus nebulosus with cessation of opercular movement for up to one minute. Ferreira4 observed 'restless behaviour' in tilapia and carp when exposed to anaesthetics. However, the behaviour observed in the current study was specific in that O. mossambicus was consistently able to stop ventilation for up to 15 minutes. We postulated that this behavioural response originated voluntarily. Since such a long period of apnea has not been observed in fish before, it warranted farther investigation. The objectives of this study were to find out a) if the cessation of opercular movement was an immediate anaesthetic effect, b) what percentage of tilapia responded to the anaesthetic through cessation of gill ventilation, c) how the presence of the anaesthetic was detected by the fish, and d) how the cessation of opercular movement affected the oxygen consumption of the fish. The first of two experiments aimed to quantify the percentage of tilapia that stopped gill ventilation when exposed to either 2phenoxyethanol or a non-anaesthetic disinfectant mixture such as a formalin/malachite green solution, commonly used in aquaculture. In addition to this, the experiment also tested whether the cessation of opercular movement was due to olfactory recognition. The second experiment was designed to test whether the cessation of opercular movement was accompanied by a drop in oxygen consumption. The tilapia, O. mossambicus, were raised at the experimental fish farm of the Department of Ichthyology and Fisheries Science, Rhodes University. All fish were acclimatized to 28°C for five days in 100-litre glass aquaria with in situ box-type filters and fed to satiation once a day. They were not fed for 24 hours prior to the experiments. Thirty fish (3.87 ± 0.43 g) were anaesthetized in a 0.5 ml 1-' 2phenoxyethanol solution in aerated 6-litre, compartmentalized, glass aquaria, with one fish in each of five compartments filled with 4 1 of anaesthetic solution. The anaesthetic solution was made up of water from the acclimation aquaria and thoroughly mixed with the appropriate volume of 2-phenoxyethanol. The temperature in the observation aquarium was kept constant at
Vol. 92 March 1996
28 °C by placing it in a water bath. All observations were made under an artificial light source. The time taken for the fish to attain total loss of equilibrium was recorded for each fish individually, at one-minute intervals for 15 min. Total loss of equilibrium was reached when the fish were inverted and/or showed no reaction to pressure around the caudal peduncle. All observations were performed on five fish per tank in triplicate tanks. To quantify the rate of oxygen consumption, 15 O. mossambicus were placed in a 1.5-1 sealed respiration chamber and exposed to three treatments as follows: Group A were exposed to 2-phenoxyethanol at 0.5 ml 1~'; group B were exposed to 250 g formalin: 1 g malachite green at 0.7 ml 1~'; and group C was the control, not exposed to foreign substances. The water was saturated with oxygen at the start of the experiment and oxygen concentrations did not drop below 5 mg I'1. The substances used were injected with a syringe through a rubber seal from the top of the chamber. Oxygen consumption was recorded at one-minute intervals for 15 min using an oxygen probe. The water in the respiration chamber was circulated using a magnetic stirrer and the temperature was set at 28°C. After the experiment, the total mass of the fish in the chamber was recorded and oxygen consumption was standardized to mg per g fish per minute. The results were analysed using a two-way analysis of variance (ANOVA) including the test for significant interactions between the oxygen consumption over time and substance. To determine the pathway for detection of 2-phenoxyethanol and a formalin/malachite green mixture, 60 O. mossambicus of an average mass of 6.14 ± 0.32 g were divided into three groups of 20 fish each and treated according to the three following procedures: Group A underwent anaesthesia with 0.09 mg I"1 benzocaine and cauterization of nares with a hot needle; group B underwent anaesthesia with 0.09 ml H benzocaine and cauterization of nares with a cold needle; group C was the unanaesthetized control. After the recovery period, the fish were placed in a 2litre observation aquarium filled with 0.5 ml I"1 2-phenoxyethanol at 28°C. Another aquarium was filled with a solution of 2 ml I"1 formalin/malachite green solution. If opercular movement stopped within 20 s after exposure, the fish were considered to have ceased opercular activity. Observations were performed twice on the same fish by two independent observers and analysed using the non-parametric Kruskall-Wallis test. Fifty per cent of the fish were in the state of total loss of equilibrium after an exposure time of 7 - 8 min, but 32.5% of the fish were not anaesthetized at the end of the 15-min exposure 70
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