Respiratory and circulatory responses of rainbow trout larvae to carbon monoxide and to hypoxia.

The purpose of this investigation was to explore the ontogeny of certain respiratory responses of rainbow trout. Adult trout respond to hypoxia with an increase in the flow of water over the respiratory surfaces, and with a reflex bradycardia (Holeton & Randall, 1967). When adult rainbow trout are exposed to 5% carbon monoxide (CO) and thus rendered anoxaemic while at the same time their arterial blood and the environmental water are at a high POj, they do not show the reflex bradycardia. In fact their heart rate increases considerably (Holeton, 1971). One possible interpretation of the differences in the response of the hypoxic adult compared with the CO-poisoned fish is that the response to CO is the basic response to anoxaemia, and that the response to hypoxia is the same but with the bradycardia reflex superimposed upon this. By examining the responses to hypoxia and to CO by larval trout, it was hoped that a temporal or ontogenetic separation in the development of responses would indicate whether more than one set of responses was involved, and if so, which was the more ' primitive' or basic. The use of CO on newly hatched trout presupposes that haemoglobin blockage will have some effect, and therefore that the haemoglobin in the larval fish is serving an oxygen transport function. The hazard of tissue metabolism interference by 5 % CO was investigated by Holeton (1971) and discounted. Thus if the respiration of the larval fish is affected by CO, then it is reasonable to assume that their haemoglobin is serving a respiratory function. An investigation such as the present one is greatly facilitated by the transparency of the newly hatched trout. The breathing movements and circulatory dynamics can be observed visually without disturbance to the fish.