The energetics of stress tolerance in the early life stages of the goldfish Carassius auratus L

Abstract

A short-term fish early life-stage toxicity test using sac-try of the goldfish Carassius auratus L. was developed using a simple multichannel flow-through system. The physiological mechanisms behind the routinely used sublethal test endpoints of growth were investigated by examining energy partitioning within the developing sac-fry, and alternative endpoints of ammonia excretion and oxygen consumption were also investigated. A technique was developed for goldfish broodstock management which gave the potential for year-round production of eggs and sac-fry for laboratory experimentation. This was achieved by holding two females and one male each in isolation in 120 litre tanks with 14L : lOD photoperiod at 20°C. Females were conditioned by induced ovulation by spiking the tank water with 3 pg.l ' ot goldfish ovarian fluid, avoiding the need for stressful handling and/or temperature shocking normally required for induced spawning. Additionally, this technique allowed the repeated spawning of the same fish throughout the year. Eggs obtained by this method hatched after five days incubation at 20°C and reached a maximum body mass (in the absence of an external food supply) 4 days after hatching.

Three chemicals: cadmium chloride, 3,4-dichloroaniline (DCA) and chlorpyrifos, differing in water solubility and exhibiting different modes of action, were used as reference compounds to examine the physiological responses to toxic chemical stress in goldfish sac-fry during the period prior to first feeding. In preliminary studies of acute lethal toxicity, cadmium was the least toxic (96 h LC50 = 3404 pg.l '), DCA was intermediately toxic (96 h LC50 = 2359 pg l ') and chlorpyrilos the most toxic (96 h LC50 = 22.6 pg.l '). Further studies with DCA which examined the effect of exposing from the egg stage (age 30 h) indicated that for this compound at least, including the egg stage reduced test sensitivity (96 h EC50 = 3332 pg.l '). It was concluded that the egg stage was generally resistant to toxic exposure when compared with the sac-fry stage, since the former was less likely to accumulate toxic chemicals due to its possession of a relatively impermeable chorionic membrane. A simple conceptual model of the effects of toxic chemical exposure on energy partitioning in developing fish sac-fry was developed and tested. Among the physiological endpoints investigated, growth, yolk utilization efficiency and oxygen to nitrogen ratio (O : N) were equally sensitive for all three chemicals tested, with no-effect concentrations (NOECs) of 500 pg.l ' for cadmium, 25 pg.l ' for DCA and 0.25 pg.l ' for chlorpyrifos. Oxygen consumption was significantly (P<0.05) affected by two of the chemicals tested: cadmium (NOEC = 500 pg l ') and DCA (NOEC = 25 pg.l ') but was unaffected at all concentrations of chlorpyrifos tested. In contra.st, increased ammonia excretion was consistently the most sensitive parameter measured for all three chemicals tested: cadmium (NOEC = 250 pg.l '), DCA (NOEC = 25 pg.l ') and chlorpyrifos (NOEC = 0.25 pg l ').

More detailed studies of patterns of oxygen consumption under sublethal chemical exposure revealed that oxygen consumption was indeed .sensitive to low-level exposures. The complex of interacting factors determining oxygen consumption resulted in a non-monotonic response which was difficult to interpret due to confounding causal factors. In contrast ammonia excretion gave a simple monotonie response increasing with exposure, in line with predictions on increased protein breakdown under sublethal toxic exposure, indicating its potential value as an endpoint in fish early life-stage tests.