Effects of cyanide exposure on Dascyllus aruanus, a tropical marine fish species: lethality, anaesthesia and physiological effects

Abstract

The lethality, anaesthetic and physiological effects of ‘pulsed’ cyanide (CN-) exposures to a common tropical marine fish Dascyllus aruanus were assessed. Cyanide (25 and 50 mg l-1) was applied as pulses (10, 60 and 120 s) to fish under non-stressed and stressed (by chasing and/or placing fish under hypoxic stress) conditions. Following treatment, the time until recovery and the percent survival were determined. The fish were allowed a 2.5 week recovery period from the treatments at which time four physiological end-points were measured: (1) the blood haemoglobin content, (2) the percent blood O-2 content, (3) the liver rhodonase activity and (4) the liver O2 consumption rate. The greater the CN- concentration and exposure time, the longer the recovery time. Non-stressed fish exposed to 10 s pulses of 25 mg l-1 of CN- fully recovered within 4 min whereas fish exposed for 60 s and 50 mg l-1 of CN- required up to 50 min for full recovery. Exposure for 120 s at 25 and 50 mg l-1 of CN- proved lethal. Under stressed conditions, previously non-lethal exposures (60 s and 50 mg l-1) were lethal. Of the physiological end-points, only the liver O2 consumption rate was indicative of previous CN- exposure. Under non-stressed conditions, pulsed exposures at 25 and 50 mg l-1 of CN- for 10 and 60 s significantly reduced the liver O2 consumption rates (ANOVA; p < 0.05). Under stressed conditions, the liver O2 consumption rates were significantly greater (maximum rates up to 17-fold greater) than the control fish (ANOVA; p < 0.05). We conclude, therefore, that environmentally relevant exposures of CN- can adversely effect fish and this affect can be measured 2.5 weeks post-exposure. Importantly, the combined effects of exposure and stress both increased the mortality and placed an appreciable metabolic load on the fish as indicated by the elevated liver O2 consumption rates. Handling stress in combination with anaesthetic CN- doses could in part explain the delayed mortality reportedly associated with CN- use in the tropical fish trade. The significance of the elevated liver O2 consumption rates on the survival of CN--exposed fish is unknown