The acute toxicity of a 1-h pulse-exposure with the synthetic pyrethroid pesticide, esfenvalerate, to Australian crimson-spotted rainbow fish (Melanotaenia fluviatilis) of different ages was measured. The effects of esfenvalerate were measured on embryos 1, 2, 4, 8, 24, 72, and 144 h post-fertilisation, and on larvae <2 days, 3-4, 7-8, 14-16, 28-32, and 90 (sexually mature) days old. The effects of rainbow fish age and esfenvalerate exposure on weight-specific metabolism were also recorded and we tested the hypothesis that there was a significant correlation between the 1-h pulse-exposure 96-h LC50, and weight-specific respiration rate. The eggs of the rainbow fish were not sensitive to esfenvalerate, although the pesticide was highly toxic to larvae less than 1 week old. The mean (s.e.) 96 h LC50's following a 1-h pulse-exposure to esfenvalerate were 2.32 (0.70), 1.07 (0.26), 28.11 (14.46), 33.42 (10.21), 48.04 (13.34) and 3960 (157) μg 1-1 for rainbow fish <2 days, 3-4, 7-8, 14-16, 28-32, and 90 days old, respectively. There was a significant negative correlation between weight-specific respiration rate and the toxicity of pulse-exposed esfenvalerate, although esfenvalerate did not affect the respiration rate directly. We postulate that the observed decrease in toxicity of esfenvalerate with increasing age was due to lower uptake of the pesticide on a weight basis.
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