Inhibition of acetylcholinesterase in rainbow trout following dichlorvos treatment at different water oxygen levels

Abstract

This study was designed to investigate the influence of water oxygen level on the degree of acetylcholinesterase (AChE) inhibition in rainbow trout ( Oncorhynchus mykiss) treated with dichlorvos against salmon lice. Two groups of rainbow trout weighing 135±40 g were subjected to a normal dichlorvos treatment (1.136 ppm for 60 min) at a water temperature of 9°C. During treatment, the water oxygen levels were 15 mg/l (TH) and 3 mg/l (TL), respectively. Afterwards both groups were kept at 6 mg/l O 2. Samples (five fish) were taken immediately afteer treatment, and after 2, 4, 7, 14 and 21 days. Control samples were taken at day 0, 7 and 21 in the same period. Brain, plasma, and liver samples were analysed for AChE activity. Immediately after treatment, the TH group showed significantly higher AChE activity in brain tissue than the TL group. The average degree of AChE inhibition was 45% (TH) and 58% (TL). Average TH values were above TL values for more than 14 days. However, both groups reached normal values at about the same time. No differences between groups could be demonstrated in plasma and liver, probably due to low AChE activity in these tissues. The results show that oxygen deficiency increases the AChE-inhibitory effect of dichlorvos, and that satisfactory oxygenation during delousing reduces the risk of intoxication of fish. They also show that the AChE activity may be inhibited for more than 14 days. Repeat treatment within this period might involve a higher mortality risk.