Assessing the risks to zooplankton grazers of continuous versus pulsed cypermethrin exposures from marine cage aquaculture

Survival responses of adult stages of the calanoid copepod A. tonsa during and after pulse exposures to cypermethrin were used to evaluate the environmental risks of cypermethrin treatments in the marine environment. To control sea lice, whole cages of salmon are exposed to cypermethrin over 1 h, after which the effluent is released into the environment and subsequently dissipates. Here we simulate this short presence of the chemical in the water column. The present study revealed that delayed toxicity in A. tonsa adults, here expressed as decreased survival, occurred following short term pulse exposures to cypermethrin. Persistent, sensitive responses to cypermethrin during and after pulse exposures of 24 h were similar to those reported in conventional acute tests, suggesting that damage caused by accumulated levels of cypermethrin during the first 24 h of exposure could explain long-term toxicity responses in A. tonsa. Delayed toxicity following 1 h pulse exposures to cypermethrin impaired male survival and therefore changed copepod sex ratios. The proposed delayed toxicity tests were considered sensitive and suggest potentially detrimental effects of cypermethrin on copepod populations within the vicinity of the cages. By monitoring toxic responses following pulsed exposures, delayed toxicity tests are especially suitable to assess environmental risks of poorly water-soluble substances that dissipate rapidly within the water column.