Neurotoxicity of pesticides to salmon: Physiology to ethology

Pesticides routinely found in the environment can adversely affect neurological systems of salmon. The major focus of the studies presented here is on the impairment of the relatively exposed olfactory sensory neurons (OSNs), since their functionality is critical to several indispensable behaviours. The responses of OSNs to various behaviourally-relevant odorants were impaired following exposure to several pesticide classes, including triazine (e.g. atrazine), carbamate (e.g. IPBC), organophosphorus (e.g. dimethoate), and phenylurea (e.g. linuron) pesticides, as well as a pesticide formulation (i.e. Roundup). In many cases, within minutes of exposure to environmentally realistic (part per billion) concentrations, impairments of greater than 50% in OSN responses were noted. Pesticides can selectively affect different OSN classes, since one pesticide (linuron) negatively affected amino acid but not bile salt responses. Lengthy (four-day) exposure to a mixture of pesticides modelled after that found in the Nicomekl River (British Columbia) also reduced OSN responses, confirming that pesticide mixtures found in salmon-producing waterways may be injurious. In fact, the ability to discern a change in odorant intensity was reduced by 25%. This may have partially been due to inadequate neuroprotection, as glutathione-S-transferase, an enzyme that protects against cellular damage, was not upregulated to any greater of an extent than it was in fish given a 10-fold lower exposure concentration. To relate changes in OSN responses to behavioural responses, experiments were conducted on two olfactory-mediated behaviours: an amino acid-evoked searching behaviour, and an anti-predator alarm response. In the first case, exposure to three different pesticides disproportionately impacted searching behaviour with respect to neurological impairment, as the searching response was completely eliminated at greater than 60% impairment of neurological response. In the second case, the freezing behaviour salmon perform after detecting harm in a conspecific was progressively reduced with exposure to increasing concentrations of a carbamate. The work here endeavoured to assess how pesticides affect salmon neurophysiology, then relate the effects to altered behaviours, and finally place the findings within a context of environmental realism, regulations, and the status of salmon within Canada and the world.