Pain in laboratory fishes: What we know and what we need to know.

The scientific exploration of the capacity of fishes to experience pain has grown since the first identification of nociceptors in rainbow trout in 2002. In the early years up to 2010 anatomical and electrophysiological evidence was gathered that demonstrated that the characteristics of peripheral and central nervous system responses were comparable to mammals. Combined with behavioural investigations where fishes exhibited anomalous responses to painful treatment that were ameliorated by pain-relieving drugs subsequent studies were fuelled by this convincing evidence. More contemporary studies have explored several species and investigated a wider variety of pain types, with zebrafish proposed as a model for the study of nociception and pain. Both operational and laboratory welfare indicators are detailed for several species which can be evaluated by the tank side or by additional behavioural scoring software and laboratory equipment. Even with this progress over the last 24 years many aspects still need to be uncovered. For efficient pain management for fishes used in science, a wider range of species need to be assessed during invasive procedures; the mechanism of action and pharmacokinetics of pain-relieving drugs must be elucidated so scientists can make informed choices, as well as identifying any possible side-effects; combinatorial strategies where more than one drug is administered at lower doses to reduce side-effects should be investigated; and pain assessment tools using intensity scales or monitoring using artificial intelligence evaluation software provides promise as an aid to identifying pain and allowing researchers to intervene to improve fish welfare.