As poikilothermic vertebrates, fish can experience changes in water temperature, and hence
body temperature, as a result of seasonal changes, migration, or efflux of large quantities of effluent
into a body of water. Temperature shifts outside of the optimal temperature range for an individual
fish species can have negative impacts on the physiology of the animal, including the immune system.
As a result, acute or chronic exposure to suboptimal temperatures can impair an organisms’ ability to
defend against pathogens and thus compromise the overall health of the animal. This review focuses
on the advances made towards understanding the impacts of suboptimal temperature on the soluble
and cellular mediators of the innate and adaptive immune systems of fishes. Although cold stress can
result in varying effects in different fish species, acute and chronic suboptimal temperature exposure
generally yield suppressive effects, particularly on adaptive immunity. Knowledge of the effects
of environmental temperature on fish species is critical for both the optimal management of wild
species and the best management practices for aquaculture species.