Temperature and salmonid reproduction: implications for aquaculture.

Fish reproduction is likely to be affected by increasing water temperatures arising from climate change. Normal changes in environmental temperature have the capacity to affect endocrine function and either advance or retard gametogenesis and maturation, but above-normal temperatures have deleterious effects on reproductive processes. In Atlantic salmon Salmo salar, exposure to elevated temperature during gametogenesis impairs both gonadal steroid synthesis and hepatic vitellogenin production, alters hepatic oestrogen receptor dynamics and ultimately results in reduced maternal investment and gamete viability. Exposure to high temperature during the maturational phase impairs gonadal steroidogenesis, delaying or inhibiting the preovulatory shift from androgen to maturation-inducing steroid production. There are also deleterious effects on reproductive development of female broodstock of rainbow trout Oncorhynchus mykiss and Arctic charr Salvelinus alpinus when they are exposed to elevated temperature. Less is known about temperature effects on male fishes but inhibition of spermiation has been observed in S. salar and O. mykiss. Among wild stocks, the response to elevated temperature will involve behavioural thermoregulation with consequent change in geographical ranges and the possibility of local extinctions in some regions. For domesticated stocks, containment in the culture environment precludes behavioural thermoregulation and aquaculturists will be required to develop adaptive strategies in order to maintain productivity. The most direct strategy is to manage the thermal environment using one or more of a range of developing aquaculture technologies. Alternatively, there is potential to mitigate the effects of elevated temperature on reproductive processes through endocrine therapies designed to augment or restore natural endocrine function. Studies largely on S. salar have demonstrated the capacity for synthetic luteinizing hormone-releasing hormone to offset the inhibitory effects of elevated temperature on maturational events in both sexes, but the potential for hormone therapy to provide protection during gametogenesis is still largely unexplored.