Halcyon days of summer may seem idyllic when warm summer nights are spent lobbing rocks at kayakers who carry banners as they chant and pass by the cage system. However, to the broodstock in the cages, warm summer nights can mean changes in their love life.

The effects of elevated temperatures and the effect on spawning performance of Atlantic salmon are well documented. In 1985 Manning and Kime demonstrated that elevated water temperatures inhibited the conversion of testosterone to estrogen – the hormone that governs vitellogenesis (making eggs). This enzyme, P450 aromatase, is temperature sensitive and is thought to be the safeguard mechanism to prevent vitellogenesis from occurring during unfavourable conditions (Watts et al., 2004). Watts and coworkers (2004) note that temperature-related damage for Atlantics occurs rapidly with even a brief spike in temperatures, usually above 14°C. The mechanism of this damage is a change in estrogen-mediated membrane permeability of the developing ova. In other words, vitellogenin (yolk) can’t get into the egg. So, vitellogenesis is delayed during periods of elevated temperatures. Therefore, with inhibition of P450 aromatase, changes in gonado-somatic index (GSI) and egg sizes occur. To the salmon farmer, the fecundity of the fish is decreased and appearance of the gonad is to view eggs of varying sizes.

King and coworkers (2003) observed that chronically elevated temperatures reduced estrogen and vitellogenin levels with a corresponding decrease in egg quality. They found that elevated temperatures compromised the chorionic membranes of the egg. In addition, they noted a depression of GSI of the value of 2.5% in the autumn when temperatures were elevated during the summer. This change in GSI was noted as a smaller mean diameter of eggs. Therefore, with elevated SW temperatures, egg development is inhibited or arrested. As detailed above, the result is decreased and delayed progression of GSI, smaller egg size, decreased fecundity and varied egg size.

A spin-off of higher summer temperatures is delayed spawning date and reduce fertility rate. Taranger and Hansen (1993) demonstrated that fish held at higher temperatures than controls had delayed spawning dates compared to fish held at cooler temperatures. Egg survival was likewise lower in fish held in warm water compared to control or cool water fish. Similarly, Taranger and coworkers (2000, 2003) observed that fish held at elevated SW and FW temperatures had inhibited, delayed or incomplete ovulation, and lower fertility rates when subjected to temperatures of 14°C during vitellogenesis and prior to spawning. The observed delay or inhibition of ovulation followed by the increase in duration of spawning season can really hurt the bottom line of hatcheries.

All of this information on what happens is fine, but one wonders what a farmer can do about it. After all, short of trading in our SUV’s, it’s hard to change the weather.

OK, so temperatures above 14°C are bad for broodstock. However, it’s hard to measure temperature. In a research tank, the water temp is the same throughout; in a net pen it varies. It may be that the fish are living at the bottom of the pen, or they migrate at night or it may be the internal temperature of the fish that counts. It’s virtually impossible to measure temperatures in cages without data loggers.

Lest all seem like hot water, there is some chilling news. It’s clear that a water temperature decrease is a cue for spawning to begin. Taranger and Hansen (1993) demonstrated that when high temperature messed up the spawning season, decreasing temperatures served to promote spawning and shorten the season. This effect was most noted in temperatures decreasing from 14 to 8°C. As well, King and Pankhurst (2000) observed that ovulation and effectiveness of GnRH injections to induce ovulation were restored in high temperature-stressed fish 2 weeks after exposure to cooler water. Later, Taranger and coworkers (2003) showed that transferring fish from 14-16°C to waters of 4-6°C advanced spawning dates and shortened the duration of the spawning season. Finally, Watts and coworkers (2004) demonstrated that 1 month of decreased temperature prior to the normal spawning period was sufficient ensure normal spawning.

Take home message: there is a temperature trigger to spawning time. This trigger serves in part to restore the maturation process and improve egg quality.

In a practical sense, fish held in recirc systems still need the temperature decrease. If brood are held at constant temperatures and even if they are photomanipulated (say that after three beers), they still need the temperature drop of about 4°C to trigger large-scale maturation of the population. If fish are spawned out of SW, the fish are at nature’s mercy – until the rains come and the temperature drops, spawning will be in dribs and drabs.

Induced maturation can restore temperature inhibited spawning (King and Pankhurst, 2000; Taranger et al., 2003). However, as always with induced maturation, the spawning success is lower in these fish. The best results are seen when induction is done after the temp trigger.

In summary, be wary of SW temperatures above 14°C for Atlantic salmon; it shuts down egg growth. Allowing fish access to cooler water will restore the maturation process and later, trigger spawning (if done during the normal cycle). Constant or high temperatures tend to delay and prolong the spawning season, but a temperature drop of 4°C should set the fish straight.

Literature Cited

King, H.R. and Pankhurst, N.W. 2000. Ovulation and GnRH responsiveness in Atlantic salmon (Salmo salar) maintained at elevated temperatures is restores by short-term temperature reduction. Poster and abstract, 4^th International Symposium on Fish Endocrinology, Seattle WA USA Jul 31-Aug 3, 2000.

King, H.R., Pankhurst, J.M., Watts, M. and Pankhurst, N.W. 2003. Effects of elevated summer temperatures on gonadal steroid production, vitellogenesis and egg quality in female Atlantic salmon. J. Fish Biol.63: 154-167.

Manning N.J. and Kime, D.E. 1985. The effect of temperature on testicular steroid production in the rainbow trout, Salmo gairneri, in vivo and in vitro. Gen. Comp. Edocrinol. 57: 377-382.

Taranger, G.L. and Hansen,T. 1993. Ovulation and egg survival following exposure of salmon Atlantic salmon, Salmo salar L., broodstock to different temperatures. Aquacult. Fish. Manag. 24: 151-156.

Taranger, G.L., Anderson, E., Stephanson, S.O., Hansen, T. and Norberg, B. 2000.Endocrine changes in photoperiod and temperature manipulated female salmon (Salmo salar L.) broodstock . Poster and abstract, 4^th International Symposium on Fish Endocrinology, Seattle WA USA Jul 31-Aug 3, 2000.

Taranger, G.L., Vikingstad, E., Klenke, U., Mayer, I, Norberg, B. Hansen, T., Zohar, Y. and Anderson, E. 2003. Effects of photoperiod, temperature and GnRHa treatment on the reproductive physiology of Atlantic salmon (Salmo salar L.) broodstock. Fish Physiol. Biochem. 28: 403-406.

Watts, M., Pankhurst, N.W. and King, H.R. 2004. Maintenance of Atlantic salmon (Salmo salar) at elevated temperature inhibits cytochrome P450 aromatase activity in isolated ovarian follicles. Gen. Comp. Edocrinol. 135: 381-390.