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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. [ABSTRACT FROM AUTHOR]

In order to compare the effects on reproductive performance of short-term or prolonged exposure to elevated temperatures during vitellogenesis, female Atlantic salmon Salmo salar were held at a water temperature of 22° C for periods of 4 or 6 weeks during the austral summer and autumn. Plasma levels of 17β-oestradiol (E2), testosterone (T) and vitellogenin (Vtg) were monitored and reproductive success was compared to that in groups of fish maintained at 14 or 22° C for 12 weeks from mid-January. Significant endocrine effects were observed within as few as 3 days of the commencement of exposure to 22° C, when plasma levels of E2 ( c. 0·5 ng ml−1) and Vtg ( c. 1·4 mg ml−1) were approximately half those observed in fish maintained at 14° C ( c. 1·0 ng ml−1 and 2·7 mg ml−1 respectively). The fertility and survival to the eyed stage of ova from fish held at 14° C exceeded 85 and 70% respectively, whereas ova from fish held at 22° C for 6 or 12 weeks exhibited significantly reduced fertility (<70 and <45% respectively) and survival ( c. 40 and 13% respectively). In spite of significant endocrine effects at all stages, a 4 week exposure to 22° C only generated significant reductions in egg fertility (<65%) and survival ( c. 30%) when it occurred between mid-February and mid-March. Together, these data confirm that high temperature spikes can affect reproductive success as strongly as more prolonged exposures, and indicate that there is a critical period of reproductive sensitivity to elevated temperature in late February and early March in this stock of Atlantic salmon. [ABSTRACT FROM AUTHOR]

The pattern of oocyte development in association with changes of plasma concentrations of vitellogenin (Vtg), 17β-oestradiol (E2) and testosterone (T) was investigated in maturing female greenback flounder Rhombosolea tapirina over the first part of a reproductive season (February to June). Examination of oocyte size-frequency distributions showed that the oocyte developmental pattern in R. tapirina is multiple group synchrony, and that reproductively mature fish were present at all sampling times. There were no significant temporal variations in the gonadosomatic index (IG), hepato-somatic index (IH), or plasma concentrations of Vtg, E2 and T during the sampling period, which indicates that reproductive development is not synchronized within the population. Significant increases in IG, IH and plasma concentrations of Vtg, E2 and T, however, were observed in vitellogenic fish, and in fish undergoing final maturation. A positive relationship was also found between the growth of oocytes and plasma concentrations of Vtg, E2 and T, although the patterns of increase were different for each variable. Plasma concentrations of Vtg and E2 rose steadily across oocyte sizes from 100 to 450 mm, but the rate of increase of plasma E2 was slower than that of Vtg, and both reached a saturated concentration at oocyte sizes of c. 450 mm. In contrast, plasma concentrations of T showed no marked increase until oocytes grewbeyond 400 µm. [ABSTRACT FROM AUTHOR]

Groups of Tasmanian female Atlantic salmon Salmo salar L. were maintained at 14, 18 and 22°C for 3 months from mid-summer (January). Blood plasma levels of 17β-oestradiol (E[sub 2]), testosterone (T), cortisol and vitellogenin (Vtg) were measured at regular intervals, and in autumn (April) temperatures were reduced to 8°C to facilitate spawning and egg incubation. Maintenance at 22° C during vitellogenesis was associated with a general reduction in plasma E[sub 2] levels and an early reduction in plasma Vtg levels relative to those observed in fish held at 14 and 18° C. Significantly reduced oocyte diameters in ova from fish held at 22° C (5·4 mm cf. 5·7 mm) confirmed reduced maternal investment, and an increase in the incidence of previously undescribed chorion damage suggested that zonagenesis may also have been impaired. As a result, the fertility and survival of ova from fish exposed to 22° C (69 and 42%, respectively) were significantly reduced relative to those of ova from fish maintained at 14° C (93 and 86%) and 18° C (86 and 84%). [ABSTRACT FROM AUTHOR]

Nesting male demoiselles Chromis dispilus were captured and blood sampled under water by divers, and plasma levels of reproductive hormones correlated with behavioural status at the time of capture. Plasma gonadotropin-II (dGtH-II) was measured using a heterologous assay based on carp GtH-II standards and an antibody to the β-subunit of cGtH-II. Plasma samples diluted parallel to standards in the assay, and there was a significant transient increase in dGtH-II in response to treatment with gonadotropin releasing hormone analogue (GnRHa). Male demoiselles had elevated plasma levels of dGtH-II during periods of reproductive display, but low levels of dGtH-II during periods of brooding, irrespective of egg stage. Plasma testosterone levels followed a similar pattern whereas plasma 11-ketotestosterone and 17,20β-dihydroxy-4-pregnen-3-one levels tended to remain elevated during the early phases of egg brooding. The results are consistent with earlier studies showing that reproductive hormone levels in demoiselles are behaviourally mediated, and support the view that behavioural alterations of reproductive endocrine status in fishes are mediated through the action of GnRH. [Copyright &y& Elsevier]