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 (E2), 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 E2 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%).

A syndrome characterized by atypical swimming behaviour followed by rapid death was first reported in captive southern bluefin tuna Thunnus maccoyii (Castelnau) in the winter of 1993. The cause of this behaviour was found to be a parasitic encephalitis due to the scuticociliate Uronema nigricans (Mueller) Based on parasitological and histological findings, it is proposed that the parasites initially colonise the olfactory rosettes and then ascend the olfactory nerves to eventually invade the brain. Possible epidemiological factors ~nvolved in the pathogenesis of the disease include water temperature (<18"C) and the immune status of the fish.

In fish all the pre-requisites to mount a specific immune response are present, but the main differences from the mammalian system are that the secondary response is relatively minor and IgG is not present. In teleosts mainly IgM is present, and IgD has been recently described but its function is, as yet, unknown. However, different forms of fish IgM and its observed flexibility of structure may compensate for a lack of Ig class diversity. The innate immune response of teleosts is highly developed. Multiple forms of key constitutive and inducible components, such as lysozyme, C3, alpha2-macroglobulin and C-reactive protein, are present, and may enhance immune recognition. Low ambient temperature appears to have an impact on all aspects of the immune response, particularly the T-dependent specific immune response due to the non-adaptive lipid composition of T-cell membranes. Temperature effects on the nonspecific immune system are less well characterised, but there is evidence that low temperatures are also suppressive. Knowledge of immune system function becomes essential for disease prevention strategies such as the development of vaccines, selection for increased disease resistance and identification of genes suitable for trangenesis.

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 17b-oestradiol (E 2 ), 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 E 2 (c. 0Á5 ng ml ÿ1 ) and Vtg (c. 1Á4 mg ml ÿ1 ) were approximately half those observed in fish maintained at 14°C (c. 1Á0 ng ml ÿ1 and 2Á7 mg ml ÿ1 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.

Atlantic salmon (Salmo salar) broodstock were transferred from natural (12-16 o C) to controlled temperatures of 14, 18 or 22 o C for 3 months during vitellogenesis. Fertility and survival were significantly reduced in eggs from broodstock held at 22 o C relative to 14 or 18 o C. Endocrine mechanisms were disrupted after only one month at 22 o C, as evidenced by decreased plasma vitellogenin (Vtg) and increased plasma testosterone (T) levels and, at later stages, decreased levels of plasma 17β-estradiol (E 2 ) . In vitro incubations of isolated ovarian follicles were carried out at monthly intervals, with follicles exposed to human chorionic gonadotropin, N, 2-0-dibutyryladenosine 3,5-cyclic monophosphate, and the gonadal steroid precursors 17-hydroxyprogesterone, androstenedione and T. After one month of exposure to controlled temperature, T synthesis was generally enhanced in response to all treatments at all temperatures, but E 2 synthesis was inhibited at 22 o C, suggesting temperature impairment of cytochrome P450 aromatase (P450 arom ) synthesis or activity. The effect became less marked as follicles matured suggesting that temperature sensitivity is stage dependent. The results of this study suggest that the inhibitory effects of elevated temperature on E 2 and Vtg synthesis, and subsequent egg development found in the present and earlier studies, arise at least partly, from temperature modulation of P450 arom .