No abstract

Oral administration and intraperitoneal implantation can be used to administer cortisol to the brown trout. Both techniques produce a dose-dependent increase in plasma cortisol concentration and the experimental procedures reduce the stress response in control fish to a minimum level. The plasma levels of the steroid are within the physiological range for the fish and the time of the response to acute oral administration is similar to that for such stresses as crowding, pollution and confinement. The evidence strongly indicates that elevated cortisol levels can significantly increase the susceptibility of salmonid fish to a wide range of infectious diseases including furunculosis and fungal infection. The present study has shown that this can occur at cortisol levels within the normal physiological range of the fish.

Potential yearling (1+) smolts were maintained under 8.25 h light: 15.75 h dark and constant temperature (10.0 °C) from late December. Groups were subjected to an abrupt increase to 16 h light: 8 h dark on December 31 (group A), February 1 (group B), March 1 (group C), or March 31 (group D). Group E was maintained under constant 8.25 h light: 15.75 h dark for the duration of the experiment and group LDN was maintained under a simulated natural photoperiod cycle (45°N). Plasma osmolality levels following 24-h, 29‰ salinity challenge tests indicated a photoperiod-independent development of hypoosmoregulatory mechanisms preceding completion of smoltification that was significantly correlated with fish body size. As judged by 96-h, 37.5‰ salinity tolerance tests and changes in condition factor, completion of smolting occurred in sequence; in groups A and B it was advanced to late February, while groups C and D completed smolting in mid-March and mid-April, respectively, compared with late May for group LDN. Group E, maintained under constant 8.25 h light: 15.75 h dark, developed salinity tolerance in late May, but unlike other groups exhibited no large reduction in condition factor. Following smolting, an increase in condition factor and a loss of salinity tolerance occurred in all groups. The results support the hypothesis that changes in photoperiod entrain an endogenous circannual rhythm involved in controlling the completion of smoltification and subsequent loss of some smolt characteristics.

The present work investigates the importance of circadian and circannual rhythms in the photoperiodic control of reproduction in the rainbow trout. Maintenance of groups of 20-30 female trout under continuous light (LL), constant long (18L:6D) or short (6L:18D) days and conditions of constant temperature (8.5-9.0°C) and feeding rates (0.5% body weight, day(-1)), starting in February, produced markedly different spawning periodicities during the first and subsequent years of treatment. At the end of the first year, spawning was advanced by up to 2 months in the fish under LL and 18L:6D and delayed by up to 5 months in those under 6L:18D when compared with the December spawning of control fish under ambient light. Continued exposure of the fish to the same photoperiod regimes produced cycles of spawning and peak levels of testosterone, oestradiol-17β and calcium (as an index of vitellogenin) at intervals of 150-170 days under LL and 18L:6D and 320-420 days under 6L:18D. The functional importance of these spawning cycles and their relationship to circannual rhythms and the control of reproduction is discussed.Exposure of fish to skeleton (6L:4D:2L:12D, 6L:6D:2L:10D and 6L:8D:2L:8D) and resonance (6L:42D, 6L:48D and 6L:54D) procedures produced ranges of spawning times up to two months in advance of control fish. Results with the resonance regimes, where fish received only a half the light-dark cycles and a quarter of the total daylight hours of those on ambient light cycles show that trout do not measure daylength and time by counting daily cycles or by hour-glass mechanisms. Collectively, these data support the proposition that rhythmic processes of photosensitivity are involved in the modulation of reproductive development in the rainbow trout and that circadian and circannual rhythms cooperate in the timing and entrainment of this cycle.

By identifying individual Atlantic salmon (Salmo salar) smolts (age 1+, mean weight 95 g), a tank study with four treatments (n = 175 fish per tank) indicated that maturation as grilse is associated with changes in somatic growth over many months, from July of the first summer in seawater onwards. The grilse rate in both sexes was associated with a relatively high condition factor from July onwards, but was independent of fork length. Food deprivation in either early winter , late winter (February-April, 0.7-4.0°C), or both early and late winter (double deprived) reduced the female grilse rate (p < 0.01) to 4, 7, and 2% versus 18% in the control, whereas male grilse rate was reduced by a lesser degree (p > 0.05), 43, 39, and 33% versus 44% in the control. Mean condition factor among both maturing and immature fish decreased by a similar degree due to food deprivation but from spring onwards increased by a faster rate among incipient grilse. In a large experiment in four sea cages (each n = 3885 fish) using similar food deprivation treatments, the reduction in grilse rate was statistically significant in both sexes: among males, 28-36 versus 43% in the control; among females, 3-4 versus 6% in the control.Résumé : Dans une étude en bassins avec quatre traitements (n = 175 poissons par bassin) dans laquelle des smolts de saumon atlantique (Salmo salar) (âge 1+, poids moyen de 95 g) étaient identifiés individuellement, on a observé que la maturation précoce (madeleineaux) était associée avec des changements dans la croissance somatique sur de nombreux mois, à partir du mois de juillet du premier été en eau de mer. Le taux d'apparition des madeleineaux chez les deux sexes était associé à un coefficient de condition relativement élevé à partir de juillet, mais était indépendant de la longueur à la fourche. La privation de nourriture au début de l'hiver (novembre à décembre, 8,2 à 4,1°C), à la fin de l'hiver (février à avril, 0,7 à 4,0°C) ou durant ces deux périodes (double privation) a réduit le taux d'apparition des madeleineaux femelles (p < 0,01) à 4, 7, et 2% contre 18% chez les témoins considérant que le taux d'apparition des madeleineaux mâles a été réduit par un peu de degré (p > 0,05), 43, 39 et 33% contre 44% chez les témoins. Le coefficient de condition moyen parmi tous les deux mfrissant et non mfrs les poissons a diminué par un degré semblable df à la privation de nourriture mais à partir du printemps a agumenté à une vitesse plus rapide parmi les madeleineaux naissants. Dans une expérience de grande envergure effectuée dans quatre cages marines (n = 3 885 poissons par cage) utilisant les mêmes traitements de privation de nourriture, la réduction du taux d'apparition des madeleineaux a été statistiquement significative chez les deux sexes : dans le cas des mâles, il a été de 28 à 36 contre 43% chez les témoins, et dans le cas des femelles, de 3 à 4 contre 6% chez les témoins.[Traduit par la Rédaction] Duston and Saunders 207