J. Duston

Photoperiodic mechanisms and rhythms of reproduction in the female rainbow trout

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.

The entrainment and gating of the endogenous circannual rhythm of reproduction in the female rainbow trout (Salmo gairdneri)

SummaryThe endogenous circannual rhythm of maturation in the female rainbow trout and associated changes in serum oestradiol-17β and vitellogenin (as calcium) can be entrained by abrupt changes in the photoperiod. Groups of virgin fish (which naturally spawn in December) maintained under LD 18∶6 from mid-January (Year 1), and then subjected to a reduction to LD 6∶18 either on 1st March (Group A), 1st April (Group B), 1st May (Group C) or 1st June (Group D), commenced spawning on 31st July, 13th and 30th August, and the 16th September, respectively. Fish maintained on LD 18∶6 throughout the experiment (Group E) commenced spawning on 10th October. These advances in the timing of spawning can be described in the form of a phase-response curve analogous to the entrainment behaviour of circadian oscillators. It is concluded that under natural conditions the annual change in photoperiod serves to continuously entrain the circannual clock thus ensuring that maturation and spawning occur at the optimal season for the survival of the species.In Groups A, B, C and D, respectively, 74%, 48%, 23% and 8% of the fish failed to mature in Year 1. Maintaining a sample of both maturing and non-maturing trout under constant LD 6∶18 for a further year resulted in all the fish spawning in August of Year 2, approximately one year after the spawning members of this group matured in Year 1. It is proposed that the reduction from LD 18∶6 to LD 6∶18 phase-advanced a circannual timing mechanism in all the fish which then ‘free-ran’; this advance was only overtly expressed by the earlier spawning of a certain percentage of the fish in Year 1, but in all the fish in Year 2. These results indicate that the internal timing mechanism can be dissociated from the neuroendocrine mechanisms controlling maturation and thus it can be considered as an autonomous circannual clock. The relationship between the timing of the reduction in photoperiod and the percentage of virgin fish attaining maturation can be explained by a gating model. It is hypothesized that the energetically demanding process of gonadal maturation is only allowed in virgin fish which have reached a certain threshold stage of development and when the clock is at a specific (‘gate open’) phase of the circannual cycle.

Constant photoperiod regimes and the entrainment of the annual cycle of reproduction in the female rainbow trout (Salmo gairdneri)

The effect of constant photoperiod regimes on the timing of maturation and the associated changes in serum oestradiol-17 beta (E2), testosterone (T), and total calcium (Ca2+, as an index of vitellogenin) were investigated in virgin female rainbow trout (Salmo gairdneri). From mid-January four groups of fish were maintained on constant long days (18L:6D) and one group on constant short days (6L:18D; group A). On May 8 the photoperiod of three of the groups on long days was abruptly reduced from 18L:6D to 6L:18D (group B), to 10L:14D (group C), or to 14L:10D (group D); Group E remained on 18L:6D throughout the experiment. The reduction in photoperiod per se appeared more important than the magnitude of the reduction in the advancement of the time of spawning with the first fish in groups B, C, and D spawning on September 3, 13, and 24, respectively; spawning of the remaining fish in each group was spread over 6-week periods. In contrast, spawning in group E began on October 24 and was spread over a period of 12 weeks. Spawning in group A was delayed until the following March. Although a long photoperiod earlier in the year was clearly necessary for the advancement of spawning, as evidenced by the delay in maturation under constant short days, no clear increases in E2, T, and Ca2+ were detected until after the reduction in photoperiod. Thereafter, the sequence of changes in serum E2, T, and Ca2+ levels was similar in all groups although the timings of these alterations were modified in relation to the time of spawning. Thus, peak levels of the three serum parameters were attained earliest in the year by fish in group B followed by those in groups C, D, E, and A, respectively. Collectively, these data support the hypothesis that photoperiod entrains an endogenous rhythm of maturation rather than having a direct, driving influence on the control of reproduction.

Developmental differences in the responsiveness of gill Na+, K+-ATPase to cortisol in salmonids

The ability of cortisol to increase gill Na+,K(+)-ATPase activity was examined in several salmonid species during development. Coho salmon (Oncorhynchus kisutch) parr were unresponsive to cortisol in vitro (10 micrograms/ml for 2 days) in November. Responsiveness was significant from January to March, peaking in January just prior to seasonal increases in gill Na+,K(+)-ATPase activity. Gill tissue became unresponsive to in vitro cortisol in April when in vivo gill Na+,K(+)-ATPase activity peaked. The ability of cortisol to stimulate gill, Na+,K(+)-ATPase activity in postemergent fry (2-3 months after hatching) was examined in chum (O. keta), chinook (O. tschawytscha), coho, and Atlantic salmon (Salmo salar). Initial levels of gill Na+,K(+)-ATPase activity were elevated in chum salmon, which normally migrate as fry. Cortisol (10 micrograms/ml for 4 days in vitro) increased gill Na+,K(+)-ATPase activity in chum salmon fry (48% above initial levels), had a limited but significant effect in chinook salmon fry, and had no effect in coho and Atlantic salmon fry. In an in vivo experiment, Atlantic salmon previously exposed to simulated natural photoperiod (SNP) and continuous light (L24) received four cortisol injections of 2 micrograms.g-1 every third day. SNP fish responded with increased gill Na+,K(+)-ATPase activity (+66%), whereas L24 fish were not affected. Atlantic salmon presmolts with initially low levels of gill Na+,K(+)-ATPase activity responded to cortisol in vitro, whereas smolts with initially high levels of gill Na+,K(+)-ATPase activity were unresponsive.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of salinity on survival and growth of Atlantic salmon (Salmo salar) parr and smolts

Abstract The two main findings were that stunting in parr transferred to seawater can be temporary, and the growth rate of smolts is largely independent of salinity. In late-May, large lower-modal group parr ( x wt.=26 g ) including some with partial smolt characteristics, and upper-modal group smolts ( x wt. = 36 g ) were randomly allocated among four tanks at salinities of 10, 20, 31 ppt (seawater), or fresh water (control). Smolts exhibited 0% mortality. Mortality of parr in seawater was size dependent and reached 47% in late-June, with few mortalities thereafter. Mortality was 20 ppt showed elevated gill Na + K + - ATPase activity, normal plasma osmolality and muscle water content, but retarded (“stunted”) growth. The degree of stunting was dependent on salinity and body size; the smallest parr in seawater remained 11–12 cm fork length from May to September, intermediate sized parr were stunted for 1–2 months, whereas the largest members of the lower-modal group exhibited growth rates similar to smolts. Growth of smolts in 20 ppt and seawater was temporarily inhibited compared to smolts in ≤ 10 ppt, but by September there was no significant difference in body weight between the smolts in the four salinities.

Circannual Rhythms of Gonadal Maturation in Female Rainbow Trout (Oncorhynchus mykiss)

Individually identified, 2-year-old female rainbow trout were maintained for up to 51 months on a constant schedule of 6 hr light and 18 hr darkness (LD 6:18), constant temperature (8.5-9.0°C), and constant feeding rate. The fish exhibited free-running circannual rhythms of gonadal maturation and ovulation, which were self-sustaining for up to three cycles. The periodicity of the rhythm showed variation between fish and in successive cycles for the same fish, ranging from approximately 11 to 15 months.

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.

Importance of body size for acclimation of underyearling Atlantic salmon parr (Salmo salar L.) to seawater

Abstract Starting on 4 October, four size groups (fork length Salmo salar L.) were subjected to a stepwise increase in salinity (5–10 ppt, every 4 days) up to full seawater (32 ppt). Fish 9.5 cm survival increased from 5% in freshwater to 75–90% after acclimation to full seawater. All groups exhibited significant increases in gill Na + K + -ATPase activity in association with the increase in salinity above 15 ppt, with mean levels 3- to 8-fold higher than initial freshwater values after 16 days in full seawater. Mean plasma osmolality in fish >9.5 cm was relatively stable (300–320 mOsmol kg −1 ), but in fish −1 by the end of the experiment. Muscle water content was maintained at 74–76% in fish >9.5 cm, but in fish

Long-to-short photoperiod in winter halves the incidence of sexual maturity among Arctic charr

To better understand how photoperiod and body size interact to affect age at maturity, PIT-tagged 2-year-old Salvelinus alpinus (ca. 300 g, n=330) were reared in fresh water from February 3 under four treatments, either 18-h light/6-h dark (LD18:6) for 42 days then LD8:16 (long–short); LD18:6 for 42 days then simulated natural (long–ambient); constant LD18:6 (constant–long); or simulated seasonal photoperiod cycle (control). Temperature was 10 °C. In November, the incidence of maturity in the long–short treatment was reduced to 44% among males and 20% among females, compared to 83% and 50% in the control (p 1 kg and immature) by over four-fold. The long–ambient treatment also reduced maturation (p<0.05), but to a lesser degree, 66% and 32% in the respective sexes. The constant–long treatment had no effect on males and increased maturation among females. The arrest of sexual maturation was associated with a marked alteration in seasonal pattern of growth suggesting it was due to a suppression of somatic growth. By contrast, initial body size and condition factor were poor indicators of incipient maturing fish.

Emamectin benzoate: an effective in-feed treatment against the gill parasite Salmincola edwardsii on brook trout

Abstract Emamectin benzoate 0.2% premix (SLICE™) coated onto pellets and fed to brook trout (Salvelinus fontinalis; 700–900 g) resulted in a 40–60% reduction in the mean number of adult female Salmincola edwardsii, whereas controls exhibited a 20% increase in parasite numbers. In each of two lab-based experiments, ca. 40 trout were tagged and the number of adult female parasites on the gills were counted, then the fish were randomly allocated to one of four tanks (300 l, flow through freshwater, 10–12 °C). The fish in the two tanks were offered medicated feed (nominal dose 50 μg kg−1 bw day−1) for a 7-day period, the other two tanks were fed a control diet. Finally at either 7 (Experiment 1) or 17–31 days (Experiment 2) after the medicated feed treatment, all fish were euthanised and the number of adult female parasites recounted. In Experiment 1, the mean number of parasites fish−1 in the treated group decreased from 118 to 49 (p