Endocrine (plasma cortisol and glucose) and behavioral (locomotor and self-feeding activity) circadian rhythms in Senegalese sole (Solea senegalensis Kaup 1858) exposed to light/dark cycles or constant light

The existence of daily rhythms under light/dark (LD) cycles in plasma cortisol, blood glucose and locomotor and self-feeding activities, as well as their persistence (circadian nature) under constant light (LL), was investigated in Senegalese sole (Solea senegalensis). For the cortisol and glucose rhythms study, 48 soles were equally distributed in 8 tanks and exposed to a 12:12 LD cycle and natural water temperature (experiment 1). After an acclimation period, blood was sampled every 3 h until a 24-h cycle was completed. Blood glucose levels were measured immediately after sampling, while plasma cortisol was measured later by ELISA. In experiment 2, the fish were exposed to LL for 11 days, and after this period, the same sampling procedure was repeated. For the study of locomotor and self-feeding rhythms (experiment 3), two groups of sole were used: one exposed to LD and the other to LL. Each group was distributed within 3 tanks equipped with infrared photocells for the record of locomotor activity, and self-feeders for feeding behavior characterization. The results revealed a marked oscillation in cortisol concentrations during the daily cycle under LD, with a peak (35.65 ± 3.14 ng/ml) in the afternoon (15:00 h) and very low levels during the night (5.30 ± 1.09 ng/ml). This cortisol rhythm persisted under LL conditions, with lower values (mean cortisol concentration = 7.12 ± 1.11 ng/ml) and with the peak shifted by 3 h. Both rhythms were confirmed by COSINOR analysis (p < 0.05). The synchronizing role of temperature and feeding schedule, in addition to light, is also discussed. Diel rhythms of glucose were not evident in LD or LL. As to locomotor and self-feeding activity, a very marked rhythm was observed under LD, with higher activity observed during the night, with acrophases located at 2:14 and 3:37 h, respectively. The statistical significance of daily rhythms was confirmed by COSINOR analysis. Under LL, both feeding and locomotor rhythms persisted, with an endogenous period (τ) around 22.5 h. In short, our findings described for the first time the existence of circadian cortisol and behavioral circadian rhythms in flat fish. Such results revealed the importance of taking into account the time of day when assessing stress responses and evaluating physiological indicators of stress in fish.     

Photic and circadian regulation of self-feeding activity in ayu

ABSTRACT:   The self-feeding rhythms of ayu Plecoglossus altivelis altivelis were examined. Individual ayu (mean body weight 40 g) were held in 60-L glass tanks equipped with self-feeders. Six of 14 fish learned self-feeding during the experiment. Under two different light–dark (LD) conditions (16 h:8 h and 8 h:16 h LD), self-feeding was synchronized to the LD cycle, and feeding occurred almost exclusively during the light phase. During exposure to constant light (LL), circadian feeding rhythms were observed. These results indicate self-feeding rhythms in ayu are restricted to the light phase under LD conditions and are controlled by the circadian clock under LL conditions.     

Food-entrainable circadian oscillator in goldfish: multiple daily feeding times and food-anticipatory activity

The existence of a food-entrainable circadian oscillator (FEO) was examined in goldfish Carassius auratus. Single goldfish were exposed to light–dark (LD) 12:12 (lights on 06:00–18:00 h) and fed at 12:00 h (phase I). The photoperiod regime was shifted to constant light (LL) (phase II). The fish were then fed at 06:00 or at 18:00 h (phase III), and all fish were fasted (phase IV). In both groups, three out of eight fish exhibited food-anticipatory activity (FAA) in phases II and III, the free-running period (τ) of locomotor activity was approximately 24.0 h, and circadian rhythms of locomotor activity started to free-run from the previous FAA in phase IV, indicating the existence of an FEO. Next, single goldfish were exposed to LD 12:12 and were fed twice daily at 12:00 and 00:00 h (phase I). The photoperiod regime was shifted to LL (phase II), and the fish were fasted (phase III). In phase II, FAA was observed in three out of six fish for both feeding times. In phase III, a circadian rhythm of locomotor activity was observed in five fish, and circadian rhythms started to free-run from the previous feeding time (either 12:00 or 00:00 h) in three fish. These results suggest that individual goldfish can remember two daily feeding times.     

Diel Rhythms of Serum Metabolites and Thyroid Hormones in Red Porgy Held in Different Photoperiod Regimes

Diel rhythms in serum glucose, lactate, cholesterol, triglycerides and thyroid hormones were studied in red porgy, Pagrus pagrus, held under different photoperiod regimes (constant darkness – DD, 8L:16D,12L:12D), at a constant temperature (17.1–18.7 °C) and fed with commercial pellets, by means of a self-feeder. A clear diurnal rhythm in feeding activity, regardless of the photoperiod regime was demonstrated. All serum components showed significant diel rhythms, although they were not necessarily consistent or circadian in periodicity. As well as this, differences in the average values among the varying treatments were observed. Fish held under the 12L:12D protocol displayed significantly higher average T₄, T₃ and lactate levels during the day rather than at night. Maximum glucose values occurred 8–12 h after dawn and maximum lactate concentrations at 06:00 or 14:00 h. Diel variations in cholesterol were evident only in the DD group with peak values inversely correlated with the rhythm of food intake. Triglycerides displayed a similar pattern of changes. Significant diel fluctuations in T₄ serum levels were observed only in fish exposed to the 12L:12D protocol, with peak values at dawn. A clear diurnal peak (10:00 h) in T₃ concentrations was observed in fish subjected to the 12L:12D regime, while fish exposed to the 8L:16D protocol showed two peaks: one in the photophase (10:00 h) and another in the scotophase (02:00 h). The light–dark alternation and the general activity rhythm of fish seem to be the main synchronizers of the diel rhythms observed in this study.     

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-17 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.     

Circadian rhythms and individual variability of self-feeding activity in groups of rainbow trout Oncorhynchus mykiss (Walbaum)

An experiment was conducted to examine whether circadian self‐feeding rhythms exist in groups of rainbow trout Oncorhynchus mykiss (Walbaum) and to investigate individual variability of self‐ feeding and its influence on growth. Ten trout of 144.9 ± 12.0 g (mean ± SD) were transferred to each of eight 290‐L tanks and given free access to feed via a self‐feeder. Fish in four of the eight groups were individually tagged, and the individual activating the trigger was identified using video observations. The self‐feeding activity of each group was recorded under a light:dark cycle of 16.5:7.5 h and under continuous light (LL) conditions. Although diel self‐feeding rhythms in the groups of trout were, in general, diurnal, night‐feeding activity was observed in the initial LD phase, but there was a progressive reduction and eventual cessation in most groups. Endogenous circadian self‐feeding rhythms were detected in the groups of trout under LL conditions. The trout formed dominance hierarchies of self‐feeding activity, with one or two individuals accounting for the majority of trigger actuations. However, the dominants did not occupy the area surrounding the trigger. Growth rates did not differ significantly between the dominants and the subordinates within the groups.     

Circadian melatonin release <Emphasis Type="Italic">in vitro</Emphasis> by European sea bass pineal

Melatonin production by the fish pineal is under the control of photoperiod. In addition, in some fish species pineal melatonin production is self-sustained by an intrapineal circadian pacemaker, not in salmonids, though. The aim of this research was to test melatonin release in vitro by the pineal of sea bass held under LD 12:12 and DD conditions to determine the endogenous nature of melatonin rhythm. Circadian rhythmicity ( = 28.3 h) was found since rhythmic melatonin release persisted on DD.     

Circadian heart rate changes and light-dependence in the Mediterranean seabream Sparus aurata

Despite the abundant literature on circadian heart rate rhythms in fish, few studies have attempted to elucidate if such pattern is endogenous or if it is modulated by environmental cues. In this study, heart rates were continuously recorded in Sparus aurata acclimated to different light regimes (normal or shifted LD 12:12 cycle and a pulsed 0.75:0.75 LD cycle) to investigate the endogenous nature and the role of light on the circadian pattern of heart rate. Under LD 12:12, the daily pattern of heart rate showed higher values at night and lower rates during the day (average amplitude of the heart rate change of 10.6 ± 5.7 beats min^–1). The circadian pattern was maintained in animals under constant light but the amplitude was decreased (2.4 ± 1.1 beats min^–1). Light masks the expression of the rhythm because under a pulsed LD cycle, heart rate tracks the pulsed LD phases, increasing during the dark phase and decreasing during the light phase. Besides the ultradian pattern imposed, an underlying circadian rhythm was detected, which indicates that the rhythm is endogenous. However, the strong link between heart rate and light obtained under pulsed LD cycles suggest the timing mechanism to be mostly based on external light reception.     

Feeding activity rhythm in Japanese sea catfish <Emphasis Type="Italic">Plotosus japonicus</Emphasis>

To examine the feeding activity characteristics of Japanese sea catfish Plotosus japonicus, access to feed box (feeding activity) and the number of food pellets consumed (food intake) were recorded under laboratory conditions. When fed ad libitum under a 12:12 h light–dark (LD) cycle, all fish exhibited feeding activity and consumed food during the dark period. Feeding activity increased in the presence of food, and this increased activity level continued for several days after food removal. During restricted food availability within the light period of the LD cycle, seven of nine fish exhibited food-anticipatory activity (FAA) both before and after the 12:00 food-restricted time. This FAA persisted under constant light conditions (in nine of nine fish). The average circadian free-running period was 24 h. These results suggest that P. japonicus exhibits a nocturnal feeding activity rhythm, which may be synchronized by a food-restricted time.     

Circadian locomotor activity in Japanese sea catfish <Emphasis Type="Italic">Plotosus lineatus</Emphasis>

To find out whether or not the locomotor activity of the Japanese sea catfish, Plotosus lineatus, shows a circadian rhythmicity, the locomotor activity of this catfish was recorded in a laboratory. All of the catfish recorded (n = 20) showed a nocturnal locomotor activity rhythm under a light–dark (LD) cycle (LD 12:12). The locomotor activity started after the light was turned off, and declined during the period of darkness. In five of the six catfish we examined, locomotor activity shifted gradually over the course of 3–4 days in order to synchronize to the LD cycle after it was advanced by 6 h. Locomotor activity persisted within a constant darkness (DD) in all eight of the catfish. The average free-running period of locomotor activity under DD lasted for 24.2 ± 0.4 h (mean ± SD). Thus, the present results demonstrate that the Japanese sea catfish has an endogenous circadian oscillator entrained by an LD cycle.     

Diel pattern of haematocrit, serum metabolites, osmotic pressure, electrolytes and thyroid hormones in sea bass and sea bream

The diel pattern of haematocrit, serum metabolites, electrolytes and thyroid hormones are described in sea bass, Dicentrarchus labrax, and sea bream, Sparus auratus, held under 12L:12D light cycle (0600–1800 h light). No diel rhythmicity in haematocrit, total proteins, potassium and chloride was observed. Maximum values of glucose, osmotic pressure and sodium were recorded at dusk (1800 h) in both species, irrespective of differences in the size of fish and feeding regime used. Maximum and minimum triiodothyronine values were found during the scotophase in bass and during the photophase in bream. Maximum thyroxine levels occurred at 1400 h (bass) and 1800 h (bream) and minimum during the scotophase, in both species. The light/dark alternation seems to be an important synchronizer of the observed diel changes. Results are discussed in relation to the timing of feeding and the general activity of the fish.     

Effects of starvation,subsequent feeding and photoperiod on flesh quality in farmed cod (Gadus morhua)

The study was designed to investigate the effect of four cycles of 5 weeks starvation followed by 10‐week refeeding compared with daily feeding under either natural photoperiod or continuous light (LL) regime on body composition and flesh quality in Atlantic cod in sea cages, northern Norway. The fish were sampled for body composition and flesh quality parameters at the start of the trial, twice at the end of a 10‐week feeding period and twice at the end of a 5‐week starvation period. There was effect of both feeding and light regime on growth, the two starving groups losing weight during starvation and regaining weight during refeeding, and the group under LL being heavier. But, the mean overall growth did not vary between groups. Starvation/refeeding regime resulted in higher slaughter yield, but no overall effect was seen on hepatosomatic index, water content, water holding capacity (WHC), muscle pH, hardness or flesh colour compared with control groups. Continuous light increased gutted weight and slaughter yield, lowered WHC and depressed maturation compared with fish under natural light regime. Increased growth rate resulted in softer fillets and lower muscle pH.     

Effects of skeleton photoperiods on melatonin secretion in the rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract Plasma melatonin levels were measured at hourly intervals in rainbow trout, Oncorhynchus mykiss (Walbaum), maitained under three different skeleton photoperiods: 8L:2D:2L:12D;8L:7D:2L:7D; and 8L:12D:2L:2D. Blood samples were taken before, during and after the 2-h light pulses. Melatonin levels increased rapidly to mean scotophase (dark period) values of 261 ± 7 pg/ml after the first light-dark transition and had returned to mean photophase (light period) values of 51 ± 2 pg/ml 30min after the end of the scotophase. Light pulses during the early, mid- and late scotophase elicited similar reductions in melatonin levels, followed by rapid increases after the light pulse, reaching previous scotophase values within 90 min. The inability of any of the light pulses to modulate or truncate melatonin secretion under a LD 8:16 photoperiod provides further support for the hypothesis that melatonin secretion in the rainbow trout is a direct response to darkness, and is not under endogenous circadian control as in other vertebrates examined.     

Influences of feeding frequency of extruded pellet and moist pellet on growth and body composition of juvenile Japanese flounder <Emphasis Type="Italic">Paralichthys olivaceus</Emphasis> in suboptimal water temperatures

This study was carried out to investigate the influences of feeding frequency of extruded pellet and moist pellet on growth and body composition of juvenile flounder (initial mean weight 6.3 g) in sub optimal water temperatures. A 2 (diets: extruded pellet and moist pellet)×2 (feeding frequencies: two and three times daily)×2 (water temperatures: 12 and 17°C) factorial design with three replications was used. After 60 days of feeding, the feeding frequency did not significantly affect growth performance of fish. Weight gain, daily feed intake, feed efficiency and protein efficiency ratio were significantly (P<0.05) higher for fish reared at the higher water temperature. At the same water temperature, weight gain, feed efficiency, and protein efficiency ratio were significantly (P<0.05) higher for fish fed the extruded pellet than moist pellet. The results of this study indicate that feeding frequency of two times daily is sufficient for optimal growth of juvenile flounder when reared in suboptimal water temperatures, and an increase in water temperature from 12 to 17°C improved growth and feed efficiency.     

The effect of diet and photoperiod on ammonia efflux rates of the African catfish, Clarias gariepinus (Burchell, 1822)

Total ammonia efflux rates of the African catfish, Clarias gariepinus (Burchell), were measured under different combinations of photoperiod (LD 24:0. LD 0:24, LD 12:12) and feeding regime (50% protein diet, 41% protein diet and zero food). A time-dependent variation of ammonia efflux following feeding was observed, with postprandial excretion rates in the first 12 h being greater than those in the following 12 h. Ammonia efflux rates were directly related to the level of protein in the diet under all photoperiod conditions. Photoperiod had no effect on ammonia efflux rate, except in the LD 24:0-high protein diet group, which may indicate a stress response.     

Wild fish are not a significant reservoir for Neoparamoeba pemaquidensis (Page, 1987)

Amoebic gill disease (AGD), caused by the protozoan Neoparamoeba pemaquidensis (Page, 1987) is the most important disease affecting salmon farms in Tasmania. Reservoirs for this protozoan parasite are largely unknown. This study investigated wild fish as a potential reservoir of N. pemaquidensis . A total of 325 wild fish, comprising 12 different fish species, were caught from and around salmon farms and examined for the presence of AGD. None of the wild fish were infected with AGD. In a laboratory trial, seahorse, Hippocampus abdominalis , greenback flounder, Rhombosolea tapirina, and Atlantic salmon, Salmo salar, were challenged with N. pemaquidensis . Neoparamoeba pemaquidensis was detected on the gills on 10 of 15 (66.7%) flounder, nine of 24 (37.5%) seahorses, and six of six (100%) Atlantic salmon. However, paramoebae positive flounder and seahorse lacked the characteristic AGD gill pathology. It is concluded that AGD does not appear in wild fish and wild fish do not seem to be a reservoir of the pathogen.     

Dualistic feeding pattern of Nile tilapia (Oreochromis niloticus,L.) reared under different self‐feeding system conditions

Nile tilapia (Oreochromis niloticus) is one of the most important cultured species worldwide. The research aims to clarify the feeding characteristics, such as daily feeding pattern of Nile tilapia using a self‐feeding technique. The feeding pattern was conducted under two rearing conditions, indoor treatments under a controlled light regime (LD 12:12) and constant water temperature (25°C), and outdoor treatments under natural conditions which consisted of duplicate trials with two periods each. The outdoor treatment was carried out from early summer through late autumn in Mie, Central Japan. Daily self‐feeding activity of Nile tilapia in indoor treatments was nearly daytime feeding pattern, synchronizing with the given photoperiod (24 hr). However, the self‐feeding activity in the outdoor experiments from early summer to early autumn was almost daytime feeding pattern, but it became less clear and shifted to a nighttime feeding profile in late autumn. The results revealed that Nile tilapia has a dualistic capacity for demand‐feeding both in light and dark phases. These results might have been caused by the seasonal change in light intensity and/or water temperature. Information obtained from the self‐feeding experiments enables us to identify the influence of environmental changes on the physiological condition of farmed fish through their expression of appetite.     

Prolonged photoperiod improves the growth performance for a hatchery reared right-eyed flatfish

Rearing winter flounder (Pseudopleuronectes americanus) in captivity for aquaculture and stock enhancement is a viable option to relieve fishing pressure and aid in ongoing recovery strategies. The objective of this study was to determine the effect of photoperiod on growth and survival of young-of-the-year juvenile winter flounder. Juveniles were reared for 12 weeks at three photoperiods: 24 light/0 dark, 18 light/6 dark, and 12 light/12 dark. Twenty fish were stocked into each of 18 enclosures with six enclosures per treatment. Fish in all treatments were fed to satiation every four hours. All fish were imaged at stocking and at three-week intervals. Growth was measured as changes in standard length (SL) and body area (BA). As early as Week 3, fish in all treatments were significantly different in terms of SL and BA. In general, fish grown under continuous 24 light were the largest and fish in the 12 light/12 dark regime were the smallest. Maximum specific growth rates of 0.41 % per day for SL and 0.83 % per day for BA were achieved in the 24 light/0 dark photoperiod. Average mortality rate over the course of the experiment was 9.17 % and there were no significant differences in the number of dead fish among the treatments. In conclusion, this study demonstrates that a longer day coupled with available food will increase growth rate for winter flounder juveniles, without affecting survival. By extending the photoperiod, growth in young-of-the year winter flounder is accelerated, bringing the growth rate of winter flounder close to that of other commercially produced flatfish.     

Development of new self-feeding system for mass rearing of ayu <Emphasis Type="Italic">Plecoglossus altivelis altivelis</Emphasis> under artificial and natural light-dark cycles

Self-feeding of grouped ayu Plecoglossus altivelis altivelis was examined under an artificial light-dark (LD) cycle and natural day length using a trigger with a photosensitive sensor. In Experiment 1, approximately 15 juvenile fish (mean body weight, 0.6 g) were kept in 60-L glass tanks (four tanks) under LD 12∶12 (lighting period 06∶00–18∶00 hours) and self-feeding and locomotor activities were simultaneously recorded. Self-feeding was detected within one day and feeding activities were strongly synchronized to the LD cycle, with almost strictly diurnal feeding (98.2%). Although locomotor activities in the upper part of the tank were also greatest during the light phase (76.8%), the activity profiles were different from those of self-feeding, suggesting the validity of the photosensitive sensor. In Experiment 2,25 fish (mean body weight, 15 g) were kept in outdoor 1000-L tanks (four tanks) under natural day length and water temperatures in May, and self-feeding was recorded from mid-August. Stable self-feeding was observed from early September in all the tanks. Nearly all food demands were diurnal, and usually crepuscular. Somatic growth was seen in all tanks. These results demonstrate that the self-feeding system using a photosensitive sensor is applicable for grouped juvenile ayu as small as 0.6 g and adult fish reared under natural conditions.     

Effect of stocking density,oxygen level,light regime and swimming velocity on the incidence of sexual maturation in adult Atlantic salmon (Salmo salar)

The effects of various environmental parameters on sexual maturation of two sea-winter Atlantic salmon (Salmo salar) were tested in two separate experiments. In the first experiment Atlantic salmon with initial mean individual weight 1.5 kg (smolt 13 months before) were reared for 8 months from June to February at different oxygen levels and stocking densities using continuous light. Oxygen levels of 5–7, 7.5–9.5 and 10–12 mg O₂ l⁻¹ and stocking densities starting at about 20, 30 and 40 kg m⁻³ and increasing as the fish grew to 80–90 kg m⁻³ for the highest densities were tested in a factorial design. Only male fish matured, and incidence of maturation among males varied from 4.1% to 25% between tanks. The highest percentage of mature males was found in the tanks with low stocking density. No clear effect on oxygen level was found.The second experiment lasted 20 months from seawater transfer in May until the fish weighed 3.3–3.5 kg. Two water current speeds (14–16 and 20–24 cm s⁻¹) and two photoperiod regimes (LD 20:4) and continuous light (LL) were tested in a factorial design. Neither swimming velocity nor photoperiod affected growth rate. Continuous light reduced the incidence of sexual maturation. The average proportion of maturation among males was 8% and 25% under the LL and LD 20:4 regimes respectively. The fish reared under the LD 20:4 light regime had a significant lower condition factor and significant larger hearts than the fish reared under continuous light. Swimming velocity had no significant effect on the incidence of maturation. The results indicate that the swimming velocity must be higher than 0.5 BL s⁻¹ in order to influence the energy stores. An important finding in this study is that light cues are not required for gonadal growth. The results also indicate that environmental factors can affect maturation even after the first sea-winter.