The effect of photoperiod on growth, survival, colour and activity of juvenile southern rock lobster, Jasus edwardsii

Juvenile (1–10 g) southern rock lobsters, Jasus edwardsii, were subjected to five photoperiods [0L(light):24D(dark); 6L:18D; 12L:12D; 18L:6D; 24L:0D] during a 112‐day trial, and growth, survival, colour, food consumption and activity were examined. Lobsters grown under the 6L:18D and 24L:0D photoperiods had significantly lower (P < 0.05) final mean weight and specific growth rate than any other treatments. Photoperiod had no effect on survival or colour of lobsters. Food consumption differed significantly during the first week of the trial; after 5 weeks, food consumption was similar in all treatments. Major peaks in activity occurred during dark periods for lobsters exposed to photoperiods that had a light:dark regime. Activity of lobsters exposed to continuous light or dark regimes remained constant and at low levels during the 24‐h period. Food consumption or activity does not appear to determine the rate of growth of lobsters in the different photoregimes. One over‐riding outcome of this and other studies on photoperiod is that the growth and survival response of juvenile J. edwardsii was not significantly better than those of the standard 12L:12D cycle.     

Physiological changes in male and female pikeperch Sander lucioperca (Linnaeus, 1758) subjected to different photoperiods and handling stress during the reproductive season

Pikeperch broodstocks were exposed to different photoperiods: constant light (24L:0D), constant darkness (0L:24D), and 12 h light, 12 h darkness (12L:12D), for 40 days. Half of the broodstocks of each photoperiod were exposed to handling stress at a specific time of the day. Results showed that cortisol and lactate did not reveal any significant difference. However, glucose levels in females increased in the stress-free darkness period in comparison with stressful darkness photoperiods (0L:24D-s). Red blood cells in males and white blood cells in females showed a significant difference under different photoperiod regimes. Both sexes showed no significant difference in the differential count of leukocytes under different photoperiods and handling stress. Constant photoperiods and handling stress affected the hematological parameters, particularly, the number of lymphocytes and neutrophils in females. Our findings revealed that due to a long-term exposure to stressors, pikeperch brooders become adapted to stressful conditions.     

Reproduction,growth and stress response in adult red sea bream,Pagrus major (Temminck & Schlegel) exposed to different photoperiods at spawning season

Adult red sea bream, Pagrus major (body weight, 1.0–2.0 kg) was exposed to three photoperiods [12 h light:12 h dark (12L:12D), 16 h light:8 h dark (16L:8D) and 24 h light:0 h dark (24L:0D)] from 2 months before spawning till the end of the spawning season to investigate growth, spawning and stress response. During the spawning season, tanks were checked every morning for spawned eggs. The growth performance in fish under 24L:0D was stimulated with significantly higher feed intake than those under other photoperiods (P<0.05). The number of eggs and gonadal histology confirmed that three and five females out of six in each of duplicate tanks of the 16L:8D treatment spawned. In contrast, only two out of six females in one tank of the 24L:0D treatment spawned, and no spawns were observed in the 12L:12D treatment. At the end of the spawning period, both 17β‐estradiol and testosterone levels were significantly higher in fish exposed to 16L:8D followed by 12L:12D and 24L:0D photoperiods (P<0.05). Photoperiod manipulation did not cause significant stress response in fish (P>0.05). The results suggest that stimulating the growth performance of red sea bream at reproductive stage with a 24L:0D photoperiod is possible if the fish are subjected to this photoperiod long before the onset of the spawning season.     

Effect of Photoperiod Manipulation on Growth Performance,Physiological and Hematological Indices in Juvenile Persian Sturgeon,Acipenser persicus

The effect of five different light regimes on growth, stress and hematological indices was studied in Persian sturgeon, Acipenser persicus. Fish with average weight of 645.3 ± 11.2 g were subjected to different photoperiods (24 L, 12 L:12 D, 16 L:8 D, 8 L:16 D and 24 D) for 8 wk. Blood samples were collected at the end of the experiment for measuring cortisol, glucose and hematological features. The whole growth parameters showed no significant difference. Plasma cortisol concentration was significantly higher in 12 L:12 D, while the lowest level was observed in fish exposed to 24 D. No significant changes were observed among the treatments for glucose concentration. Lactate concentration varied significantly among the treatments. Some hematological indices including hematocrit and number of white blood cells were affected by different light regimes, but the others (hemoglobin and number of red blood cells) were not affected significantly. The results showed that photoperiod manipulation can alters some stress‐related metabolites and may enhance growth rate in fish exposing to continuous darkness.     

Photoperiod influences the growth, food intake, feed efficiency and digestibility of red sea bream (Pagrus major)

Two consecutive trials were conducted to investigate the effects of photoperiod manipulation on growth rate, food intake and feed conversion efficiency (Trial 1), and the digestibility of nutrients and energy (Trial 2) in red sea bream, Pagrus major (body weight 19–120 g). Fish were exposed to four photoperiods (6L:6D, 12L:12D, 16L:8D and 24L:0D) with light intensity 1500 lx on the water surface. The fish were fed with a commercial diet to apparent satiation. In Trial 2, 0.5% chromic oxide (Cr₂O₃) was used as an inert marker. Significantly higher weight gain and specific growth rates were observed in fish exposed to a 24L:0D photoperiod followed by 16L:8D, 6L:6D and 12L:12D photoperiods (P < 0.05). Food intake and feed conversion efficiency (FCE) were also significantly higher in fish exposed to 24L:0D followed by 16L:8D, 6L:6D and 12L:12D photoperiods (P < 0.05). Fish exposed to 24L:0D and 16L:8D photoperiods showed significantly higher lipid and energy digestibility than those exposed to a 12L:12D photoperiod (P < 0.05). The results demonstrated that the enhancement of growth performances under 16L:8D and 24L:0D photoperiods were attributed to improved appetite, greater food intake and higher feed conversion efficiency as well as higher digestibility.     

光照周期对褐牙鲆幼鱼生长、能量分配及生化指标的影响

在循环水实验系统中,设置1 L∶23 D、9 L∶15 D、12 L∶12 D、15 L∶9 D、24 L∶0 D(光L∶暗D)共5个光照周期处理组,研究其对褐牙鲆幼鱼生长及部分生化指标的影响。实验结束时褐牙鲆幼鱼的体质量为41.10~43.98 g,不同处理组间无显著差异,体质量未表现出与日光照周期长短的相关性。日生长系数只在21~30 d阶段内出现显著差异,整个实验期间的平均日生长系数为1.86~2.02,不同处理组间不存在显著差异,日生长系数随日光照周期的延长略呈下降趋势。不同阶段的摄食率在31~40 d阶段出现显著差异,12 L∶12 D处理组摄食率显著高于9 L∶15 D处理组。在整个实验期间,1 L∶23 D的褐牙鲆幼鱼摄食率为1.64,显著低于12 L∶12 D的褐牙鲆幼鱼的摄食率(1.79)。整个实验期间,1 L∶23 D的褐牙鲆幼鱼平均饲料转化效率为113.49,显著高于12 L∶12 D、15 L∶9 D、24 L∶0 D的褐牙鲆幼鱼,并略高于9 L∶15 D的褐牙鲆幼鱼,饲料转化效率随光照时间的延长呈下降趋势。实验期间摄食能的分配比例仅在排泄能上出现显著差异,15 L∶9 D最高,12 L∶12 D最低,其他各项能量分配均无显著差异。不同光照周期对耗氧率影响显著,15 L∶9 D最高,为0.252 mg/(g·h),24 L∶0 D最低,为0.173 mg/(g·h)。耗氧率随着光照时间的延长呈先上升后下降的趋势。血浆生长激素和类胰岛素生长因子I含量在不同处理组间也不存在显著差异,随光照时间延长也未表现出明显变动趋势。血浆褪黑素含量随光照时间的增加呈先下降后上升的趋势,12 L∶12 D处理组MT含量最低,但与其他处理组差异不显著。实验结束时肌肉RNA/DNA比值随光照周期延长呈先上升后下降变动趋势,其中15 L∶9 D和24 L∶0 D处理组的肌肉RNA/DNA比值显著高于1 L∶23 D处理。肝脏RNA/DNA比值以12 L∶12 D最高,15 L∶9 D最低。实验结果表明光照周期对褐牙鲆幼鱼摄食量和饲料转化效率产生显著影响,但摄食量减少的处理组由于耗氧率降低因而饲料转化效率得到提高,因此光照周期对生长没有显著影响。光照周期明显影响血浆褪黑素、肌肉和肝脏RNA/DNA比值,但生化指标的差异与生长无明显相关性。     

Effect of stocking density and photoperiod on growth and survival in cultured early juvenile pot‐bellied seahorses Hippocampus abdominalis Lesson, 1827

The effect of stocking density and photoperiod on Hippocampus abdominalis was examined in this study. Stocking densities of 45, 30, 15 and 5 seahorses 3 L^?1 were tested on newborns. Growth and survival were independent of stocking density. A second stocking‐density experiment aimed to remove the effect of an early mortality experienced in experiment 1 using older juveniles at 25, 15 and 5 seahorses 3 L^?1. There were no differences in the parameters measured. Also, the effect of photoperiod was investigated on seahorses cultured under 24:00, 16:08 and 08:16 (L:D) photoperiods. A single Artemia meal was delivered at 10:00 hours. Survival and body growth in 16:08 and 08:16 hours were higher than in 24:00 hours. In a second experiment, seahorses were cultured in the photoperiods and conditions described for experiment 1, except they were fed twice the amount in two meals delivered at 10:00 and 16:30 hours. The seahorses in 16:08 hours showed better growth than the other treatments, but there were no differences in survival. These results suggest that early juvenile H. abdominalis can be cultured at higher stocking densities than previously reported, without compromising growth and survival, and when feeding was not limiting, grew better in an extended photoperiod (16:08) but not in 24:00 hours.     

Growth performance and physiological responses in striped knifejaw, Oplegnathus fasciatus, held under different photoperiods

The growth performance and stress response in striped knifejaw, Oplegnathus fasciatus (body weight 100–300 g) reared under four photoperiods (6L:6D, 12L:12D, 16L:8D and 24L:0D) were investigated. Fish were fed a commercial diet to apparent satiation, two times a day for 8 weeks. A trial of acute handling and confinement stress was also carried out to investigate the stress-induced levels of different stress indicators in O. fasciatus. Blood was also collected from undisturbed fish which was considered as control. Although there were no significant differences in weight gain, specific growth rate (SGR) and feed conversion efficiency (FCE) in fish exposed to 6L:6D, 16L:8D and 24L:0D photoperiods, all parameters in these photoperiods were significantly higher than those of 12L:12D photoperiod (P < 0.05). There was no significant difference in protein retention efficiency (PRE) between fish exposed to 16L:8D and 24L:0D photoperiods, but PRE in both photoperiods was significantly higher than that of 12L:12D photoperiod.Acute stress significantly increased the plasma levels of cortisol (110.3 ng ml^− 1) and glucose (195.4 mg dl^− 1), and decreased plasma levels of total protein (0.8 g dl^− 1); however, all parameters were returned back to the levels indistinguishable from those of control, undisturbed fish within 24 h. The levels of cortisol, glucose and total protein in fish exposed to different photoperiods during the study were far from the stress-induced levels (P < 0.05). The results demonstrated that the growth performance of O. fasciatus reared from 100 to 300 g can be stimulated significantly by using the manipulated photoperiods where feeding time may be playing an important role to increase food intake and feed conversion efficiency. It also revealed that the artificial photoperiods did not cause significant stress response in fish.     

Enhanced growth and retarded gonadal development of farmed rainbow trout,Oncorhynchus mykiss (Walbaum) following a long‐day photoperiod

Long‐day photoperiods are considered as an effective managerial tool in manipulating of reproduction and somatic growth in a number of fish species. In this study, the effects of three different artificial photoperiods on the gonadal development and somatic growth of rainbow trout (Oncorhynchus mykiss L.) were investigated. Two years‐old immature female rainbow trout (279.94 ±2.25 g) were exposed to three artificial photoperiod regimes of 24L:0D, 18L:6D and 6L:18D and natural light (NL) regime for 5 months. The highest gonadosomatic indices were recorded in NL and 6L:18D groups while the rates were significantly lower in fish maintained under 18L:6D and 24L:0D photoperiods (P < 0.05). Mean oocyte diameters in fish exposed to 24L:0D and also to 18L:6D were significantly lower than the 6L:18D and NL groups. Photoperiods with 24L:0D and 18L:6D regimes resulted in significantly higher mean final weights and specific growth rates (SGR) than NL regime. The highest mean final weight (635.45 ± 16.19 g) and SGR (1.03 ± 0.04% day^?1) were obtained under 24L:0D photoperiod. Fish exposed to 24L:0D and 18L:6D showed the highest condition factor as 1.44 ± 0.01 and 1.44 ± 0.02 respectively, when compared with the NL (1.27 ± 0.01) and 6L:18D (1.34 ± 0.02) groups. Basically, the results suggested that continuous artificial lightning can be used as an influential factor in delaying gonadal development and enhancing somatic growth in rainbow trout during gonadal growth phase.     

Effect of combined photoperiod, water calcium concentration and pH on survival, growth, and moulting of juvenile crayfish (Procambarus clarkii ) cultured under laboratory conditions

The red swamp crayfish, Procambarus clarkii (Girard), is one of the most commonly farmed freshwater species in inland China due to its high market value and consumer demand. The aim of this study was to determine the optimum combinations of photoperiod, water calcium concentration and pH for juvenile survival, growth and moulting. In our orthogonal experiment, the three environmental factors were varied at three levels (photoperiod: 16L:8D, 12L:12D and 8L:16D; calcium concentration: 45.5, 65.5 and 85.5 mg L⁻¹; and pH: 6.8, 7.8 and 8.8). Range analysis showed that the maximum survival of juvenile crayfish occurred at photoperiods of 16L:8D or 8L:16D, water calcium concentration of 45.5 mg L⁻¹ and pH of 7.8; maximum weight gain at photoperiod 16L:8D, water calcium concentration 65.5 mg L⁻¹ and pH 7.8; maximum length increase at photoperiod 16L:8D, water calcium concentration 65.5 mg L⁻¹ and pH 7.8; and the highest moult frequency at photoperiod 12L:12D, water calcium concentration 65.5 mg L⁻¹ and pH 7.8. Analysis of variance indicated that photoperiod, water calcium concentration and pH significantly influenced only the weight gain of juvenile crayfish ( P <0.05). Taking growth into consideration, we suggest that a photoperiod of 16L:8D, calcium concentration of 65.5 mg L⁻¹ and pH 7.8 might be optimal conditions for rearing juvenile P. clarkii .     

Stimulation of growth performance without causing stress response in young red sea bream, Pagrus major (Temminck & Schlegel), by photoperiod manipulation

Three photoperiods (12L:12D, 16L:8D and 24L:0D) were used to investigate the growth performance and stress response in red sea bream, Pagrus major (body weight 200–400 g). Fish were fed a commercial diet to apparent satiation, two times a day for 8 weeks. Fish exposed to a 24L:0D photoperiod showed a significantly higher weight gain (%) than those exposed to other photoperiods (P<0.05). The best specific growth rate and feed conversion efficiency were achieved at 24L:0D and 16L:8D, without significant differences among them. Although fish exposed to 16L:8D showed a significantly higher plasma level of cortisol than those exposed to other photoperiods in the third week, there was no major variation in the cortisol level among the treatments either at the sixth week or at the end of this study. There were no significant differences either in the haematocrit level or the plasma levels of glucose, total cholesterol and total protein among the treatments during this study. The results revealed that the growth performance of red sea bream reared from 200 to 400 g can be stimulated significantly using a continuous (24L:0D) photoperiod without any measurable significant stress response in fish.     

Effect of different photoperiod cycles on metabolic rate and energy loss of both fed and unfed young tilapia Oreochromis niloticus: Part I

ABSTRACT: The influence of different photoperiod cycles (3L : 3D, 6L : 6D, 12L : 12D, and 24L : 24D) on the metabolic rate and energy loss of either fed or unfed young tilapia Oreochromis niloticus (bodyweight 8.6–9.5 g) was investigated at 28°C. A computer-operated respirometer with a closed tank was used to prevent water from evaporating into the air or condensing from the air. The photoperiods acted as strong Zeitgeber (i.e., cue or synchronizer) during the experiments with either fed or unfed fish. A photoperiod-mediated metabolic cycle was demonstrated in fed and unfed fish in which oxygen consumption was higher during the light phase compared with during the dark phase for all photoperiods. Mean oxygen consumption during the 3L : 3D, 6L : 6D, 12L : 12D, and 24L : 24D periods for fed and unfed fish were 685.06 mg/kg per h and 299.33 mg/kg per h; 658.52 mg/kg per h and 284.80 mg/kg per h; 591.09 mg/kg per h and 249.62 mg/kg per h; and 500.64 mg/kg per h and 239.14 mg/kg per h, respectively. The highest post-prandial increase in energy loss was recorded during the 3L : 3D period (145.88 kJ/kg per day), followed by 141.19 kJ/kg per day during the 6L : 6D, 128.70 kJ/kg per day during the 12L : 12D, and 99.92 kJ/kg per day during the 24L : 24D periods. The results suggest that higher energy conservation would be achieved if fish are exposed to longer rather than shorter photoperiod cycles.     

Larval survival,development and growth in the horsehair crab,Erimacrus isenbeckii,cultured under different photoperiod conditions

Larval sinking causes larval mass mortality during seed production in the horsehair crab, Erimacrus isenbeckii. Under normal light conditions, horsehair crab larvae generally show negative phototactic behaviour and sink in their rearing seawater. It has been proposed that culturing larvae in the dark may prevent larval sinking. Herein, we examined the effect of photoperiods on horsehair crab larval survival and development to facilitate the development of larval rearing techniques that prevent sinking. Newly hatched larvae were reared with Artemia to the first crab stage in 2‐L beakers under five photoperiods: 0L:24D, 6L:18D, 12L:12D, 18L:6D and 24L:0D. Larvae survived and molted to the first crab stage under all tested photoperiod conditions. The survival rate improved with increasing light period, whereas the developmental period for each larval stage decreased with increasing light period. Longer light periods increased the carapace length at the first crab stage. Our results suggest that larvae could be cultured to the first crab stage in large‐scale tanks under constant darkness. However, significantly improved larval performance under longer photoperiodic conditions indicates a need for developing alternative culture techniques to control larval behaviour in the seed production tank.     

Effect of the dusk photoperiod change from light to dark on the incubation period of eggs of the spotted rose snapper, Lutjanus guttatus (Steindachner)

Spotted rose snapper, Lutjanus guttatus (Steindachner), eggs were incubated under different photoperiods to examine the effect of photoperiod on incubation. The eggs from two fish were incubated under five artificial photoperiods: constant dark (D), constant light (L) from 06:00 hours and 6, 10 and 14 h of light from 06:00 hours. The eggs from seven other fish were incubated under a natural photoperiod. Different spawning times (21:00 – 01:00 hours) and different photoperiods combined to give the start of the dusk photoperiod change after 11–23 h of incubation. Constant light or applying the dusk photoperiod change after ≥20 h of incubation appeared to extend the hatching period. The mean hatching period for groups of eggs incubated in darkness or that received the dusk photoperiod change after ≤19 h of incubation (n=8 different groups) was 2 h 15±10 min, which was significantly lower (P<0.05) than the mean hatching period of 4 h±37 min for groups that did not receive the dusk photoperiod change or that received the dusk photoperiod change after ≥20 h of incubation (n=9 groups). However, despite these differences, the majority of the eggs hatched during a 2–3 h period from 17 to 20 h of incubation, and a sigmoid regression (r²=0.9) explained the relationship between percentage hatch and hours of incubation for all photoperiod groups.     

不同光周期下欧洲舌齿鲈幼鱼生长、摄食和肌肉营养成分的比较

选取450尾欧洲舌齿鲈（）幼鱼，体长（13.50±0.52）cm，体重（46.04±0.61）g，监测其在5种光周期（0L：24D、8L：16D、12L：12D、16L：8D、24L：0D，L表示光照时间，D表示黑暗时间）下的生长、摄食及肌肉营养成分等相关指标。实验周期为60 d。结果表明，8L：16D处理组的欧洲舌齿鲈的日增重系数显著高于其他各组（<0.05）；0L：24D和8L：16D处理组粗脂肪含量分别较16L：8D处理组分别提高了19.70%和21.67%（<0.05）。5种光周期下的欧洲舌齿鲈幼鱼肌肉中均检测到17种氨基酸，其中8L：16D和12L：12D处理组的肌肉中氨基酸总含量较0L：24D组分别提高11.60%和9.16%（>0.05）；肌肉中二十碳五烯酸（EPA）与二十二碳六烯酸（DHA）总含量在8L：16D处理组中最高，为14.21%，在0L：24D处理组中最低，为12.30%。不同光照周期对欧洲舌齿鲈幼鱼生长、摄食和肌肉营养品质均产生了不同程度的影响，综上所述，8L：16D和12L：12D光照环境更适宜欧洲舌齿鲈的生长。     

Effects of photoperiod and stocking density on survival,growth and physiological responses of narrow clawed crayfish (Astacus leptodactylus)

Photoperiod and stocking density are critical factors influencing the performance of decapod crustaceans in culture, however, their influence on growth; survival and biochemical physiology of crayfish broodstock have rarely been considered. Analysis of biochemical physiology in crayfish broodstock during the non‐breeding season provides information on the energy storage requirements of broodstock for increased survival and reproductive output. Growth rate, moulting frequency, survival and biochemical physiology were measured in Astacus leptodactylus broodstock that were cultured at three different photoperiods (18L:6D; 12L:12D; and 6L:18D) and three stocking densities (10, 20 and 40 individuals m^?2) during non‐breeding season. Survival of crayfish was highest at 18L:6D photophase and 10 m^?2 (100%) than other treatments. Survival in the high stocking density was high when combined with 18L:6D photophase, but weight gain (WG) and specific growth rate (SGR) were higher at shorter photophase and lower stocking density. Longer photophase (18L:6D) increased stress responses, characterized by increased haemolymph lactate and glucose levels. Stocking density did not affect proximate composition of crayfish; however, individuals cultured at 18L:6D photophase had higher lipid content than other photoperiod treatments. The study demonstrated that culturing A. leptodactylus at 18L:6D photophase and 10 m^?2 is critical for increased survival of broodstock.     

Feeding interval and photoperiod influence the growth performance of striped knifejaw,Oplegnathus fasciatus

A two‐factor experiment incorporating two photoperiods (16 L:8 D and 24 L:0 D) and four feeding intervals (6, 9, 12 and 24 h) was carried out to determine the most efficient feeding interval and photoperiod for striped knifejaw, Oplegnathus fasciatus. Fish were fed a commercial diet to apparent satiation for 10 weeks. Blood was collected to measure the stress indicators. Both the photoperiod and the feeding intervals had a significant effect on the growth performance of striped knifejaw; however, their interaction did not produce any significant effect. The final body weight, weight gain (%) and feed intake at 6‐, 9‐ and 12‐h feeding intervals under 16 L:8 D were significantly higher than those at the 24‐h interval irrespective of the photoperiod (P<0.05). The specific growth rate at 6, 9 and 12 h intervals of the 16 L:8 D photoperiod was significantly higher than that of the other feeding intervals, except at 12 h of the 24 L:0 D photoperiod (P<0.05). At the end of the experiment, there were no significant differences in the plasma levels of cortisol, glucose and total protein observed in fish exposed to different photoperiods compared with the initial levels. The results suggested that a combination of a 16 L:8 D photoperiod and either of 6‐, 9‐ or 12‐h intervals can be used to stimulate the growth performance in striped knifejaw.     

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.     

Effect of photoperiod and tank colour on growth and survival of pelagic‐phase seahorse Hippocampus reidi

The effect of photoperiod and tank colour on the survival and growth of juvenile seahorse Hippocampus reidi was investigated in the first 15 days after release (DAR). In the first experiment, newly released seahorses were exposed to six photoperiods – 6 h of light (L):18 h of darkness (D), 10L:14D, 12L:12D, 14L:10D, 18L:6D and 24L:0D. In the second experiment, juveniles were kept in tanks of different background colours – blue, white, black, clear and red. No significant differences were observed in the growth and survival among seahorse subjected to 10–24 h of light (P > 0.05). At the end of 15 DAR, fish exposed to 6L:18D showed lower growth in height and weight compared to other treatments (13.8 mm vs. 20.0–22.2 mm and 13.4 mg vs. 37.2–43.2 mg respectively) and lower survival (80% vs. 88–95% respectively). Tank colour did not affect growth and survival of juveniles up to 15 DAR.     

Transpositional feeding rhythm of loach <Emphasis Type="Italic">Misgurnus anguillicaudatus</Emphasis> from larvae to juveniles and its ontogenesis under artificial rearing conditions

The diel feeding rhythm and ontogenesis during early life stage of loach Misgurnus anguillicaudatus were investigated under experimental conditions (light: L 06:00–18:00, D 18:00–06:00 h). Morphological and behavioral developments of loach from newly hatched to 40 days after hatching were observed. Larvae were able to prey on daphnia 3–4 days after hatching at 23 ± 0.5°C. As the larvae grew, they showed an increasing feeding capacity and a distinct feeding rhythm. Feeding intensity and incidence for day-4 larvae were highest at 10:00 and 16:00 h. The highest levels of feeding intensity for day-12 larvae occurred at 08:00, 12:00, and 18:00 h as did feeding incidence. By day 20, when the larvae metamorphosed, the highest levels of feeding intensity occurred at 06:00, 18:00, and 24:00 h and were concurrent with the highest feeding incidence. After metamorphosis, feeding capacity had again increased considerably and, in contrast to the earlier stages before day 20, feeding intensity for day-30 juveniles peaked at 05:00 and 20:00 h, about 1–2 h after the maximum feeding incidence. The feeding rhythm of loach juveniles at day 40 was almost the same as the day-30 juveniles. The estimated maximum daily feeding rates were 43.1%, 33.4%, 19.0%, 12.8%, and 5.8% of body weight on days 4, 12, 20, 30, and 40, respectively. Thus, loach was found to have different feeding rhythms in the pre- and post-metamorphosis stages, with the highest feeding activity in daytime during the larval planktonic stage before metamorphosis, and intensely nocturnal feeding behavior during the juvenile benthic stage after metamorphosis.