Prolonged photoperiod inhibits growth and reproductive functions of rohu Labeo rohita

Aquaculture Reports 16 (2020) 100272

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Prolonged photoperiod inhibits growth and reproductive functions of rohu Labeo rohita

T

Md. Shahjahan*, Md. Al-Emran, SM Majharul Islam, SM Abdul Baten, Harunur Rashid, Md. Mahfuzul Haque Laboratory of Fish Ecophysiology, Department of Fisheries Management, Bangladesh Agricultural University, Mymensingh, 2202, Bangladesh

ARTICLE INFO

ABSTRACT

Keywords: Rohu Light Blood glucose Hematological parameters Reproduction

Among the environmental factors, photoperiod plays an important role in growth and reproduction of fish. This study was conducted to assess the effects of photoperiod on growth of juveniles and reproductive functions of matured female rohu (Labeo rohita), a commonly cultured freshwater fish species. Two experiments were conducted each with three different photoperiod treatments, such as 6 h of light and 18 h of dark (06L:18D), 12 h of light and 12 h of dark (12L:12D), and 18 h of light and 6 h of dark (18L:06D), each treatment with three replications. In the first experiment, fingerlings (2.39 ± 0.35 g BW, 6.67 ± 1.23 cm TL) were exposed in the three photoperiod conditions for 30 days and sampled fishes were sacrificed on 7, 15 and 30 days of exposure to measure growth parameters (weight gain (g); percent weight gain; specific growth rate, SGR and feed conversion ratio, FCR) and major hemato-biochemical parameters (Hemoglobin, Hb; Red blood cell, RBC; White blood cell, WBC and blood glucose). Final weight gain (g), percent weight gain and SGR were highest in 12L:12D and the lowest in 18L:06D conditions. FCR was lowest in 12L:12D conditions. In prolonged photoperiod (18L:06D), the Hb and RBC decreased significantly, while WBC and blood glucose level increased significantly during initial days of exposure. In the second experiment, sexually matured females were exposed in the three photoperiod conditions for 30 days and the sampled fishes were sacrificed on 0, 15 and 30 days of exposure to know gonadosomatic index (GSI) and gonadal histology. The mean GSI values, oocyte diameter and proportions of vitellogenic oocyte were maximum in the fishes of 06L:18D treatment indicating stimulation of vitellogenesis. On the other hand, the same parameters were minimum in the fishes of 18L:06D, revealing the inhibition of vitellogenesis. This study indicates that prolonged photoperiod negatively affect growth and reproductive performances of rohu.

1. Introduction The growth and reproduction of fish is regulated by a number of environmental factors, such as photoperiod, temperature, salinity and nutritional status (Boeuf and Falcon, 2001; Shahjahan et al., 2013, 2017). The feeding strategies of fishes are affected by photoperiod (Reynalte-Tataje et al., 2002). In most of the fishes, feeding do not occur in a random way, rather it follows a particular type of biorhythms, i.e., circadian rhythms that are influenced by the photoperiod. Moreover, photoperiod is an important factor which controls the seasonal and diel rhythms in living organisms including fishes. Manipulation of light intensity and photoperiod are important in fisheries because they can optimize the production of a species, as the physiological response of fish can be positively affected by variations in light intensity, wavelength and daily or seasonal photoperiod (Stuart &

⁎

Drawbridge, 2011; Gunnarsson et al., 2012; Prayogo et al., 2012; Honryo et al., 2013; Wang et al., 2015). The timing of puberty and sexual maturation of fish are strongly affected by photoperiod (Norberg et al., 2004). Gonadal development and reproduction of fish are regulated by the endogenous rhythms, and synthesis and secretion of sex hormones, which are mediated by photoperiod (Elisio et al., 2014, 2015). The timing and duration of photoperiod are critically important for the sexual maturity of the fishes those maintain specific seasonality in their breeding activity (Zhu et al., 2014). Therefore, sexual maturity could be changed by reducing or extending the yearly photoperiod cycle (Bromage et al., 2001). The manipulation of the photoperiod is being used in aquaculture to induce maturation, control spawning and stimulate gonadal development in different fish species (Biswas et al., 2002; Gines et al., 2004; Rad et al., 2006). It has been reported that prolonged photoperiod increased

Corresponding author. E-mail address: [email protected] (Md. Shahjahan).

https://doi.org/10.1016/j.aqrep.2019.100272 Received 26 October 2019; Received in revised form 31 December 2019; Accepted 31 December 2019 2352-5134/ © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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fecundity in tilapia (El-Sayed and Kawanna, 2004) and stimulated ovarian maturation in topmouth gudgeon (Zhu et al., 2014). Hemato-biochemical indices of blood are frequently used for the assessment of the health status of the fish (Ruane et al., 2000; Okomoda et al., 2013). These parameters provide adequate evidence about physiological response of fish to environmental changes including any factor that affect homeostasis (Cazenave et al., 2005; Sharmin et al., 2015; Salam et al., 2015; Shahjahan et al., 2018; Islam et al., 2019; Jahan et al., 2019). Therefore, these parameters are routinely used to assess the level of stresses due to environmental and nutritional factors. It has been reported that hematological parameters have been used to assess the level of stresses due to different photoperiod in juvenile great sturgeon Huso huso (Bani et al., 2009) and Clarias gariepinus (Solomon and Okomoda, 2012). Besides, cortisol level increased in the pacamã catfish (Lophiosilurus alexandri) subjected to a long period of light (Kitagawa et al., 2015). The Indian major carp rohu (Labeo rohita) is one of the most commonly cultured freshwater fishes in Bangladesh. This species is mostly abundant in northern and central India, Bangladesh, Nepal, Myanmar and Pakistan (Talwar and Jhingran, 1991). It is considered as an important fish because of its high growth potential, nutritious and delicious attributes and high market value (Dahanukar, 2010). Its fry and fingerlings are easily available for culture, and consumers preference of its traits have made the species a suitable aquaculture candidate in Bangladesh. Furthermore, it is an ideal species for carp polyculture system and can be stocked with other carps like catla (Catla catla), mrigal (Cirrhinus mrigala), kalibaush (Labeo calbasu) etc. In Bangladesh, carps are cultured in natural photoperiods which varied with respect to the season i.e., from summer solstice of about 14 h to winter solstice of about 10 h (Boeuf and Falcon, 2001). Several photoperiod studies have been conducted on temperate species of fishes in terms of their growth and gonadal maturation, such as, farmed rainbow trout Oncorhynchus mykiss (Noori et al., 2015) and brook trout Salvelinus fontinalis (Lundova et al., 2019a, b). Similarly a number of studies have considered the effect of photoperiod on growth, feed efficiency and metabolism, physiological functions and reproduction in tropical fish, such as tilapia (Biswas et al., 2002; El-Sayed and Kawanna, 2004; Biswas et al., 2005), Amazonian ornamental fish Pyrrhulina brevis (Veras et al., 2016), pacamã catfish Lophiosilurus alexandri (Kitagawa et al., 2015) and striped knifejaw, Oplegnathus fasciatus (Biswas et al., 2016). However, no studies have been yet conducted on Indian major carps for growth and reproduction under different light regimes. Thus, the objectives of the present study were to know how photoperiod regimes affect growth performances and reproductive functions in rohu.

care and use committee of Bangladesh Agricultural University, Mymensingh.

2. Materials and methods

Weight gain (g) = final weight (g) - initial weight (g)

2.1. Experimental fish

Weight gain(%) =

2.2. Experimental design Both of the experiments were conducted with three treatments, each with three replications. Three different photoperiods were used, such as 6 h of light and 18 h of dark (06L:18D), 12 h of light and 12 h of dark (12L:12D), and 18 h of light and 6 h of dark (18L:06D). 2.3. First experiment: effects of light on growth performances of juvenile rohu This experiment was conducted during August to September 2016. In this experiment, nine glass aquaria (Length × width × height = 75 cm × 45 cm × 45 cm) were set on a cemented table in the Wet Laboratory, Department of Fisheries Management, BAU, Mymensingh. Each aquarium was filled with 100 L of tap water. The vertical sides of the glass aquaria were covered with black polythene sheets and the upper sides were exposed to the fluorescent light. During the maintenance of dark period, the upper side of each aquarium was also covered by black polythene in such a way that does not hamper aeration but hinders the entrance of day light or the illumination from other treatments. The aquaria were provided with filtration cum aeration device (Sebo-aquarium internal filter WP-850 F) for self-cleaning and aeration. The fluorescent light (LED light-40W, BLAZE, India) was hanged from the roof with a rope and maintained 45 cm distance from water. Before start of the experiment the juveniles were stocked with a stocking density of 20 fish per aquarium and maintained natural photoperiod conditions for 15 days. Then the fishes were exposed to above mentioned three different photoperiod conditions for 30 days. Photoperiod was controlled using fluorescent light with a constant intensity of 1200 lx (LED light-40W, BLAZE, India). The fish were fed a commercial diet containing 35% protein at the rate of 5% of the body weight per day, half at 9:00 am and the other half at 5:00 pm. Fish were sampled from each aquarium after 7, 15 and 30 days of exposure. 2.4. Analysis of growth performance The fishes from each aquarium were counted and weighed after 7, 15 and 30 days exposure to different photoperiod conditions. Growth parameters such as, weight gain, percent weight gain, specific growth rate (SGR), feed conversion ratio (FCR) and survival rate were calculated according to the following formulae:

In August 2016, a total of 300 apparently healthy 30 days old active juveniles of rohu were collected from the Field Laboratory Complex of the Faculty of Fisheries, Bangladesh Agricultural University, Mymensingh, Bangladesh. The average weight and total length of fish were 2.39 ± 0.35 (1.40–4.92) g and 6.67 ± 1.23 (5.1–7.6) cm, respectively. The collected juveniles were divided into two groups. In one group, 200 juveniles were acclimatized in the laboratory for 15 days. These juveniles were used to study the effects of light on growth performance in the first experiment. The juveniles were fed commercial diet containing 35% crude protein (Mega Fish FeedsLtd., Bangladesh) twice daily at 9 a.m. and 5 pm up to satiation. In another group, 100 juveniles were stocked in a research pond and reared for 2.5 years in natural conditions following carp polyculture system practiced in India and Bangladesh (Alam et al., 2002). These fishes were used to study the effects of light on reproductive function in the second experiment. The experimental procedures followed the guidance approved by the animal

final weight(g)- initial weight(g) × 100 initial weight(g)

Specific growth rate (SCR%) =

ln final weight(g)- ln initial weight(g) number of days

× 100 Feed conversion ratio (FCR) = Feed given (dry weight) / Body weight gain (wet weight)

Survival rate (%) =

Final no. of harvested fish × 100 Initial no. of fish - Sampled no. of fish

2.5. Blood sampling and measurement of hemato-biochemical parameters On each sampling day, the sampled fishes (n = 6) were anesthetized with clove oil (5 mg/L) immediately after collection and blood samples were collected from the caudal vein using heparinized plastic syringe. 2

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Table 1 Changes in water quality parameters (mean ± SD) recorded in different sampling dates at different photoperiods. Water quality parameters

Temperature (ºC) Dissolved Oxygen (mg/L) Free CO2 (mg/L) pH Total Alkalinity (mg/L)

Photoperiods (h) (Light : Dark)

Days of exposure 7

15

30

06 12 18 06 12 18 06 12 18 06 12 18 06 12 18

31.0 ± 0.05 31.0 ± 0.05 31.0 ± 0.05 6.20 ± 0.15 7.30 ± 0.13 6.20 ± 0.11 10.0 ± 0.05 10.0 ± 0.06 6.0 ± 0.04 7.80 ± 0.05 7.90 ± 0.07 7.50 ± 0.06 144.0 ± 12.2 110.0 ± 13.5 114.0 ± 13.12

29.0 ± 0.05 29.0 ± 0.05 29.0 ± 0.05 6.10 ± 0.12 6.20 ± 0.15 5.10 ± 0.25 8.0 ± 0.07 12.0 ± 0.02 8.0 ± 0.05 8.10 ± 0.02 7.50 ± 0.05 7.00 ± 0.04 102.0 ± 7.5 104.0 ± 6.6 110.0 ± 9.5

28.0 ± 0.05 28.0 ± 0.05 28.0 ± 0.05 5.90 ± 0.15 6.00 ± 0.11 4.90 ± 0.17 12.0 ± 0.05 10.0 ± 0.04 12.0 ± 0.03 7.80 ± 0.07 8.30 ± 0.02 8.00 ± 0.05 124.0 ± 13.1 112.0 ± 12.2 118.0 ± 11.3

:18 :12 : 06 :18 :12 : 06 :18 :12 : 06 :18 :12 : 06 :18 :12 : 06

least three fish were analyzed from each treatment group. From each fish, three slides were prepared and from each slide 2000 oocytes were counted and classified. Subsequently, the photographs of the oocytes were taken using an ISCapture 3.9.0.601 versions and measured using the following formula: Oocyte diameter (mm) = (long axis length + short axis length)/2.

Collected blood was gently pushed into a sterilized microfuge tube containing anticoagulant (20 mM EDTA). Blood samples were mixed gently and discarded when encountered difficulty in taking them or clots seen in the vial during examination at the laboratory. Whole blood withdrawal process took less than one minute per fish which was considered important to avoid stress eff ;ects in order to minimize any error in normal blood values. The red blood cells (RBCs) and white blood cells (WBCs) were counted using an improved Neubauer Hemocytometer under a light microscope. Hemoglobin (Hb) levels (g/dL) and blood glucose levels (mg/dL) were measured using hemoglobin strips and glucose strips, respectively in a digital EasyMate® GHb, blood glucose/hemoglobin dual-function monitoring system (Model: ET- 232, Bioptik Technology Inc. Taiwan 35,057).

2.7. Water quality parameters In both of the experiments, the water temperature, dissolved oxygen (DO), pH, free CO2 and total alkalinity were measured on each sampling day. Temperature, DO and pH were measured using a mercury thermometer, DO meter (Model DO5509, Lutron, made in Taiwan) and portable pH meter (Model number- RI 02895, HANNA Instruments Co.), respectively. The free CO2 of water was determined by titrimetric method using phenolphthalein indicator and 0.0227 N NaOH titrant. Total alkalinity of water was determined by titrimetric method using methyl orange indicator and 0.02 N H2SO4 titrant.

2.6. Second experiment: effects of light on reproductive function of matured female rohu This experiment was conducted during April to May 2019 in three cemented cisterns (Length × width × height = 2 m × 2 m × 0.75 m), each with 1000 L water. Fifteen female fishes were stocked in each tank and reared for 30 days. The cisterns were provided with filtration cum aeration device (Sebo-aquarium internal filter WP-850 F) for selfcleaning and aeration. The fluorescent light was hanged from the roof with a rope and maintained 45 cm distance from the water. During the maintenance of dark period, the upper side of each cistern was covered by black polythene in such a way that does not hamper aeration but hinders the entrance of day light or the illumination from other treatments. At the beginning of the experiment, six female rohu fishes were sacrificed to examine the initial condition of gonad by measuring GSI and histological studies which served as initial control sample at day 0 (IC). From each photoperiod treatment, fishes (n = 6) were sacrificed on 15 and 30 days of exposure. The gut of the sampled fishes were opened using a pair of fine scissors and the ovaries were removed carefully, weighed using an analytical balance (0.1 mg) and then fixed them by immersion in Bouin’s solution. The gonadosomatic index (GSI) of the sampled fishes was calculated using the formula: GSI = (gonad weight /body weight) × 100. The gonad samples were processed routinely and embedded in paraffin wax. The embedded gonads were sectioned at 5−10 μm thickness by a sledge microtome machine and then the sections were stained using hematoxylin and eosin (H&E). The slides were examined under a light microscope (MCX 100) which was connected to a video camera (AmScope 1000). Vitellogenic, previtellogenic and immature oocytes proportions were established. At

2.8. Data analysis Values are expressed as means ± standard deviation (SD). The normality and homogeneity of variance test have been done before statistical analyses in all groups of data. Two-way analysis of variance (ANOVA) was carried out to test the statistically significant difference among the different photoperiod conditions and the days of exposure. Statistical significance was set at p < 0.05. Statistical analyses were performed using PASW Statistics 18.0 software (IBM SPSS Statistics, IBM, Chicago, USA). 3. Results 3.1. Water quality parameters The values of the measured water quality parameters, such as water temperature (⁰C), dissolved oxygen (mg/L), free CO2 (mg/L), pH and total alkalinity (mg/L) are presented in Table 1. All the parameters were found to be within the acceptable ranges for fish culture in all treatments. No distinct changes were observed in any water quality parameters irrespective of any photoperiod conditions in both of the experiments (data of the second experiment are not presented). 3.2. Effects of photoperiod on growth performances of rohu The growth performance and feed utilization of L. rohita juveniles 3

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Table 2 Effects of photoperiods on growth rates and feed utilization efficiency in rohu Labeo rohita fingerlings. Photoperiod (Light:Dark) 06:18 12:12 18:06

Initial Weight (g) a

2.41 ± 0.35 2.44 ± 0.27a 2.43 ± 0.72a

Final Weight (g) ab

6.21 ± 0.11 7.60 ± 0.08b 5.32 ± 0.39a

Weight gain (g)

% Weight gain

ab

SGR (%/day) a

3.8 ± 0.27 4.54 ± 1.25b 2.99 ± 1.21a

162.41 ± 3.66 215.50 ± 6.60b 128.05 ± 9.30a

FCR

a

2.39 ± 0.09b 1.90 ± 0.09a 3.19 ± 0.09b

1.47 ± 0.07 1.78 ± 0.04b 1.28 ± 0.05a

SGR=specific growth rate and FCR=feed conversion ratio. Values in a column with different alphabetical superscripts are significantly (p < 0.05) different. All values are expressed as mean ± SD.

Fig. 1. Weekly growth increments of rohu Labeo rohita exposed to different photoperiod regimes. Values accompanied by different letters are significantly different (p < 0.05) among the treatments. Asterisk (*) indicates significant (p < 0.05) difference among days of exposure. All values are expressed as mean ± SD (n = 6).

Fig. 2. Gonadosomatic index (GSI) of rohu Labeo rohita before and after exposition to different photoperiods, during the 30 days of experiment. Values accompanied by different letters are significantly different (p < 0.05) among the treatments. Values with different numeric superscripts are significantly (p < 0.05) different among days of exposure. All values are expressed as mean ± SD (n = 6).

after exposure to three photoperiod conditions are shown in Table 2. The highest (p < 0.05) final weight gain, percent weight gain and SGR of L. rohita juveniles were in 12L:12D condition than those of in 18L:06D conditions. Feed conversion ratio differed significantly among the treatments and 12L:12D treatment showed the lowest (p < 0.05) FCR among the three treatments (Table 1). The weekly increments of growth are shown in Fig. 1. When the growth increments were compared among days of exposure, the growth in the day-15 was significantly higher than those in the day-7 in 06L:18D and12L:12D conditions. On the other hand, the growth in the day-30 was significantly higher than those in the day-7 in all the treatment groups.

fishes treated with longer photoperiod (18L:06D) compared to two other treatments after 7 days of exposure (Table 3), whereas after 15 and 30 days of exposure, no significant change was observed among the treatments (Table 3). The RBC numbers decreased significantly (p < 0.05) in 18L:06D treatment than those in 06L:18D but not those in 12L:12D after 7 days of exposure (Table 3) and did not show any significant difference among the three treatments after 15 and 30 days of exposure. The number of WBC increased significantly (p < 0.05) in prolonged photoperiod after 7 days of exposure (Table 3) but after 15 and 30 days of exposure the values of WBC did not differ significantly (Table 3). Blood glucose levels were significantly (p < 0.05) increased

3.3. Effects of photoperiod on hemato-biochemical parameters The hemoglobin levels were significantly (p < 0.05) decreased in

Table 3 Changes in hemato-biochemical parameters of rohu Labeo rohita fingerlings after exposure to different photoperiods. Hematological parameters

Photoperiods (h) (Light : Dark)

Days of exposure 7

Hemoglobin (g/dL) RBC (×106 /mm3) WBC (×103 /mm3) Blood glucose (mg/dL)

06 12 18 06 12 18 06 12 18 06 12 18

15 b

:18 :12 : 06 :18 :12 : 06 :18 :12 : 06 :18 :12 : 06

9.03 ± 0.18 8.93 ± 0.10b 8.13 ± 0.38a 1.69 ± .024b 1.42 ± 0.05ab 1.28 ± 0.04a,A 1.49 ± 0.06a,A 1.54 ± 0.12ab,A 1.77 ± 0.13b,A 61.00 ± 9.04a,A 70.83 ± 8.97ab,A 80.00 ± 7.52b,A

30 a

8.83 ± 0.68 9.16 ± 0.74a 9.60 ± 0.44a 1.63 ± 0.15a 1.69 ± 0.10a 1.67 ± 0.10a,B 1.23 ± 0.09a,B 1.20 ± 0.10a,B 1.34 ± 0.05a,B 71.50 ± 6.89a,A 62.00 ± 4.52a,A 78.20 ± 5.22a,A

9.67 ± 4.73a 8.67 ± 0.54a 9.50 ± 0.40a 1.74 ± 0.07a 1.65 ± 0.02a 1.65 ± 0.12a,B 1.19 ± 0.06a,B 1.18 ± 0.12a,B 1.27 ± 0.16a,B 93.33 ± 5.82a,B 83.00 ± 6.89a,B 97.67 ± 4.73a,B

RBC= red blood cell and WBC=white blood cell. Values of a single hemato−biochemical parameter in a column with different alphabetical (small letters) superscripts are significantly (p < 0.05) different. Values with different alphabetical (capital letters) superscripts in a row differ significantly (p < 0.05) among days of exposure. All values are expressed as mean ± SD (n = 6). 4

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Fig. 3. Histology of ovarian tissue of rohu Labeo rohita exposed to different photoperiod regimes and at initial condition. IMO - immature oocytes, PVO - previtellogenic oocytes and VO - vitellogenic oocytes.

in prolonged photoperiod treatment after 7 days of exposure (Table 3). On the other hand, after 15 and 30 days of exposure blood glucose levels showed no significant difference among the treatments (Table 3).

largest size and highest number of vitellogenic oocytes (Fig. 3).

3.4. Effects of light on reproductive functions of rohu

Photoperiod is known to affect various physiological processes in fish similar to some other environmental factors. Although fish showed no mortality in three different photoperiod regimes in the present study, we observed photoperiod induced distinct changes in growth, hemato-biochemical indices and gonadal development (GSI and gonadal maturation), indicating the important role of light in the physiology of rohu Labeo rohita, a sub-tropical fish species. In this study, prolonged photoperiod (18L:06D) resulted poor growth performance in rohu fingerlings. Increased light phases reduced metabolic rate in rockfish, Sebastes diploma (Boehlert, 1981) supported the present findings. On the other hand, it has been demonstrated that longer photoperiod have positive effect on growth performance in Atlantic salmon (Saunders et al. (1985)), Atlantic cod (Folkword and Ottera, 1993), turbot (Imsland et al., 1995, 1997), haddok (Trippel and Neil, 2002) and tilapia (El-Sayed and Kawanna, 2004). The effects of photoperiod might have varied due to the physiological condition of the concerned species and its habitat. One possible reason for the

4. Discussion

After 15 days of exposure, the mean GSI values had no significant change and no significant difference among treatments, whereas after 30 days of exposure the mean GSI values increased and varied significantly (p < 0.01) among the treatments. The highest GSI was recorded in the 06L:18D group followed by the 12L:12D group (Fig. 2) at the end of the experiment. On the other hand, the GSI mean value in the fishes exposed to 18L:06D was almost same with the value found at the start of the experiment. Similarly, histological analyses revealed significant differences (p < 0.01) among treatments in oocyte size and the proportion of previtellogenic and vitellogenic oocytes (Figs. 3 and 4) at the end of the experiment. The fishes of 18L:06D photoperiod showed absolutely immature gonads with the smallest oocyte size (Figs. 3 and 4). The 12L:12D photoperiod advanced gonad development of the fishes (Fig. 3) but presented an intermediated proportion of vitellogenic oocytes. The fishes of 06L:18D photoperiod showed the 5

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situation stored intracellular glycogen is used due to depression of respiratory metabolism. Consequently, the hyperglycemic hormone is released for the degradation of glycogen leaking glucose into the blood which causes hyperglycemia (Banaee et al., 2011). However, in the later stages of exposure in the present study, blood glucose levels showed no distinct changes irrespective of different photoperiod conditions. It may be inferred that gradual adaptation of fish to the light intensity made this possible in the later stages of the treatment with prolonged photoperiod. Nevertheless, the growth of fishes under this treatment remained lower when compared with the fishes under the treatments with neutral photoperiod. This warrants further study with the analyses of blood cortisol level and secretion and synthesis of growth hormone with much longer exponential period. The present experiment showed that photoperiod treatments also affect reproductive functions of rohu. In prolonged photoperiod (18L:06D) treatment, fish did not respond to gonadal development, observed similar GSI and proportion of vitellogenic oocytes as initial values throughout the experimental period. These findings are well supported by those found in European perch, Perca fluviatilis (Jourdan et al., 2000), Atlantic cod, Gadus morhua (Hansen et al., 2001; Taranger et al., 2006) and Nile tilapia, Oreochromis niloticus (Rad et al., 2006), where prolonged photoperiod caused negative effect on reproductive functions. On the other hand, there was a gradual progress in development of gonadal maturation in terms of GSI and proportion of vitellogenics oocytes in 12L:12D treatment, indicating neutral effect of this light-dark period on reproductive functions of the fish. Similar results were also found in tilapia, Oreochromis niloticus where under 12L:12D treatment, the tilapia spawned in a normal way (Biswas et al., 2002). In the case of prolonged darkness (06L:18D), a stimulatory response was observed in terms of GSI and gonadal histology. The mean GSI, oocyte diameter and vitellogenic oocyte proportions were highest in this treatment indicating stimulation of vitellogenesis. Our results are supported by the studies on Perca fluviatilis (Jourdan et al., 2000), Oreochromis niloticus (Rad et al., 2006) and Cheirodon interruptus (García et al. (2019)). In these fish species, ovarian maturations were enhanced with increasing darkness. However, several studies reported that prolonged darkness has delaying effects on ovarian maturation, while prolonged photoperiod has accelerating effects on ovarian maturation (Duston and Bromage, 1986; Hansen et al., 2001; Zhu et al., 2014). The variation of the effects of photoperiod may depend on the physiological condition of the concerned species and their habitat. Rohu (L. rohita) is the natural inhabitant of the freshwater sections of the rivers. The spawning season of this fish species generally coincides with the southwest monsoon (Talwar and Jhingran, 1991). Spawning takes place in flooded rivers during rainy and cloudy weather in absence of shiny sunlight.

Fig. 4. (A) Oocyte diameter (mm) and (B) percentage (%) proportion of immature, vitellogenic and previtellogenic oocytes at the end of the experiment after exposition to different photoperiods for 30 days.

retardation of growth performance of our tested species rohu in longer light phase is due to the sensitivity of this species to the prolonged light or the negative alteration of different stress indicators. Deviation of results from different study may be related to the differences in species, culture systems, fish size, sex ratio and light type and intensity. Nevertheless, more study is needed to authenticate the effects of photoperiods on rohu in different culture conditions. In several studies photoperiod manipulation induced a significant increase in stress responses in fish (Leonardi and Klempau, 2003). For example, cortisol level increased in the pacamã catfish exposed to an extended light period (Kitagawa et al., 2015). In the fishes treated with prolonged photoperiod (18L:06D), the Hb level decreased significantly during early days of experiment, but later on with the increase of the experimental days, the Hb level did not show any significant change irrespective of any treatments. Similar result was recorded by Bani et al. (2009) in juvenile great sturgeon Huso huso. In the present study, adaptation of fishes to chronic photoperiod regimes may be supported by the presence of similar level of Hb in fishes of all the three treatments. Similar negative pattern of changes in longer light phase also observed in case of erythrocyte numbers. RBCs decreased significantly in prolonged light periods compared to other two photoperiod regimes possibly due to highest stress response occurred in 18L:06D condition. The number of erythrocyte decreases in the blood of freshwater bream, Abramis brama in response to stress (Martem-yanov, 1995) supported the results of the present study. Increase of leukocyte numbers in the prolonged light regime during the first few days of exposure is possibly due to the initial stress response produced by the fish, and with gradual adaptation of fishes in the extended days of exposure showed no distinct changes among the treatments. Initially prolonged photoperiod (18L:06D) significantly increased blood glucose levels. Chronic stress generally results in high levels of cortisol, lactate and glucose in fish (Pottinger et al., 1999). Similar phenomena were observed in fishes exposed to extreme photoperiods in the present study. This increase may be due to the transformation of glycogen into glucose to fulfil their higher energy requirements under stress caused by longer photoperiod (Winkaler et al., 2007). In this

5. Conclusion The present study confirms that extended artificial light regime negatively affects growth performance, major hematological indices and reproductive functions of rohu, Labeo rohita. These findings are the first step with some baseline information for the development of optimum photoperiod regimes for rohu and other Indian carps through further research. Declaration of Competing Interest The authors have no conflict of interests. The authors themselves are responsible for the content of the paper. Acknowledgment We are grateful to Prof. Dr. Mohammod Kamruj Jaman Bhuiyan, Department of Agricultural Statistics, Bangladesh Agricultural 6

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University, Mymensingh, 2202, Bangladesh for supporting statistical analysis of the data. This study was supported by a grant (2017/282/ BAU) from Bangladesh Agricultural University Research System to the corresponding author which is gratefully acknowledged.

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