Original article 659
The effect of water quality on the reproductive performance of the silver carp Mostafa A. Mousa, Mohamed F. Kora and Noha A. Khalil Fish Reproduction Laboratory, National Institute of Oceanography and Fisheries, Alexandria, Egypt Correspondence to Mostafa A. Mousa, PhD Fish Reproduction Laboratory, National Institute of Oceanography and Fisheries, Qayet Bay, 21411, Anfoushy, Alexandria, Egypt Tel: +20 100 108 2930; fax: +20 348 01174; e-mail:
[email protected] Received 15 May 2015 Accepted 22 October 2015 The Egyptian Journal of Histology 2015, 38: 659-669 58 (1555-2015)
Introduction Eggs of the silver carp (Hypophthalmichthys molitrix) absorb water after release from the female, causing them to become turgid and increase substantially in size. Prior observations have suggested that silver carp eggs may swell and burst in soft waters. Identifying water hardness threshold values is needed for the successful incubation of silver carp (H. molitrix) eggs. Aim of the work The present experiments were designed to determine the effect of water quality on the reproductive performance of the silver carp. Material and methods In this study, we tested the effect of water hardness from three sources – tap, ElSerw Fish Farm, and Lake Manzalah – on silver carp egg enlargement and hatching success. Results Water chemistry variables, including temperature, dissolved oxygen, and pH, were consistent among tests. The observed results, during induction of spawning, indicated that the application of human chorionic gonadotropin, at a dose of 1500 IU/kg, was effective for inducing the final stages of maturation, ovulation, and spawning in silver carp in ripe spawners. Water hardness during egg hardening had a significant effect on the hatching success of eggs originating from the same parent fish. Hatching success rates were highest for eggs hardened in the hardest water treatment (132 mg/l) and lowest for eggs hardened in the softest water treatment (45 mg/l). Both embryonic and larval development have been described, and there was no difference in the tested waters. Conclusion Our research suggests that water hardness of 45–132 mg/l is suitable for the reproduction of silver carp. Keywords: embryonic development, fish larvae, Hypophthalmichthys molitrix, spawning, water quality Egypt J Histol 38: 659-669 © 2015 The Egyptian Journal of Histology 1110-0559
Introduction
The silver carp (Hypophthalmichthys molitrix) has valuable flesh, which is consumed as an important form of protein in many countries, especially in tropical regions. In Egypt, silver carp fingerlings are reared successfully in freshwater polyculture ponds, together with thin-lipped grey mullet (Liza ramada), common carp (Cyprinus carpio), grass carp (Ctenopharyngodon idella), and Nile tilapia (Oreochromis niloticus). However, culture of silver carp is largely dependent on the availability of their fry. Like females of many commercially important fishes, the silver carp does not undergo final oocyte maturation, ovulation, or spawning in captivity. Consequently, it is desirable to develop and establish practical techniques for artificial propagation. 1110-0559 © 2015 The Egyptian Journal of Histology
The problems of standardization and cost of hormone preparations are partly solved with the use of mammalian gonadotropin preparations. Two are available in the purified form, human chorionic gonadotropin (HCG) and pregnant male serum. The dosage used varies widely between species and may be related to how closely HCG and pregnant male serum resemble the endogenous gonadotropin in each species [1]. HCG has been successfully used in most carp species [2,3]. The effect of water chemistry on the hatching and viability of major Chinese carps is of great interest to aquaculture researchers. Premature hatching of Chinese carp (silver and bighead carps) eggs and poor survival of the larvae when eggs are incubated in soft water can be a 10.1097/01.EHX.0000473750.89193.b9
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660 Egyptian Journal of Histology
major problem for fish farmers [4]. Carp eggs, like those of other freshwater fish, are hyperosmotic to their medium [5–8]. The membrane of a fertilized carp egg absorbs water and swells rapidly. If the incubating medium has a lower ionic concentration (hypo-osmotic) than the egg, premature bursting of the egg from excessive water absorption may occur. It is therefore necessary to control the ionic concentration of the medium to minimize premature bursting. Conversely, a hyperosmotic medium will prevent proper swelling of the egg or even dehydrate and shrink the egg, causing spinal damage to the larvae. The major cations of natural water and body fluids are calcium, magnesium, sodium, and potassium. Of these, calcium and sodium are normally the dominant ions [9], but the bivalent cations, principally calcium and magnesium, contribute significantly to the total hardness of natural water [10]. Although most of the physiological calcium in fish is bound to proteins, it is the free calcium ions and not the protein-bound calcium that contribute to osmotic pressure [11]. The calcium ions reduce the hydration of polar organic molecules by reducing the repulsive forces between anions of the chorion. It is this characteristic of calcium ions that allows closer packing of the membrane molecules and decreases membrane porosity [12]. If the incubating medium has a lower ionic concentration than the egg, the egg absorbs water, swells rapidly, and may burst during embryo development [6]. The osmotic gradient – the difference in total dissolved solid concentrations between eggs and water – may be more important in determining egg viability than water hardness, but water hardness is generally a good indicator of total dissolved solids [13]. The aim of the present study was to estimate the effect of water hardness levels of different waters on the reproductive performance of silver carp.
Materials and methods Study site
The present study was carried out in 2014 during the spawning season (May to July) at El-Matareyya Research Station, Dakahlia Province, Egypt. Induction of spawning
Before spawning, mature breeders of silver carp, at least 2 years old, with average weights ranging from 2000 to 3500 g, were kept in two ponds (each pond, 4200 m2). The males and females were separated since January into two different ponds at El-Serw Fish Research Station (freshwater habitat). They were fed daily to ensure good quality and quantity of eggs. The fish were fed a diet containing 25% crude protein, 5% lipid, 0.2% vitamins, and 0.2% minerals at a daily rate of 3% of their body weight. At the beginning of May, artificial spawning was carried out in 2000 l circular fibreglass tanks. The fish obtained from El-Serw Fish Research Station were anesthetized in a solution (40 mg/l) of clove oil (Sigma) before handling, according to the procedure followed by Mousa [14]. Mature female silver carp were selected on the basis of
the presence of a soft, swollen abdomen and protruding genital papillae. Ripe males, in which milt could be easily extruded by gentle pressure on their bellies, were used. In brief, selected breeders were transferred to spawning tanks equipped with constant running water and aeration (one female+two males/tank) for induction of spawning with HCG (pregnyl; Nile Co., Cairo, Egypt). The water temperature ranged from 27 to 29°C. Pregnyl was injected into the dorsal musculature of the fish, adjacent to the dorsal fin, as a priming injection at a dose of 500 IU HCG/kg body weight, followed 22 h later by a resolving injection of 1000 IU HCG/kg for both males and females. The used doses were calculated empirically depending on a series of preliminary experiments determining the optimal dose. When spawning occurred, some of the fertilized eggs were collected and transferred for hatching into glass funnels that were supplied with a current of running water. Incubation of eggs and embryonic development
The eggs were collected from each spawning tank approximately 2 h after the estimated time of spawning. The fertilization rate was calculated for each spawner using the following equation:
Fertilization rate(%)=
Number of fertilizedeggsof eggsample × 100. Totalnumber of eggsample
The fertilized eggs were incubated in the laboratory under normal conditions. During the experiment, the flow rate of water into the incubator funnels was 100% per hour. Before use, fresh water was filtered and treated with antibiotics. Both penicillin (10 IU/ml) and streptomycin (0.01 mg/ml) were added daily, which effectively reduced bacterial growth. To determine the common embryonic developments, 30 eggs were taken from each incubator every 30 min until the morula stage, and then at hourly intervals. Whenever an evident difference appeared during embryonic development, photographs were taken before or after the due time. After hatching, the hatching rate was calculated using the following equation:
Hatching rate(%)=
Number of hatched fryfor eggsample × 100. Number of fertilized eggsample
At the end of the experiments, the survival rate was calculated for each breeder as follows:
Survivalrate(%)=
Number of survived fryof sample × 100. Number of totalhatched fryfor thesample
The effect of water quality on embryonic and larval development
To point out the effect of water quality on silver carp development, the fertilized eggs were incubated under control conditions: in dechlorinated tap water and in both El-Serw Fish Farm and Manzalah Lake waters. The mean
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Effect of water quality on silver carp reproduction Mousa et al. 661
values of water chemistry during the experiment (from May to July) are recorded in Table (1). The morphology
of the embryos was examined daily, and deformation and mortality were recorded.
Table 1. The mean values of water physicochemistry during the experiment Determinant (ppm unless stated)
Tap water
El-Serw Fish Farm water
Lake Manzalah water
28
28
28
pH
7.4
7.8
7.9
Ca2+
34*
58
80
Temp. (°C)
2+
11*
34
52
Na
36.8*
255.3
425.5
K
5.07*
15.21
15.21
3
4
8
89.87*
366.51
580.71
Mg
PO4
Total dissolved solid Total hardness Conductivity (µS/cm) Salinity (‰)
45*
92
132
Soft
Fairly soft
Slightly hard
134.13*
547.03
865.26
0.09*
0.37
0.58
*Mean of tap water was significantly lower than that of both El-Serw Fish Farm water and Lake Manzalah water (P
