Fish like all animals, reproduce to maintain survival of the species. They must not only reproduce but they must reproduce when maximum reproductive success is possible (Lam, 1983; Whittier and Crews, 1987; Munro ,1990). Developmental and maturational events are dominated and coordinated by seasonal changes in photoperiod, temperature, rainfall etc. (Sanchez-vazquez et al., 2000). Photoperiod and temperature are generally considered the most important factors. Seasonal reproduction is influenced by biotic factors such as temperatures, water quality and photoperiod and biotic factors such as food availability, predators and pathogens being at optimum conditions (Bergman, 1987; Whittier and Crews, 1987; Hontela and Stacey, 1990; Munro, 1990; Sumpter, 1990). Seasonal activities often appear linked with the changes in environmental photoperiod and influence the seasonal activities (Vinod kumar and Follet, 1993).

Fenwick (1970 a) exposed goldfish Carassius auratus to various photoperiods at several different times of the year, long photoperiods stimulated gonadal maturation in Carassius but only during spring. A long photoperiod warm temperature regime also promotes sexual maturation in other fishes Carassius auratus (Kawamura and Otsuka, 1950).

Photoperiod is widespread among teleostean fish and has been studied in at least nine orders, in various species. Photoperiod may provide the go/no-go signal for seasonal dormancy as well as in migration, sexual maturation and associated physiology and behaviour in migratory fish. Fish with short gonadal maturation cycles usually require sequentially changing day lengths (Bromage et al., 2001).

Holmes et. al. (1994) demonstrated that the presence of photopeiodism and circannual rhythms in echinoderms indicates that photoperiodism and its regulation of circannual rhythmicity either appeared early in deuterostome evolution and were lost in cephalochordates, hag fishes, lampreys and cartilaginous fish, or evolved separately in echinoderms and bony vertebrates, where photoperiod provides pivotal go/no-go signals for the seasonal timing of life history events in teleost fish. Photoperiod may exert the dominant regulatory role of sexual cycles in many teleost fishes, day length changes in combination with various temperatures seem to be important in controlling reproduction in cyprinid and centraorchid fishes (De Vlaming, 1972a) the long photoperiods are required for the final stages of gonadal maturation in Phoxinus (Bullogh, 1939). The long photoperiods with warm temperature during winter and spring brought about sexual maturity in female goldfish (Kawamura and Otsuka, 1950).

Animals use the length of day or photoperiod to time their seasonal development reproduction migration and dormancy (Bradshaw and Holzapfel, 2007). Seasonal activities often appear linked with the changes in environmental photoperiod (Vinod kumar, 1986).

Photoperiod would regulate the axis photoreceptors, central nervous system, hypothalamus pituitary gonadotropin, ovarian estrogen secretion, oogonial proliferation and endogenous yolk formation. In some vertebrates photoperiod acts as a chief proximate factor in the regulation of seasonal reproduction. Photoperiod changes accurately and reliably around the year, so that animals can use this use for prediction of seasonal changes and accordingly programme their gonadal development (Fraile et al., 1989). When goldfish, Carassius auratus were exposed to various photoperiods at low temperatures several times in the year it was noticed that the long photoperiods stimulated gonadal maturation only in Spring, the effect of photoperiod on gonadal activity of goldfish, Carassius auratus vary with season (Fenwick, 1970 a).

Clocks have powerful endogenous components and daily light dark cycles entrain them to 24 hour(Vinod kumar et al.,2010;Chandrashekhar et al.,1985). Day length interacts with the endogenous daily photosensitive rhythms and regulates growth and development of gonads. There is also evidence of endogenous circannual rhythm. This causes evidence of photoperiodic response, there is also evidence of the endogenous rhythmicity regulating gonadal cycles in few species (Chandola et al.,1985;Vinod kumar et al.,2001).

It has long been assumed that annual rhythms in reproduction and other seasonal events are directly driven by seasonal proximate factors, particularly daylength (Gwinner,1986). This concept has been challenged in recent years and there is now a growing body of evidence that in many long lived species seasonally changing daylength interacts does not actively drive or induce reproductive development, or rather acts as a zeitgeber to entrain an endogenous circannual rhythm of reproductive function (Duston and Bromage, 1986). Annual reproductive cycle for many species of fishes have most frequently been described in terms of seasonal changes in the GSI or histological changes in the testis or ovary (Peter and Crim , 1979). The adaptive value of biological rhythms is tied at least in part to their being synchronized to the light phases of the external cycle, which in most cases in the light dark (LD)cycle of the environment, in fact a stable and daily change in light characteristics at dawn or dusk times serve as the most reliable indicator phase of the day (Vinod kumar et al.,2002; Chandrashekhar et al.,1985). By measuring (Photoperiod) organisms are able to measure the passage of time and coordinate their biological processes with favourable environmental conditions (Sumpter, 1990). Environmental cues, particularly photoperiod appear to have a more direct role than simply entraining the clock to calendar time (Chandola and Singh, 1981).

Recent advances in molecular biology have begun to clarify the role that light plays in the timings of biological functions at a daily and an annual level. This hypotheses is that light acts as an environmental cue or "Zeitgeber" that entertains endogenous oscillation of clock genes (Roenneberg and Merrow, 2003). Clock genes in turn regulate other genes that require rhythmic daily expression. A number of genes have been implicated in daily and circannual time keeping. These processes and the genes involved are not completely understood at present and their complexity varies across species (Dunlop, 1999). Because photoperiod has been shown to have a significant effect on maturation spawning time and development in salmonids, circadian rhythms genes are likely candidates for influencing the traits clock and is common to both invertebrates and vertebrates (Stanewsky, 2003). It is accepted that light dark cycle of environment synchronizes the biological clocks in most organisms which have been studied (Chandola et al.,1985; Thapliyal and Chandola , 1974).

Photoperiod entrains the rhythms via the circadian rhythm of melatonin secretion. Further only a portion of the annual photoperiodic cycle is needed to entrain the circannual reproductive rhythm; in this regard, photoperiodic input during spring and summer is especially crucial if thyroid hormones are absent during a restricted "Window" of time around the end of the reproductive period in winter. A hierachy of biological periodicities and multiple hormonal components of the hypothalamus pituitary axis thus contribute to the regulation and expression of the circannual reproductive rhythm (Stanewsky, 2003).

Davies and Bromage (1991) reported the seasonally changing photoperiod might have been expected to re-entrain the endogenous circannual rhythm after exposure to continuous light, thus adjusting spawning back to natural time.

Pevet (2000) demonstrated that circadian rhythms are fundamental features of all living organisms. The functional mechanism involved is built around the internal biological clock and hormone melatonin one of its critical components. The use of unusual light dark regimes and how they are interpreted by the animal will help researchers to determine if the circadian system is involved in measurement of photoperiodic time (Turek et al., 1984).

There are two types of skeleton photoperiods (Pitterdrigh, 1965). There are 'symmetrical' photoperiods in which each light pulse of the same duration and there are 'asymmetrical' photoperiods in which the light pulses are of different lengths although it is not known how bright a light pulse must be to stimulate a response in fish (Sumpter, 1990). However it has been shown that exposure to light of low intensity can be effective (Bergman 1987). When using skeleton photoperiods it is important without the second light pulse is non-stimulatory (Baggerman, 1990).

Data from catfish suggests that the eyes and the pineal contribute to synchronize the behavioural rhythms by a light dark cycle but that the eyes and the pineal contribute to synchronize the behavioural rhythms by a lights dark cycle but that other photpreceptors and oscillators also contribute (Vinod kumar et al.,2001; Chandola et al.,1985).

The environmental cue most likely to influence biological rhythm is photoperiod, biological rhythms have fundamental role privileging individuals with capacity of anticipation of favourable-unfavourable conditions (Marques, 2003).

Ovarian growth, maturation and egg laying is controlled by hormones which in turn are regulated principally by photoperiod (Stephens, 1952). In seasonally breeding fish species altered fecundity fertility and spawning intervals are associated with changes in environmental cues such as photoperiod (Koger, et al., 1999).

Photoperiod strongly affected the timing of puberty and sexual maturation (Norberg, et al., 2004). The photoperiod related information is transmitted, via the pineal hormone, Melatonin (Reiter, 1995).

In teleost fish, growth development and reproduction are influenced by daily and seasonal variations of photoperiod and temperature. Early in vivo studies indicated that the pineal gland mediates the effect of these external factors most probably through the rhythmic production of Melatonin (Falcon,e t al., 2003). Photoperiod has a pronounced effect on the teleost pineal. The functional relationship between the pineal and reproductive centers may also therefore be dependent on day length. Melatonin treatment inhibits the increase on gonadal size stimulated by long photoperiods in Carassius (Fenwick, 1970 a).

Rhythmic process of photosensitivity are involved in the modulation of the reproductive cycle in Rainbow trout and the circannual and circadian rhythms co-operate in the timing and entrainment of reproductive cycle (Duston, and Bromage, 1986). The annual gonadal cycle consists of alternating periods of recrudecence and regression.

In many fish species gonadal recrudescence and regression are to a large extent controlled by the length of photoperiod (Baggerman, 1972). Both photoperiod and temperature affected melatonin production in the senegasole transducing seasonal information and controlling annual reproductive rhythms (Vera et al., 2007).

In 1952 Stephens published a complex hypothesis of endocrine control of the female reproductive cycle of the crayfish Orconectes virilis. On the basis of eye stalk ablation, implantations of nervous tissue and histological preparations of ovarian tissue she concluded that four hormones controlled ovarian growth maturation and egg laying and that the hormones in turn regulated principally by photoperiod.

Studies suggest that genotype hormones and physiological conditions are equally important endogenous regulators of growth (Dutta 1994). Photoperiod would regulate the axis photoreceptor, pituitary gonadotropin, ovarian estrogen secretion, oogonial proliferation and endogonous yolk formation (Hansen et. al., 2001).

It was observed by Immelman(1963), that reproductive cycles are affected by exogenous and endogenous environmental cues. Gross et al.(1965), observed that fish growth and development is also improved by increasing the photoperiod. The circannual rhythms of hormones to be closely related with circannual variations in ambient temperature day length and gonadal steroids (Pavlidis et al., 2000).

This seasonal periodicity is believed to be entrained by environmental cues such as photoperiod and temperature (Leder, 2006). Many poikilothermic animals use photoperiod to time seasonal events such as migration and reproduction (Stephen, et al. 2000).

Modified photoperiod regimes can be used to advance or delay the spawning period and a year round supply of eggs can be ensured by replacing, increasing and decreasing components of the seasonally changing day length with periods of constant long and short day length (Taylor and Migand 2008). Reproduction in most fish is typically a seasonal process. Fish reproduction cannot be considered an exclusively annual phenomenon because spawning may also show daily rhythmicity (Meseguer et al. 2008). Photoperiodism is the ability of an organism to assess and use the day length as an anticipatory cue to time seasonal events in their life histories (Bradshaw and Helzafel, 2007).

Developmental and maturational events are dominated and coordinated by seasonal changes in photoperiod, temperature, food supplies, rainfall, etc. (Porter et al., 1998; Sanchez-Vazquez et al., 2000). Photoperiod and temperature are generally considered the most important factors in growth in fishes (Dutta, 1994). The effect of photoperiod on teleostean reproduction are mediated via retinal pathways and/or by extraretinal receptors. Electrophysical data confirm that the teleostean pineal organ is a photoreceptor (Hanya and Niwa, 1970).

The melatonin daily rhythms provide the organism with photoperiod related information and represents a mechanism to transduce information concerning time of day. Photoperiod and temperature affected melatonin production transducing seasonal information and controlling annual reproductive rhythms (Vera, 2007). Pineal melatonin production was clearly dependent on light perceived by eyes as opthalmectomy resulted in basal plasma melatonin levels during the dark period (Davie, et al., 2007).

References