Environmental Modulation of Annual and Daily Rhythms Associated with Reproduction in Teleost Fishes

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Abstract
Both temperate and the cold water teleosts are seasonal breeders and production of young fish occurs at an optimal time of the year thereby maximizing survival. Teleost reproductive activity can be divided into annual periods of prespawning, spawning, and postspawning gonadal events which correlate with wave-like endocrine fluctuations. Long-term (circannual) endocrine patterns in the hypothalamo-pituitary-gonad axis (HPG) are observed with maximum hormone levels occurring with spawning and minimal values relating to gonad regression during the postspawning period. Recently, short-term (circadian) endocrine rhythms have also been described. During the period of gonadal recrudescence the basal gonadotropin (GtH) level rises and daily variations of GtH are maximal. Light and temperature variations primarily cue seasonal gonad development in teleosts although the relative importance of these and other environmental triggers differs among various fish species. Clearly HPG activity is influenced by environmental conditions. Temperature has a direct effect upon circulating GtH levels but other physiological mechanisms are implicated as well. Endocrine organ sensitivity undergoes annual variations and cyclic hormone release may respond accordingly. But, daily fluctuations in gonadal responsivity to GtH treatment suggests that phasing of endocrine variations may play an important role and should be considered in future studies of environmental regulation of fish reproductive cycles.Key words: teleost fishes, hormone rhythms, environmental modulation, reproduction

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