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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Photoperiod and growth in fish (2001)
Auteurs :	G. Boeuf, Auteur ; J. Falcon, Auteur
Type de document :	Article
Dans :	Vie et Milieu - Life and Environment (51(4) décembre 2001)
Article en page(s) :	247-266
Langues:	Anglais
Résumé :	Light is a complex external and ecological factor whose components include color spectrum (quality), intensity (quantity) and photoperiod (periodicity). An aquatic environment has peculiar and extremely variable characteristics. Moreover, "receptivity" of fish to light chancres profoundly from one species to another and, within the same species, from one developmental stage to another. The present review focuses on the periodic aspect of light supply and its consequences on fish growth. Not only growth, but virtually all biochemical processes, physiological functions and behaviors are rhythmic in nature and synchronized by the 24 h light/dark (L:D) cycle. Available data indicate that fish growth follows a pattern which varies as a function of variations in daylength. Generally, larvae need a minimal threshold intensity to be able to develop normally and grow. This is probably related to the aptitude to localize, catch and ingest prey, and long daylength improves larval rearing quality. The synergistic effect of "food availability and daylength" appears to be determining at this stage. Older fish (marine and salmonid species), also react to photoperiod treatments and long daylength stimulates growth. The most studied species is the Atlantic salmon, which is very sensitive to changes in photoperiod, in fresh and seawater, and particularly during the parrsmolt transformation. Flatfish appear as a noticeable exception because growth was not affected by photoperiod in several cases in the species investigated. The question raises to know how are the effects of photoperiod mediated? Early physiological studies suggested a role for the pineal gland. The fish pineal organ is a light sensor. Each of its cone-type photoreceptor cells contains a circadian clock, synchronized by the 24 h light: dark cycle which, in turn, produces rhythmic messages. Actually, the pineal gland resembles a very simple retina with only very few, inter-neurons. Pineal and retinal photoreceptors produce the "time-keeping molecule" melatonin. Retinal melatonin is used and metabolized in situ, whereas pineal melatonin has neurohormonal properties. The present paper reviews the information regarding the fish melatonin generating system and melatonin receptors. We also discuss recent evidence indicating the hormone might affect fish growth hormone secretion either directly (on the pituitary) or indirectly (hypothalamus). Melatonin may also influence thyroid hormone metabolism as well as food ingestion, two other factors that affect growth. Future studies combining "classical" physiological approaches in vitro and in vivo) together with pharmacological and molecular approaches should provide insights into the mechanisms underlying the control of fish (growth by photoperiod. This studies have great potential interest for aquaculture because they should provide indication on the best photoperiod conditions for optimizing fish "natural" growth all year long.
En ligne :	https://wwwphp.obs-banyuls.fr/Viemilieu/images/Archives_1950-2002/vie-et-milieu/VOLUME_2001_51_fasc4.pdf

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