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Acid -base disturbances caused by environmental factors and physiological events including feeding have been well documented in several fish species, but little is known about the impact of dietary electrolyte balance (dEB). In the present study, we investigated the effect of feeding diets differing in dEB (2100, 200, 500 or 800 mEq/kg diet) on the growth, nutrient digestibility and energy balance of Nile tilapia. After 5 weeks on the test diet, the growth of the fish was linearly affected by the dEB levels (P, 0·001), with the lowest growth being observed in the fish fed the 800 dEB diet. The apparent digestibility coefficient (ADC) of fat was unaffected by dEB, whereas the ADC of DM and protein were curvilinearly related to the dEB levels, being lowest and highest in the 200 and 800 dEB diets, respectively. Stomach chyme pH at 3 h after feeding was linearly related to the dEB levels (P,0·05). At the same time, blood pH of the heart (P,0·05) and caudal vein (P,0·01) was curvilinearly related to the dEB levels, suggesting the influence of dEB on postprandial metabolic alkalosis. Consequently, maintenance energy expenditure (MEm) was curvilinearly related to the dEB levels (P,0·001), being 54 % higher in the 800 dEB group (88 kJ/kg 0·8 per d) than in the 200 dEB group (57 kJ/kg 0·8 per d). These results suggest that varying dEB levels in a diet have both positive and negative effects on fish. On the one hand, they improve nutrient digestibility; on the other hand, they challenge the acid -base homeostasis (pH) of fish, causing an increase in MEm, and thereby reduce the energy required for growth.

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RÉSUMÉLa mise au point d'un test de sexage des esturgeons sibériens, basé sur la détection de la vitellogénine (VTG) plasmatique, a permis de mettre en évidence cette protéine complexe oestrogéno-dépendante dans le plasma des poissons femelles, mâles et immatures de cette espèce. L'hypothèse d'une contamination oestrogénique d'origine environnementale a été confirmée par un test basé sur l'administration de différents aliments. Chez les témoins sans oestrogènes, la VTG disparaissait en quelques Semaines du plasma des poissons. Avec un aliment du commerce ou un aliment à base de soja, les taux de VTG augmentaient de manière significative. Plus tard, un effet oestrogénique du soja administré par voie orale, quoique faible, a été mis en évidence in vivo chez la truite.

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