| Abstract: | Tilapia hybrids of Oreochromis niloticus × O. aureus with a starting weight of 90 g were grown for 57 days in 18 floating net‐cages set up in a fishpond (1.8 ha × 3 m deep; water temperature around 27 °C) at the Aquaculture Research Station in Dor, Israel. They were fed pellets of a commercial diet which contained 8.1 ppm natural l ‐carnitine. This diet was then supplemented with 150 or 450 ppm of l ‐carnitine. The results show that at the end of the 8‐week feeding trail, l ‐carnitine supplementation did not influence weight gain. The subsequent histological characteristics of the pond fish were compared with those of a separated group of their siblings kept in an indoor tank in clean flow‐through water. Normal and contact epi‐microscopy, contact fluorescent microscopy and contact microfluorometry with various fluorescent probes revealed that the fish fed the diet with 150 ppm of supplementary l ‐carnitine showed the lowest permeability to fluorescein in gills, gut and skin epithelia, the highest activity in the system of active transport for organic anions, and the highest levels of the multixenobiotic resistance transporter (MXRtr) for lipophilic/amphiphilic xenobiotics. The MXRtr activity in the liver bile canaliculi and renal proximal tubules of this group of fish was much higher than that of the other groups except those from the indoor tank. The intralysosomal accumulation of neutral red in their head kidney macrophages was also significantly higher. The results suggest that a low level of l ‐carnitine enrichment provides several protective effects in fish reared under intensive pond‐culture conditions. |
