Effect of inland water salinity on growth, feed conversion efficiency and intestinal enzyme activity in growing grey mullet, Mugil cephalus (Linn.):: Field and laboratory studies

Two experiments were conducted to investigate the effect of inland water salinity on growth performance, feed conversion efficiency and intestinal enzyme activity in grey mullet. In experiment I, a 90 day monoculture of grey mullet at different salinity levels ( 0, 10, 15, 20 and 25 parts per thousand) was carried out. The fingerlings were stocked at 5000 per hectare and fed on a supplementary diet at 5% BW d(-1). This Study revealed that fish growth mean body weight (90.5 +/- 4.5 g) and mean length (21.6 +/- 0.4 cm), SGR (4.70%) and growth per day (0.99 g d(-1)) were significantly ( p < 0.05) enhanced in fish maintained at 10 parts per thousand salinity in comparison with other treatments. Nutrient levels, phytoplankton population, NPP and chlorophyll 'a' all decreased with an increase in salinity (> 10 parts per thousand). In addition, zooplankton populations increased with an increase in the salinity level. Most of the other hydrochemical characteristics remained at optimal levels in all other treatments. Fish weight gain showed a significant positive correlation with productivity indicating parameters viz. alkalinity ( r = 0.53), turbidity ( r = 0.62), NPP ( r = 0.75) and chlorophyll 'a' ( r = 0.46), clearly revealing that fish growth is also related to the trophic status of the ponds. In the second experiment ( Experiment II), mullet fry were exposed to five different salinity levels ( 10, 15, 20, 25 and 30 parts per thousand) and maintained for 70 days in the laboratory. Significantly ( p < 0.05) high growth, ( SGR and per cent increase in body weight), feed conversion efficiency and intestinal enzyme activity were observed in the group maintained at 10 parts per thousand salinity in comparison with other groups maintained at similar salinity levels. Carcass composition, muscle and liver glycogen levels were also significantly ( p < 0.05) affected by salinity changes.