No abstract

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&) was carried out. The fingerlings were stocked at 5000 per hectare and fed on a supplementary diet at 5% BW d À1 . 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 À1 ) were significantly (p < 0.05) enhanced in fish maintained at 10& salinity in comparison with other treatments. Nutrient levels, phytoplankton population, NPP and chlorophyll 'a' all decreased with an increase in salinity (>10&). 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&) 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& salinity in comparison with other groups maintained at similar salinity levels. Carcass composition, musc1e and liver glycogen levels were also significantly (p < 0.05) affected by salinity changes.

To investigate the effect of inland groundwater salinity on growth performance, feed conversion efficiency, nutrient retention and intestinal enzyme activity in milkfish, two experiments were conducted. In the first experiment (Expt I), a 100-day monoculture of Chanos chanos [mean body weight (BW): 2.2 g] at different salinities (0, 10, 15, 20 and 25&) was carried out in ponds fertilized with cowdung (about 10 000 kg ha )1 year )1 ) and poultry droppings (about 3000 kg ha )1 year )1 ). The fish were fed a compounded supplementary diet (containing 40% protein) at 5% BW day )1 . Studies have revealed that growth increased with each increase in the salinity level; the highest values in weight gain and energy assimilated were observed in ponds maintained at 25& salinity [weight: 322.2 g and specific growth rate (SGR): 8.3]. Highest values of condition factor (0.7) and exponential value (n) of the length-weight relationship (LWR; n ¼ 3.25) were also observed in ponds maintained at 25& salinity. Dissolved oxygen (DO), biological oxygen demand (BOD), pH and nutrient release remained at the optimal level during the culture period. High values of chlorophyll a, net primary productivity (NPP), phytoplankton and zooplankton population coincided with the highest values of alkalinity and turbidity in ponds maintained at 25& salinity. Multivariate analysis revealed a significant positive correlation of chlorides (r ¼ 0.91), conductivity (r ¼ 0.89) and hardness (r ¼ 0.96) with fish growth. Productivity indicating parameters viz. NPP (r ¼ 0.45), nitrate (r ¼ 0.94) and o-PO 4 (r ¼ 0.52) also showed a significant positive correlation with fish weight gain. In the second experiment (Expt II), milkfish (mean BW: 3.7 g) fry were exposed to different levels of salinity (0.0, 10, 15, 20, 25 and 30&), and maintained for 90 days in the laboratory. Significantly (P < 0.05) high growth (percentage increase in BW: 183.1 and SGR: 1.2), feed conversion efficiency (64.5%) and intestinal enzyme activity (protease 5.1, amylase 4.1 and cellulolytic 3.2) were observed in the group maintained at 25 ppt salinity in comparison with other groups similarly maintained at low or high salinity levels. Carcass composition, muscle and liver glycogen levels were also significantly (P < 0.05) affected by salinity changes. The significance of these findings is discussed in this paper.

Two experiments were conducted to investigate the effects of varying dietary protein (35-45%) and energy levels (17.34-19.44 kJ g )1 ) on the growth performance of milkfish (Chanos chanos) maintained under laboratory (experiment 1) and field conditions (Experiment 2) in inland saline groundwater. The results of experiment 1 (initial weight of fish: 0.25 g) revealed that, irrespective of the protein source (fish meal or processed full fat soybean), fish fed a diet containing 40% protein showed significantly (p < 0.05) higher growth in terms of live weight gain and specific growth rate, low feed conversion ratio [1.71 (fish meal) and 1.58 (soybean)], high nutrient retention (gross protein retention: 28.59 and 31.05%; gross energy retention: 24.23 and 26.04%), apparent protein digestibility (81.74 and 85.91%) and digestive enzyme activity (specific protease and amylase). An observation on the postprandial excretion of metabolites (N-NH 4 and o-PO 4 ) indicated significantly (p < 0.05) low levels of these metabolites in aquaria where the fish were fed 40% dietary protein irrespective of the protein source. The results of experiment 2 also indicate (initial weight of fish: 4.43 g) a significantly (p < 0.05) higher growth performance (weight gain, biomass, specific growth rate, growth day )1 and fish production) in ponds where the fish were fed 40% dietary protein. Irrespective of the experimental conditions (laboratory/field), feeding the fish higher dietary protein levels (beyond 40%) not only repressed growth performance but also affected proximate composition by lowering protein accumulation and energy assimilation. An investigation on the effects of feeds on water quality parameters revealedthat values for total alkalinity, NH 4 -N, NO 3 -N, turbidity, total dissolved solids and parameters indicative of productivity (chlorophyll a, net primary productivity and plankton population) increased significantly (p < 0.05) from treatments 1-3 (35-40% protein) and declined thereafter, in treatment 4 (42% dietary protein), indicating that water quality characteristics correlated well with fish growth. The results suggest that in order to obtain a high-yield in milkfish culture system the fish should be fed supplementary diets containing appropriate (40%) protein levels, especially when the stocking rates are high.

The present study attempts to assess the potential of artificial substrates to enhance fish production in inland saline groundwater ponds through periphyton production. Grey mullet, Mugil cephalus, was cultured for 100 days in ponds with substrate (treatment ponds) and without substrate (control ponds). To enhance the surface area, bamboo poles were used as substrate. The periphyton population, pigment concentration and hydrobiological characteristics of pond water were monitored. The studies revealed little difference in most of the water quality parameters observed in the two treatments. However, turbidity (27.0 ± 0.1-35.0 ± 0.1 Nephalo Turbidity Unit (NTU)), chlorophyll ÔaÕ (6.6 ± 0.6-7.6 ± 0.6 lg L )1 ), plankton population (phytoplankton 8.4 · 10 3 -9.4 ·10 3 numbers L )1 ; zooplankton 4.0 · 10 3 -5.1 · 10 3 numbers L )1 ) and NH 4 -N (2.0 ± 0.2-2.3 ± 0.1 mg L )1 ) were high in the treatment with no additional substrate; however, in the treatment with substrate the total Kjeldahl nitrogen (9.8 ± 0.8-10.8 ± 0.7 mg L )1 ) and o-PO 4 (0.1 ± 0.01-0.1 mg L )1 ) remained significantly (P < 0.05) higher. Highest periphyton biomass in terms of dry matter (DM) (0.8 ± 0.01-1.4 ± 0.01 mg cm )2 ), ash free DM (0.4 ± 0.0-0.6 ± 0.01 mg cm )2 ), chlorophyll ÔaÕ (3.1 ± 0.2-8.1 ± 0.8 lg cm )2 ) and pheophytin ÔaÕ (1.9 ± 0.4-3.9 ± 0.5 lg cm )2 ) was observed at 50 cm depth in ponds provided with additional substrate. Fifteen plankton genera showing periphytic affinity colonized the bamboo substrates. Fish growth (mean fish weight 524.3 ± 8.7 g and SGR 2.5 ± 0.1) was significantly (P < 0.05) higher in ponds provided with additional substrate compared with control ponds (387.2 ± 6.0). Length-weight relationship (LWR) (W ¼ cL n ) also showed that the exponential value (ÔnÕ) of length was high in substrate-supported ponds (n ¼ 2.36) in comparison with controls (n ¼ 1.09). These studies suggest that a periphyton-supported aquaculture system can be used successfully for the culture of herbivorous brackishwater fish species like M. cephalus in inland saline groundwaters and thus could contribute to the development of sound and sustainable aquaculture technology.