The fate of organic matter derived from small-scale fish cage aquaculture in coastal waters of Sulawesi and Sumatra, Indonesia

To determine the fate of organic matter derived from fish cage aquaculture, carbon and nutrient cycling processes in waters and sediments, and water circulation, were examined at two fish cage farms of different size and utilizing different food types, on Sulawesi and Sumatra islands, Indonesia. Mass balance models of C, N and P were constructed to assess the pathways of waste utilization and dispersal. At the smaller farm in Sulawesi (64.5 m² cage area; 3 t annual net production) using pelleted food, there was proportionally less waste (≈ 40% of total C input) than at the larger Sumatran farm (2432 m² cage area; 9 t annual net production) using trash fish (≈ 70% of total C input) as food. At the small farm, the mass balance and hydrographic models indicated a maximum dispersal area of 1180 m², equivalent to 18 times the farm area. Within this area, 30% of total organic matter input (farm waste + fixed phytoplankton carbon) was buried in sediments with 30% mineralized in the water-column and 40% mineralized in the seabed. At the larger Sumatran farm, the models indicated a maximum dispersal area of 29  220 m², roughly 12 times the farm area. Within this area, 30% of total organic matter input (farm waste + natural pelagic input) was buried in sediments with 50% mineralized in the water-column and 20% mineralized on the seabed. There was some evidence of benthic enrichment at both farms, mostly as enhanced dissolved nutrient release, but sulfate reduction accounted for < 25% of total C flux, and denitrification/ammonium oxidation accounted for only 4–17% of N lost from sediments. There was no clear evidence of impact with distance from the cages at either farm. Phytoplankton gross primary production accounted for 60–77% of the total organic input to the receiving environment, suggesting that the relative importance of fish cage wastes must be assessed against natural inputs of organic matter.     

Optimum dietary protein and lipid specifications for grow-out of humpback grouper Cromileptes altivelis (Valenciennes)

Fingerling Cromileptes altivelis of less than 50 g have been shown to require feeds of 50–56% crude protein (CP) and 9–15% lipid. The requirements of larger, market‐size fish have not been reported. A total of 324 hatchery‐produced C. altivelis were weight sorted into three groups of 136, 175 and 225 g start weight and equally (12 seacage^?1) and randomly distributed to floating net seacages in accordance with a 3 × 3 factorial arrangement of CP (42%, 47% or 53%; estimated digestible CP of 40%, 46% or 52%) and lipid (8%, 12% or 16%; equivalent to estimated digestible energy (DE) contents of 14.0, 15.8 or 17.5 MJ kg^?1). Changes in dietary CP and lipid content were achieved at the cost of wheat flour by proportionally varying the protein mixture (essentially a 0.62:0.22:0.16 ratio of fish meal, mysid meal and casein respectively) and oil mixture (a 2:1 ratio of fish oil and soybean oil respectively). Fish were fed twice daily to satiation for 180 days. There was no significant (P>0.05) interaction between the main effects of dietary protein and lipid for any growth, nutrient retention or whole‐body composition measurements. Increasing dietary CP significantly improved the survival rate (80.6%, 88.9% and 87.0%), specific growth rate (SGR; 0.24%, 0.28% and 0.31% day^?1), feed conversion ratio (FCR; 2.77, 2.21 and 2.00) and DE retention (18.2, 21.3 and 23.2%), respectively, but did not significantly affect digestible protein retention. Increasing dietary lipid increased SGR (0.25, 0.29 and 0.29% day^?1) and the whole‐body lipid (and energy) composition, and reduced the survival rate (87.0%, 88.9% and 80.6%), respectively, but FCR and retentions of digestible protein and DE were not significantly affected. These results indicate that humpback grouper of 150–400 g require a dietary specification of not less than 51% digestible protein (～53% CP), 10–12% lipid and digestible protein:DE of 31–32 g MJ^?1 for optimal growth.