Optimization of fish to grass ratio in a recirculating fish-grass system
循环流水鱼草共生生态系统的鱼草配比试验研究

The effect of fish-to-grass ratio on fish production in a recirculating integrated fish-grass system has been studied. The experimental system is mainly comprised of fish tanks, six grass culture tanks, a water pump and lines. Five experimental systems were used. Effluent from each fish tank was recycled and utilized for NFT (nitrogen film technology) culture of grass (Lolium multiflorum Lam.). The original stocking density of fish (Oreochromis niloticus) was 2.00 kg m~(-3) for group I, 1.61 for group II, 1.38 for group III, 1.20 for group IV and 0.00 for the control group ck. The biomasses of the grass in all groups were identical at the beginning of the experiment. The period of this experiment was 30 days, from March 25, 2001 to April 25, 2001.Water quality (pH, dissolved oxygen, unionized ammonia, chemical oxygen demand, total nitrogen and total phosphorus) in group III and group IV met the requirements of the Fishery Water Quality Standard of China except for total phosphorus. Water quality parameters of group I exceeded that required by the Chinese Fishery Water Quality Standard except for pH. Water quality in all groups deteriorated, especially in groups with higher fish stocking densities. At the end of the test, conditions of dissolved oxygen and unionized ammonia in group III were near the limits of the Chinese Fishery Water Quality Standard.In terms of coarse protein, fat, fibre, total nitrogen, total phosphorus, and biomass, the grass in group III was similar to grass cultured in an open field, thus indicating there were sufficient nutrients in the effluent to satisfy the requirements of the grass, while nutrients from the effluent in group IV were less than that in group III and slightly more in group II. Since there were no fish in the control and therefore no nutrient was added, the grass in the control group ck received no nutrient and grew poorly.As a result of water quality, the average growth rates of fish in group III (28.3 g) and group IV (28.4 g) were higher than that in group II (24.9 g) and group I (18.2 g). The feed efficiencies of group III (54.4%) and group IV (54.7%) took first place. The values of integrated fish productivity showed that group III was the best among all 5 groups.The differences among the 5 groups at the end of the test may be explained in part by the fish:grass ratio in weight, 0.232 for group I, 0.233 for group II, 0.221 for group III and 0.235 for group IV. In group III, not only did the fish grow quickly, but the grass also did well, while in other groups, either the fish or the grass, or both grew badly.