Browsing by Author "Becker, Klaus"
Effects of low nitrogen-phosphorus ratios in the phytoplankton community in Laguna de Bay, a shallow eutrophic lake in the Philippines The effects of low nitrogen-phosphorus ratios on microalgae from a large eutrophic freshwater lake in the Philippines were investigated. Natural microalgal populations from Laguna de Bay, the largest lake in the Philippines, were cultured using three different nitrogen-phosphorus weight ratios (2N:1P; 6N:1P and 12N:1P) at two phosphorus concentrations (0.25 and 0.5 mg l–1) in each case. The growth and genera composition of the cultures under the different treatments were followed for a 12-week period. Community level responses were assessed based on species richness (s), Shannon-Wiener Index (H’), Simpson Index (λ) and Evenness (J’). Among the different microalgal groups, only the chlorophytes showed a significantly higher density in response to the 12N:1P treatment at the higher P concentration, indicating that the nutrient ratio had a significant interaction with the nutrient levels used in the experiments. The genera found in the different treatments were generally similar; however, the degree of dominance of some varied with treatment during the experiment. The succession of dominant genera also differed among the N:P treatments. The diatoms like Fragilaria, Aulacoseira (= Melosira) and Nitzschia dominated the lowest N:P. On the other hand, chlorophytes (Kirchneriella and Scenedesmus) dominated the highest N:P treatment, particularly from the second to the seventh week of the experiments with the diatoms becoming co-dominant only towards the eighth week until the end of the experimental. The 6N:1P treatment showed a mixed dominance between the diatoms and the chlorophyte genera. The various indices of diversity indicate significantly lower diversity only in the 12N:1P at 0.5 mg l–1 P and not in 12N:1P at 0.25 mg l–1 P.
Survey on the use of natural food and supplemental feed in commercial milkfish farms on Panay, Philippines C Lückstädt, U Focken, R Coloso & K Becker - In Deutscher Tropentag 2000: International Agricultural Research ; a Contribution to Crisis Prevention, October 11-12 2000, University of Hohenheim, 2000 - Stuttgart UniversityThis study evaluated the feed intake of the milkfish (Chanos chanos Forsskål) in commercial brackishwater ponds under different management regimes. Feed intake and growth were compared between a rather intensive culture management in a fish farm of 1 ha pond size and a semi-intensive one, with a total pond area of 30 ha. The data suggested a direct consumption of only 12 % of the supplemental feed in the intensive farm, leading to a wastage of high quality fish feed and a significantly lower specific and metabolic growth rate (P< 0.001) than in the semi-intensive system without any supplementation and only relying on abundant natural food through fertilization. These results suggest that a heavy reduction in, or even the abandonment of the use of, supplemental feed for milkfish culture would be more cost-effective.