The effects of shading on phytoplankton photosynthesis in rice-fish fields in Bangladesh

The combined culture of rice and fish in ricefields (rice-fish culture) has been promoted by development agencies in several developing countries as a means of increasing incomes for rice farmers and the amount of protein in the diets of subsistence rice farming communities. The purpose of the present research, carried out in Bangladesh, was to test a 'low-tech' method for improving fish yields in rice-fish systems. The method, termed 'border planting' (BP), involves alternate planting of closely and widely spaced rows to allow more light to reach the water surface, with the aim of increasing levels of aquatic photosynthesis and thus the supply of autochthonous food for fish, ultimately increasing fish yields. The effects of using BP and different rice cultivars on shading and phytoplankton photosynthesis were investigated in trial rice-fish plots. The rice cultivars used were 'BR11' (a high yielding cultivar) and 'Joshoa' and 'Kolojira' (tall local cultivars). Rice was planted to follow either a regular planting (RP) pattern, 26 cm within/between rows, or in one of two BP patterns (using Joshoa only). In one treatment, no rice was planted. One of the BP patterns (BP1) had relatively narrow maximum spaces between planted rows (36 cm), whereas in the other (BP2), the maximum spaces were wider (75 cm). Rice yields ranged from 1.3 to 1.8 mt ha(-1) and did not differ significantly among treatments, suggesting that BP will not lead to a decrease in rice yields. Measurements of photosynthetically active radiation showed that shading was significantly lower (by approximately 120%) within the wide spaces in BP2 compared with the narrow spaces. The BP2 produced open areas within rice-fish fields in which there was reduced shading and increased phytoplankton photosynthesis, and there was no decrease in phytoplankton photosynthesis within the closely planted areas of the treatment. The BP2 is therefore capable of producing an overall increase in aquatic primary production within rice-fish fields, which would be expected to increase levels of secondary production, including fish production. The BP2 may therefore have potential as a low-tech method for enhancing fish production in ricefields in Bangladesh and other developing countries. Further research in this area is recommended.