This work was supported partly by the grants from the Ministry of Environment and Forests, Government of India and Indian Institute of Science. We thank Prof. Madhav Gadgil and Prof. N.V.Joshi for support and fruitful exchange of views. We thank Indian Institute of Science, Jawaharlal Nehru Centre for Advanced Scientific Research and Assam University for extending cooperation to carry out this research. We wish to acknowledge the help received from the fisherfolk and colleagues during the field work and analyses.