Studies on temporal variations of exploited fishery resources and their trophic levels in a tropical estuary

Estuaries are dynamic ecosystems that warrant detailed examination of trophic interactions due to diversified prey items contributed simultaneously from marine and freshwater environments. Most coastal and estuarine fisheries resources are either fully exploited or overexploited due to the involvement of increased number of fishermen, employment of more efficient fishing gears and adoption of mechanization. The present study elucidates trophic level status of fish resources exploited from Vembanad lake, S. India as an indicator of fishery-induced impacts in food web structure and thereby delineating its usefulness in describing the state of fisheries in this ecologically fragile ecosystem. Results showed that fishery resources of Vembanad lake has dwindled considerably from 115 species contributing 3068.29 tonnes during 1987-1988 to 80 species constituting 1192 tonnes during 2012-2013. The species richness has reduced to 3.78 in latter period from 6.40 in former period. A perceptible variation in composition of trophic groups was also observed in exploited fishery resources of the two periods. The number of species representing herbivores-omnivores-detritivores (Trophic Level-2.0-2.99) reduced from 31 to 28, mid-level carnivores (Trophic Level-3.0-3.99) from to 66 to 43 and the same of high-level carnivores from 18 to 9 species. Chi-square test revealed significant (p<0.05) difference in reduction of both mid-level carnivores and total species exploited from the lake. Large marine migrant predators, which showed pre-dominance during 1987-1988, declined along with a significant influx of freshwater and lower trophic fishes in fish catch of 2012-13. The significant reduction in the diversity of fishes represented in exploited fishery bring about theories like 'fishing down the food web' and 'top down effect' and change in ecology of the lake. These ecological changes are brought about by a combination of natural as well as anthropogenic causes which demand continuous monitoring of fish catch in order to ascertain the ecosystem health of Vembanad lake.