Fish community structure and functional guild composition in a large tropical estuary (Vembanad Lake, India)

The fish guild structure of 96 fish species inhabiting the Vembanad Lake, an open, large, well-mixed coastal estuary along the south-west coast of India, was studied based on samples from upper (freshwater), middle (estuarine), and lower (saline) zones representing three major seasons (monsoon, post-monsoon, and pre-monsoon). Functional group assessment revealed seven ecological guilds (marine estuarine opportunists, freshwater, marine estuarine dependent, estuarine, catadromous, anadromous, and amphidromous), six feeding guilds (detritivore, zooplanktivore, herbivore, zoobenthivore, omnivore, and carnivore), three vertical distribution guilds (demersal, benthopelagic, and pelagic), five reproductive-mode functional groups (oviparous with pelagic eggs, oviparous with benthic eggs, oviparous guarders, viviparous and ovoviviparous), and five flow-preference guilds (rheophilic A1, eurytopic, rheophilic A2, rheophilic B, and rheophilic C). The estuary was characterized by high species diversity, low individual species abundance, complex guild structure, varying fish diversity from upper to lower estuary and higher proportion of marine rather than 'true' estuarine species. The absence of a significant relationship between season and fish guild structuring could be due to high resource availability in the estuary and low ecological connectivity between upper and lower estuarine zones as a result of anthropogenic interventions in the middle zone. Salinity variations considerably influenced the fish guild formation in the estuary. A drastic change in the hydrography of the estuary over the past sixty years due to topographical alterations has transformed the fish guilds resulting in a considerable increase in freshwater species and a reduction in marine and estuarine-dependant species.