Estimating sea snake diversity using trawler bycatch along the coast of Goa

Abstract

The sea snake community off the coast of Goa was studied using trawler bycatch. To obtain a fair idea of species richness .other fishing crafts such as shore seines and sluice nets were visited at regular intervals to check for snakes that were opportunistically caught in them. Catch per unit effort was calculated to compute the sea, snake abundance. Food habits of sea snakes were studied using a gut content analysis, where in the fish families commonly fed on by sea snakes were recorded. An attempt was made to understand the causes of mortalities in sea snakes. This was done relating the magnitude which was the number of dead sea snakes and intensity which was the proportion of dead sea snakes with the duration of the trawl, weight of the catch and mean depth in meters. Sea snakes were measured for several parameters and length-weight relationships were computed. A total of six species were encountered of which four were caught through systematic sampling using trawlers while the remaining two. were obtained through opportunistic searches using shore seines and sluice nets. The total catch per unit effort calculated for the entire study was 1.28 x 10-5 sea snakes per net/hr and the catch per unit efforts calculated for the individual bases varied significantly. The sea snake diversity was seen to be the highest for Vasco followed by Malim, Chapora and Betul. A cluster analysis with complete linkage showing similarities across the four different fishing bases showed that Vasco is most similar with Malim and Betul with Chapora. The composition of sea snake species varied with depth The proportion of Lapemis curtus increased and Enhydrina schistosa decreased with an increase in depth. The proportion of male and female snakes per trawl were similar in all three depths for L.curtus. In E.schistosa, however males did not occur in depths greater than 15 m. In L.curtus most individuals contained food in the stomach. The species was found to feed on five fish families of which families Clupeidae and Cynoglossidae dominated~ The consumption of clupeids decreased with increase in the length of the snake. Consumption of prey families varied slightly with the time of the day and this was mainly with the change in the proportion of Cynoglossids consumed.The mortality of sea snakes was found to be strongly related with the number of individuals caught. The magnitude and intensity of mortality were found to increase with the duration of the trawl, weight of the catch and mean depth. Mortalities of sea snakes varied with species, L.curtus having a larger proportion of dead than live individuals and E.schistosa having a larger proportion of live as compared to dead individuals, thus showing that L.curtus is more susceptible to mortalities- due to trawling as 'compared to E.schistOS8. Betul recorded the highest mortalities and the mortality of sea snakes was found to be significantly influenced by the time of the day and season. Bruises were found to vary significantly in live and dead sea snakes and were higher in dead snakes. Morphometric studies in L.curtus and E.schistosa found that snout vent length bore a strong relationship with the weight and tail length. No sexual dimorphism in terms of size was recorded in both the species which was similar to previous studies in the case of L.curtus.