This study was based on the plankton collections taken at fortnightly intervals from two fixed stations in the Cochin Backwater, one at the Fairway Buoy area and other at Aroor during the period January to December 1968. An attempt was made to determine the species of Chaetognatha present, their relative abundance, seasonal fluctuations in relation to the monsoonal changes breeding periods and vertical distributions in the backwaters.

Four species belonging to two genera were encountered in these collections. They were Sagitta bedoti Beraneck, Sagitta enflata Grassi, Krobnitta pacifica (Aida) and Segitta robusta Doncaster in the order of their dominance in the surveyed stations. S. bedoti is a neritic form and breeds continuously in the backwaters. Numerical abundance of S. enflata was very low in the Aroor station, but at the Fairway Buoy station they dominated in all the collections from September to December. K.pacifica is an oceanic form and the sporadic occurrence of the species during the months of high salinity indicates the incursion of oceanic water affecting the inshore areas.  S.robusta was recorded from only one collection taken from the Fairway Buoy station.

The distribution of chaetognaths at Aroor and Fairway Buoy stations reflects the differing characteristics of the two stations. Aroor is typically estuarine with a complete turnover of salt water into fresh water and chaetognaths were found to be absent during the period of low salinity. Fresh recruits from the sea will be brought back to the system along with the incoming waters of the post monsoon period. At Fairway Buoy area where the prevailing conditions are more marine, chaetognaths were present throughout the year.