Population dynamics of the cuttlefish Sepia elliptica Hoyle in Saurashtra waters

Abstract

Tiie cephalopod fishery in Veraval is sustained mostly by five important species which are the squid Loligo duvaucelii, the cuttlefishes Sepia elliptica, S. pharaonis, S. aculeata and Sepiella inermis. On an average 1582.3 t of cephalopods were landed in a year during 1979-84 which constituted 5% of total catch by trawlers in Veraval. The annual average catch composition of the cuttlefish Sepia elliptica was 435.3 t forming 27.5% of cephalopod landings. Age and growth, instantaneous total mDrtality coefficient (Z) and instantaneous natural mortality coefficient (M) were estimated for S. elliptica. The natural mortality coefficient is estimated to be 1.59 by regressing the Z obtained for 5 years as per the method of Alagaraja (1984) against the annual effort. The Z varied from 3.33 in 1979-80 to 5.17 in 1983-84 with an average of 3.93. Yield per recruit at constant age at first capture and varying fishing mortality coefficient for different M/K ratios are inversely proportionate. For the prevailing M/K ratio and cod end mesh size (20 mm) the F mtx which can produce the yield max of 33 g is 2.0. This study indicates that further increase in effort may not result in proportionate increase in production. The exploitation ratio E is higher than the E opt during the period of this study indicating that the stock is exposed to higher fishing pressure. An increase in the cod end mesh size from 20 imn to 30 mm may favour increased production in commensuration with the effort increase. This suggestion is valid only when the fishery is aimed at exploitation of S. elliptica subjected to modifications with reference to other commercially important fisheries like prawn fishery.