Population dynamics of siboga squid Doryteuthis sibogae (Cephalopoda/Teuthoidea) in Thoothukkudi (Tuticorin) coast, southeast coast of India

Study on the population dynamics of siboga squid Doryteuthis sibogae in Thoothukkudi coast revealed that the seasonal change has definite impact on the growth of this species. The growth parameters viz. L∞, K, t0, C, and ϕ were 39.5 cm, 0.4, -0.02, 0.5 and 2.9 respectively. Mortality parameters Z, M and F were estimated as 2.27 / year, 0.91 / year and 1.36 / year. Weight was more or less square function of length rather than in cubic function. This squid was found to have two recruitment seasons, one major season during December and a minor season during April. The length structured virtual population analysis revealed heavy fishing pressure on the length group 17 - 20 cm and hence the fishery of D. sibogae was not found to suffer due to recruitment overfishing in Thoothukkudi waters. Estimated total stock, average annual stock and maximum sustainable yield(MSY) were 529.47, 209.14 and 217.33 tonnes respectively. Present level of exploitation rate (U) and exploitation ratio (F/Z) was estimated to be 0.5372 and 0.6 respectively. The study infers that the cod end mesh size of the trawls of Thoothukkudi has to be increased from 10 to 25 mm to increase the yield per recruit. Further, the effort should be reduced and maintained at the exploitation ratio of 0.45 to get maximum yield per recruit. The study suggests reducing the present level of effort by 10 percent to sustain the stock of D.sibogae in Thoothukkudi waters.