1 Fishery and population dynamics of the sacred chank Turbinella pyrum (=Xancus pyrum Linnaeus, 1758) off Kayalpattinam in the Gulf of Mannar I. JAGADIS, G. SYDA RAO*, K. K. JOSHI * AND P. KANDAN Tuticorin Research Centre of Central Marine Fisheries Reseach Institute, Tuticorin - 628 001, Tamil Nadu, India *Central Marine Fisheries Research Institute, Kochi - 682 018, Kerala, India e-mail: ijagadis@sify.com ABSTRACT The fishery and population dynamics of the sacred chank, Turbinella pyrum exploited by bottom-set gillnet off Kayalpattinam, Gulf of Mannar were studied during 2004-2006. The size/weight range was between 80-290 mm / 60 to 1600 g. The age of the chanks was estimated to be 20+ years and it reaches 94/135, 154/453, 197/833 and 227 mm /1195 g in 1 st , 2 nd , 3 rd and 4 th year of life. The length–weight relationship (L-W) of T. pyrum was W = 0.001728 * L (2.47087) , length–maximum shell diameter (L-MSD) was W = 0.0011749 * MSD (2.851393) . The VBGF parameters estimated are L ∝ = 306.1 mm, K = 0.33 yr -1 and W α = 2.5 kg. Estimated total mortality, natural mortality and the fishing mortality are 1.6913 yr -1 , 0.45909 yr -1 and 1.2322 yr -1 respectively. From the estimates arrived at, the fishing pressure on chanks in the fishing ground is evident and about 30% reduction from the present effort of 5,943 units operated from Kayalpattinam is required to sustain the stock and return to the maximum sustainable yield (MSY) of 48.5 t. Keywords: Age and growth, Fishery, Length-weight relationship, Mortality, Turbinella pyrum The sacred chank Turbinella pyrum (= Xancus pyrum Linnaeus, 1758) forms a commercial fishery along the south-east coast of India. In the Gulf of Mannar (GoM), there are about 10 chank beds extending from off Vaipar to Tiruchendur, exploited from Tuticorin as base. These beds extend from 6-27 m depth or even beyond and chanks are found to be abundant in 16-20 m depths (Hornell, 1914, 1922; Mahadevan and Nayar, 1966). Over 95% of the catch is from skin diving in depths up to 20 m while the remaining comes from the incidental catches from various fishing gears, especially trawlers. Chanks exploited all along the GoM area are the mainstay of the bangle industry in West Bengal. Studies on Indian chanks are very limited, restricted to the early underwater observations of chank beds in GoM and studies conducted on its ecology, tagging and recapture, growth of juvenile chanks and on dynamics (Mahadevan and Nagappan Nayar, 1966; Devaraj and Ravichandran, 1988; Pon Siraimetan et al., 1988; Lipton and Selvakku, 2001; CMFRI, 2004). Kayalpattinam is an important fish landing centre along the GoM, south-east coast of India (lat. 8° 34' N; long. 78° 07' E). Shellfish fishery in this centre is supported by various species of crabs, lobsters and gastropods especially chanks. The major gear employed for exploiting these resources is bottom-set gillnets (shingi/mandal valai). Chanks form a considerable catch in this gear throughout the year, contributing over a lakh chanks annually with peak Indian J. Fish., 57(3) : 1-5, 2010 catches from October to April. An attempt was made to find out the age and growth, length-weight (L-W) as well as length-maximum shell diameter (L-MSD) relationships and important fishery parameters of exploited chanks by bottom-set gillnets to understand the current status and sustenance of the fishery. Samples of T. pyrum were measured at fortnightly intervals from the commercial catches from bottom-set gillnet units operating off Kayalpattinam during 2004 -’06. Data on total length, weight and maximum shell diameter (MSD) and other fishery data were collected from the landing centre and the monthly estimates of catch were arrived at. A total of 523 chanks in the size range of 80-290 mm total length was used for L-W and MSD-weight calculations. Length and MSD linear relationship was derived using least square method. Parameters of growth in length were estimated using the von Bertalanffy growth equation: Lt =L ∝ (1- e –K (t-t 0 ) ). The von Bertalanffy growth parameters were estimated using FiSAT (Gayanilo et al., 1988; Sparre and Venema, 1991). The pooled sample data for the period 2004-06 were used for the estimation of growth parameters of chank. The growth parameters were estimated using monthly length frequency distribution of each year. A total of 1142 chanks in the length range of 80-290 mm in total length were used for estimation of growth parameters. The L ∝ values and their associated K values were selected using FiSAT.