Journal of the Marine Biological Association of India

Dynamics of the exploited Indian mackerel Rastrelliger kanagurta stock along the southwest coast of India.

Volume 36 Issue 1&2

M. Devaraj, I. Fernandez and S.S. Kamat

Abstract

The dynamics of the fishery for the Indian mackerel Rastrelliger kanagurta (Cuvier) along the southwest coast of India has been studied from the data for the annual catch, effort and length composition for five decades from 1934 to 1984. The mean lengths (total) at ½ year, 1year and 1 ½ year were found to be 18.42 cm (weight: 56.9g), 22.6 cm (weight: 113.8 g) and 23.43 cm (weight: 133.5 g) respectively. The length growth parameters in the Bertalanffy growth function (VBGF) were estimated to be L_∞ =23.83 cm, annual k =2.84, t₀ =0.0003 year and the weight growth parameters to be W_∞ =143.0g_, annual k =2.66, t₀ =0.0009 year. The mean annual total mortality (Z) was 3.68, the mean annual natural mortality (M) 1.24 and mean fishing mortality (F) 2.44. The length at recruitment (l_r) was 9.8cm (0.2 year) and the length at first capture (l_c) 18.4 cm (0.5 year). The year class is comprised of one major brood originating in the premonsoon season (February to May) and an occasional secondary brood originating in November. For the Kerala Karnataka stretch within the southwest coast, the mean annual catch during 1934-1975 comprised 105 x10⁶ 0+ (1 to 5 months), 523.6x10⁶ ½ + (6 to 11 months), 92.7 x 10⁶ 1+ (12 to 17 months) and 25.3 x10⁶  1 ½ + (18 to 23 months) old fish. The mean annual size of the population for this period included 17,242 x10⁶ ½ 0+, 1338 x 10⁶ ½ +, 170 x 10⁶ 1 + and 35 x 10⁶ 1 ½ +old fish. The average annual stock was estimate to be 109,299 tonnes (393.89 x 10⁶ fish), the MSY 70,788 t and the recruits at t_c , 2135 x 10⁶ fish. The average annual exploitation ratio (0.68) was only marginally less than the value of 0.72 required for harvesting the MSY. The progeny was estimated to attain the maximum number (5258 x10⁶) when the number of spawners was 140 x 10⁶ , The recruits attain maximum size (3375 x 10⁶ ) at a stock size of 525 x 10⁶   fish. The MSY of 73490 t (Schaefer model) and 76197 t (Fox model) agrees very well with that based on the analytical model (70788 t) and is only marginally higher than the average annual yield of 62198 t for 1956-1984. The Schaefer model fitted to the limited data indicates the biologically optimum number of purse seiners operating for about 200 days a year to be about 1,000 which seems quite unrealistic as the fleet has stabilized at 500 vessels owing to diminishing returns.

Date : 30-12-1994