A.K. JAISWAR, R.S. BIRADAR AND D.K. GULATI Central Institute of Fisheries Education, Versova, Mumbai ABSTRACT

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1 Journal of the Indian Fisheries Association 24, 1994 (Proc. at. Symp. Aqu.ac~ops), STOCK DYAMICS OF HORSE MACKEREL, MEGALASPIS CORDYLA (LIAEUS) ALOG ORTHWEST COAST OF IDIA A.K. JAISWAR, R.S. BIRADAR AD D.K. GULATI Central Institute of Fisheries Education, Versova, Mumbai ABSTRACT The stock dynamics of horse mackerel, Megalaspis cordyla, along the northwest coast of India has been studied using length frequency data recorded from commercial landings and trawl catches of researchcum-training vessel M.F.V. Saraswati. The growth parameters for this species has been estimated to be L==54 em and K=0.49 per annum. The natural and fishing mortality for the stock have been worked out to be 0.93 and 0.91 per annum respectively. The study indicated that the stock is fished at a safer fishing mortality level F 0. 1, lower than Fmsy level. Megalaspis cordyla (Linnaeus) commonly known as horse mackerel is a commercially important species along the northwest coast of India. Total landings of this species during 1993 was t of which 45% of the landings came from Maharashtra and Gujarat. M.cordyla IS a shoaling, pelagic carnivorous fish feeding primarily on sergestid shrimps like Acetes indicus and small fish such a Stolephorus sp. Datar (1954), Chacko and Mathew (1955), Radhakrishnan (1973) and Srinivasan (1978) studied the biology and fisheries of horse mackerel, while Srinivasan (1975) evaluated its food habits. Kuthalingam (1959) investigated the effect of absence of light on the development and feeding habits of larvae and postlarvae of M.cordyla. Bapat et al. (1982) reported catch rates as high as 2564 kg/hr for M. cordyla during M. T. Murena cruises off northwest coast of India and suggested that the M. cordyla stock needs to be exploited cautiously as it is not known whether we can obtain a sustained yield with increasing fishing pressure. In the light of their suggestion, it becomes imperative to know what is the optimum fishing level required to obtain sustained yield over the years for this species. Although M. T. Murena cruises indicated tremendous scope for horse mackerel fishery off the northwest coast of India nearly 17 years back, studies on stock dynamics of M.cordyla, which are essential for optimum utilization of stock are lacking. Therefore, an attempt has been made to study the stock dynamics of M.cordyla along

2 116 OTES the northwest coast of India. Length composition of M.cordyla landings by commercial fisheries was recorded monthly from three landing centres viz : Ferry Wharf, Sassoon Docks and Versova. Length frequency data was also collected from trawl catches made during the cruises of research-cum-training vessel M.F.V. Saraswati along the northwest coast of India during September 1986 to August 1988 between lat ' to 21 ooo' Consolidated monthly length frequency data on total length of 4995 CO.0 c 0 ""I -v 0...r c:.. 0 ( w ') } 4l6\Jll1

3 OTES 117 specimen were obtained by pooling the data of two years period. The vun Bertalanffy growth equation was used to describe the growth of fish. The monthly length composition of fish were separated into normal components corresponding to different Bhattacharya (1967) method. Modal progression analysis and Gulland and Holt (1959) were used to estimate Loo and K, the growth parameters of von Bertalanf(y growth equation. The results of Bhattacharya analysis on a time series of length frequency samples formed the basis of modal progression analysis and Gulland and Holt (1959) plot. The mean lengths that appear to correspond to the same cohorts were identified and Gulland and Holt plot was then used to estimate Loo and K (Fig.1). The natural mortality M was estimated using Pauly's (1980) empirical formula which is given by log 10 M= log 10 Loo Log 10 K log 10 T where M is the natural mortality, L= and K are parameters of von Bertalanffy equation and T is the annual mean temperature (in degree centigrade) of the water in which the stock lives, taken as 27 C based on the information collected through cruises of M.F.V. Saraswati. The method of Rikhter and Effanov (1976) was also used to calculate the natural mortality using the following relationship. M=l.521 tm where tm is the age at which 50% of the population is mature, which was taken as 1.42 years. The total mortality coefficient Z was estimated using the length converted catch curve analysis (Sickle, 1977). The fishing mortality coefficient (F) was obtained using the relation F = Z-M. Asymptotic weight (Woo) corresponding to asymptotic length (Loo) was calculated using the lengthweight relationship reported by Jaiswar and Acharya (1991). The length at recruitment (lr) was taken as 12 em and was converted into age at recruitment (tr) using von Bertalanffy growth equation. The length at first capture was derived as per Sparre and Venema (1992). The yield in weight per recruit (Y/R) was calculated by Beverton and Holt formula (Sparre and Venema, 1992). The growth parameters for the species were estimated as follows Loo = 54 em and K = 0.49 per annum. The L= value of 54 em so obtained is close to the largest fish of 50 em recorded in the catch. Srinivasan (1978) studying the age and growth of this fish off Vizhingam in Kerala reported Loo = 42 em and K = 0.26 per annum. Srinivasan's (1978) study recorded a fish of 45.5 em in length. Thus he appears to have under estimated asymptotic length and K. The 't ' 0 estimated by Srinivasan (1978) was on half yearly basis. Which appears to be an over estimate.

4 118 OTES The natural mortality coefficient M is one of the difficult parameters to estimate in stock dynamics studies. Using Pauly's (1980) empirical formula M was estimated to be 0.93, while Rikther and Effanov (1976) method gave an estimate of Mas Thus it appears that the natural mortality for this species is around 1 per annum. The MIK ratios of fish have generally been found to lie between 1 and 2.5. The ratio in the present case is around 2 which is within the acceptable range. For further estimates of yield per recruit the value ofm of0.93 obtained from Pauly (1980) formula has been considered. ' The total mortality Z was estimated to be 1.84 per year by the length converted catch curve. The fishing mortality coefficient F was computed to be 0.91 per year. The yield per recruit at the present age of first capture and fishing mortality has been estimated to be 73 g. The MSY per recruit of 89 g could be attained by increasing F to 3.42 per annum, maintaining the present age of first capture. Thus by incresing F by almost four fold, the MSY per recruit is expected to be incresed by about 22%. It is always safer to fish to the left of F msy than at F msy or to the right of it. The fishery at present is operating to the left of F msy at F O.l level and hence there is little reward in increasing fishing beyond this level. The authors are grateful to Dr.S.D. Tripathi, Director, Central Institute of Fisheries Education, Bombay for his kind encouragement and interest in the study and to Dr. M. Devaraj, Jt. Director, for his valuable suggestions. REFERECES Bapat, S.V., Deshmukh, V.M., Krishnamorthi, B., Muthiah, C., Kagwade, P., Ramamirtham, C.P., Mathew, K.J., Pillai, S.K., and Mukundan, C Fishery resources of exclusive economic zone of northwest coast of India, Bull. Cent. Mar. Fish. Res. lnst. 33 : 60 p. Bhattacharya, C. G A simple method of resolution of a distribution into 'Guassian components'. Biometrics. 23: Chacho, P.D. and Mathew, M.J Biology and fisheries of horse mackerel of the west coast of Madras state. Cont. Mar. Biol. Stn. West Bill, Malabar coast o.2: Govt. Press, Madras Datar, G.G The food and feeding habits of Caranx rottleri, (Cuv.), Proc. 41st Indian Sci. Cong. 3 : Gulland, J.A. and Holt, S.J Estimation of growth parameters for data at unequal time intervals. J. Con. ClEM, 25 (1) : Jaiswar, A.K. and Acharya, P Length-weight relationship

5 of Megalaspis cordyla (Linnaeus) along northwest coast of india. J.Indian Fish,. Asso. 21 : Kuthalingam, M.D.K A contribution to the life history and feeding habits of horse macke;rel, Megalaspis cordyla and Caranx mate and a note on the effect of absence of light on the development and feeding habits of larvae and post larvae of M. cardyla. J. Madras. Uni. B, 29 (2) : Pauly, D A selection of simple methods for the assessment of tropical fish stocks, FAO Fish Circ. (729). 54 p. Radhakrishnan, Pelagic fisheries of Vizh,ingam. Indian J. Fish. 20 (2) : Ricther, V.A. and Effanav, Y.V On one ofthe approches to estimation of natural mortality of OTES 119 fish populations ICAF Res. DVI, 76I/8. 12 pp. Sickle, J. van Mortality rates from size distributions the application of a conservation law. Decologia, Berlin. 27 : Sparre, P. and Venema, S.C Introduction to tropical fish stock assessment Part I- Manual. FAO Fisheries Technical Paper o Rev. 1. Rome FAO, 376 p. Sreenivasan, P.V Observa-.. tions on the food and feeding habits of.the 'Torpedo Travelly 1 Megalaspis c'ordyla from Vizhinjam Bay. Indian J: Fish. 25 (1) : Sreenivasan, P.V Observations on the fishery and biology of 'Megalaspis cordyla 1 at Vizhingam. Indian J. Fish. 25 (1&2) :