MATURATION AND SPAWNING OF THE ANCHOVY THRYSSA MYSTAX IN THE NORTHERN ARABIAN SEA S. M. SHAMSUL HODA Centre of Excellence in Marine Biology, University of Karachi, Karachi-32- ABSTRACT Maturity studies carried out on Thryssa mystax (Bloch & Schneider), collected from commercial landings at Karachi Fish Harbour, from January to December, 1977, show that the female-male ratio is 1.26 : 1, and, as revealed by- ova-diameter polygons, S'pawnmg occurs twice a year. There are two peak spawning seasons, in January-March and June-July, as borne out by the close approximation of GSI and relative condition coefficient. The minimum size at sexual maturity was about 140 mm TL for both sexes. The relations between total length of fish and length and breadth of gonads, and between fish weight and gonad weight have been determined. The logarithmic relationships between fecundity and total length and body weight and ovary weight have also been eatablished. INTRODUCTION Anchovies make a significant contribution to the clupeiod landings on the Karachi coast throughout the year. Studies on their systematics and biology have been made in India (Day 1889, Chidambaram and Venkataraman 1949, Devanesan and Chidambaram 1948, Palekar and Karandikar 1952, Venkata­ raman 1956, Dharmamba 1959, Masurekar and Rege 1960, Dutta 1961, Rao 1963, 1964, Whitehead 1967, 1972) and in Pakistan (Hoda 1976, 1979, NORAD/Pak Fisheries Programme 1978). Thryssa mystax (Bloch & Schneider) and T. dussumieri (Valenciennes) are among the common species of anchovies caught along the Pakistan coast in the northern Araibian Sea. The present report deals with the maturity and spawning of T. mystax. MATERIALS ASID METHODS The 411 samples were collected at random from the commercial landings at Karachi Fish Harbour, West Wharf, Pakistan, from January to December 1977. Data on total length (TL), weight (acutaMy the mass), sex, mauration (Wood 1930) and goand length, breadth and weight (mass) were recorded. The ova were measured after Hickling and Ruteniberg (1936), De Jong (1940) and Clark (1945). Fecundity was determined by counting the ova in a portion of the known weight of the preserved ovary and calculating the total number of ova from the total weight of the ovary. Relative condition coefficient 214 HODA wa^ calculated after Le Cren (1951) and Blackburn (1960). To determine GSI (Gonosomatic index), gonad weight was recorded as percentage of the body weight, including gonad. OBSERVATIONS Sex ratio' Sex ratio was 1.26 female : 1 male. The observed ratios were tested against an expected 1 : 1 ratio by the method of X^ (Chi-square). X^ values of high significance occurred in January, March, June, September and November (Table 1) with a preponderance of females. The data for all other months appear to satisfy the null hypothesis. Sex ratios varied with the length (Table2)- Signi­ ficant X^ values, with dominance of females, occurred at 160, 170 and 180 mm TL. Size and weight of gonad The log-log regression between fish weight 'X' and gonad weight 'Y' calculated by the method of least square, yielded the following relationships: Testis: Log Y = -3.0576 + 1.3727 Log X (r = 0.831, S.E. = 0.137 and 1.1035 to 1.6419 for 95% C.L.) Ovary: Log Y = 0.3319 + 0.7529 Log X (r = 0.625, S.E. = 0126 and 0.5065 to 0.9994 for 95% C.L.) TABLE 1. Chi-square test for T. mystax in different months. (d.f. =1). Months Male Female Proportion of male X2 January 6 23 &.21 9.966* February 42 27 0.61 3.261 March 40 71 0.36 8.658* April g 3 0.73 2.273 May 25 25 0.50 000 June 4 12 0.25 4.00 July 9 9 050 000 August 6 6 0.50 0000 September 3 16 0.16 8.895* October 12 11 0.52 0045 November 12 16 0.43 0572 December 15 10 0.60 1.00 Pooled 182 229 0.44 5.375* * Significant at 5% level. MATURATION AND SPAWNING OF ANCHOVY 215 TABLE 2. Chi-square test for T. mystax ast different size groups. (d.f. "= !)• Size group Male Female Proportion (mm) of male X2 90 1 0.25 1.000 100 2 2 0.50 000 110 4 2 0.67 0.667 120 3 6 0.37 1.000 130 22 18 0.55 0.400 140 40 32 0.56 0.889 150 46 32 0.59 2.513 160 28 50 0.36 6.205* 170 21 42 0.33 7.000* 180 10 21 0.32 3.903* 190 5 10 0.33 1667 200 9 — 000 210 2 — 000 Pooled 182 229. 0.44 5.375* * Significant at 5% level. The 'F' values were significant (Table 3) indicating differences between the slopes of the regression lines for testes and ovaries. Therefore, two different re­ gression lines for males and females are adjusted for fish weight-gonad weight relationship. .„^.,, , A total of 115 males, measuring 90-190 mm TL, and 167. ..fehiales, measuring 90-205 mm TL, were grouped into twenty 5-mm.intervalsi and the mean values of the length and breadth of the testes and ovaries were calcui^ted TABLE 3. Analysis of covariance for differences in the regression coefficients of the weight-gonad relaitionship. Deviation from regression Source of variation d.f. Reg. coeff. d.f. ; SS MS F-value Within ' ^ ~ • : ' Testis 44 , 1.3727 43 1.2502 0.2907 ';Ovary ••- 57 • tO.7529 56 2.5244 .*.«.0451 Pooled within d :, . 99 , 3.7746 0.0381 Common 1001 0.9704 100 4-1564.1568 0.0416 10.0867 Difference between the slopes 1 0.3843 0.3843 (1. 99) 216 HODA for each group. In order to know how the sizes of the gonads are related to the length of the fish, the relationships between the length and breadth of the gonads and the total length of the fish were established by the method of least square (Table 4). It appears that the growth rates in the left lobes of the testes and ovaries are faster than in the right lobes of the testes and ovaries. TABLE 4. The equations for the regression lines to show the relationships between fish length and gonadal length and breadth. (X = fish length, Y = gonadal length or breadth) Equations 'r' S.E.of reg. coeff. Total length-gonadal length Left lobe of testes: Y = -26.0414 + 0.3542 X 0.9642 0.0230 Right lobe of testes: Y = -28.2642 + 0.3360 X 0.9206 0.0336 Left lobe of ovary: Y = -25.8940 + 0-3427 X 0.9564 0.0247 Right lobe of ovary Y = -24.1126 + 0.2937 X 0.9716 0.0167 Total length-gonadal length Left lobe of testes: Y = -11.7669 + 0.1423 X 0.9059 0.0157 Right lobe of testes: Y = -10.2957 + 0.1149 X 0.9496 0.0089 Left lobe of ovary: Y = -6.8627 + 0.1026 X 0.9514 0.0079 Right lobe of ovary: Y = -3.1234 + 0.0477 X 0.9543 0.0035 Size at first maturity Fish in stage I were considered immature, those of II and III as matur­ ing and in stages IV-VI as mature. The specimens (229 females and 182 130 160 190 «0 liO 190 130 IM TOTAL LENGTH (mm)- FIG. 1. Size at first maturity of T. Mystax. A: female (Venkataraman 1956); B: feinale (pneKot study); C: male (present study). MATURATION AND SPAWNING OF ANCHOVY 217 -males) were studied for determining the size at first maturity. It is observed that 15% mature females and 21% matures males appeared first at 130-140 mm TL and that 47% females and 52% males at 140-150 mm TL (Fig. 1 B,C). The size at which 50% at the fish were found mature is 140-150 mm TL and in the present instance, this may be considered as the size at which first maturity is attained in both the sexes. About 90% maturity is attained at 160 mm TL. Fecundity For this study 23 specimens, 130-205 mm TL, of IV-VI stages of ma­ turity, were used. The equations of regression coefficients between total length, weight of the fish and weight of the ovary and number of ova are as below: Log F (fecundity) = -4.0785 + 3.6359 Log TL (r = 0.7586, SE. = 0.2564) Log F = 2.6259 + 0.8954 Log W (weight of fish) (r = 0.7801, S.E. ==0.1567) Log F = 2.5316 + 0.4450 Log W (weight of ovary) (r = 0.5092, S.E. = 0.1641) The significance of 'r' has been tested by the statistic 't' = r V n - 2/v'l - r^ (Snedecor and Cochran 1967) and it has been found significant at 5% level The 't' values obtained are 5.335 for fecundity-total length, 5.716 for fecundity- fish weight and 2.711 for fecundity-ovary wei^t relationships- (Figs 2 & 3), The 4.C 4.4 •• - / • / ^*-2 # * o / iL . w * O4.0 1 _/' • g3.e ' f'» ^ • ••'•••' J L_J_ 2-i 22 2.3 2X 1.2 1.4 1-6 LS 2.0 LOG TOTAL LEN6TH(mm) LOG WEIGHTlmg) FIG. 2. Relation between log length and log number of ova (left) and logwei^t and log number of ova (right) in T. Mystax. number erf eggs increases by 3.6359 of the totJd length. It is seen that the number of eggs produced varies consideraUy in fish of neariy the same body and ovary lengtii and weight. This may indicate that the eggs in this species are shed in batches. The fecundity ranged from 3580 to 24180, with a mean value of 10360 in fish of 130 to 205 mm TL and weight of 14 to 64 g.