Karlou-Riga and Economidis: Ovarian atretic rates and sexual maturity of Trachaurus trachurus 



73 



20 40 60 80 100 120 140 160 180 200 220 



Days elapsed since 9 December 



May 



Jun Jul 



Figure 4 



Fraction of European horse mackerel, Trachurus trachurus, 

 spawning and postspawning females during the period from 9 

 December to 5 July (1989-90). (a) Females with active ovaries, 

 which had one or more spawning stages (open squares); trend 

 line is a logistic curve, where a = 4.699, 6 = 0. 070, r 2 = 0.98. (b) 

 Females in atretic states 2 or 3, which have finished their spawn- 

 ing season (crosses); trend line is a logistic curve, where a = 6.757, 

 b = 0.042, r~ = 0.97. The approximate spawning season for the indi- 

 vidual female (94 days ) was estimated by subtracting the 50^ spawn- 

 ing point (67 days) from the 50% postspawning point ( 161 days). 



covariance of the log^-transformation of percent ma- 

 ture females on the length and maturity-staging 

 method indicated that the difference between the two 

 ML 50 estimates was not significant (F=0.85, 

 P=0.373). 



Discussion 



Reproductive cycle 



In the present work, we found that histological atretic 

 states were useful as an index of the population's 

 reproductive capability for European horse mackerel. 

 Atretic-state 1 females (<50% of advanced yolked 

 oocytes in a-stage atresia) were apparently capable 

 of spawning, whereas atretic-state 2 females (50% 

 or more of advanced yolked oocytes in a-stage atre- 

 sia) had a lower spawning probability. It seems most 

 likely that once females are in atretic state 2 they do 

 not spawn or spawn infrequently (Hunter and 

 Macewicz, 1985b). Therefore, this state was the best 

 absolute measure of ovary resorption. Atretic-state 

 3 females (showing no yolked oocytes but late atre- 

 sia stages) were those in late postspawning condition. 

 These females could not be distinguished from imma- 



ture females by gross anatomical criteria. Conse- 

 quently, this histological state was very useful in 

 classifying mature females and estimating length 

 at first maturity. 



The high percentages of atretic oocytes ob- 

 served in European horse mackerel throughout 

 the spawning season are a normal occurrence 

 in multispawners (Hunter and Macewicz, 

 1985b; Dickerson et al., 1992), an attribute 

 which probably characterizes most species of 

 Trachurus (Macewicz and Hunter, 1993). High 

 numbers of atretic oocytes are also observed in 

 Atlantic mackerel (Scomber scombrus L.) ova- 

 ries around peak spawning (Walsh et al. 6 ). 

 Therefore, atretic state 1 is expected to be the 

 most common condition during the main spawn- 

 ing season. 



During the first spawning season, we found 

 that atretic state 2 increased near the end of 

 the season, and that it was completely replaced 

 by atretic state 3 at the end. Thus, it is useful 

 to study both of these atretic states during the 

 entire spawning season. However, it is also im- 

 portant to estimate atretic states near and af- 

 ter the end of the season. We were not able to 

 assess the atretic state in June because of a lack 

 of histological samples. However, because 

 atretic state 3 lasts for one month or more in 

 the ovary (Hunter and Macewicz, 1985, a and 

 b), it is very likely that ovary resorption observed in 

 high levels in late May continued through July, be- 

 cause all the mature females were in late postspawn- 

 ing condition by July. Low GSI values in June and 

 July supported this conclusion. Thus, high levels of 

 atretic state 2 actually forecasted the end of the Eu- 

 ropean horse mackerel spawning season as has been 

 noted for other fishes (Hunter and Macewicz, 1985b). 

 Because GSI remained low during the summer and 

 no atresia stages were observed in early September 

 1990, vitellogenesis probably did not occur in the time 

 interval between July and September. 



Females were inactive until December 1990, when 

 the next reproductive season began. By the middle 

 of this second season (March 1991), high levels of 

 atretic states 2 and 3 were noted, and subsequently 

 percentages of females in atretic state (yolked but 

 no atretic oocytes) increased, resulting in an unex- 

 pected pattern. It was not possible to explain either 

 the "delay" of onset for this reproductive season or 

 the mid-season increase in atretic rates. Variation 



fi Walsh, M., P. Hopkins, P. Witthames, M. Greer-Walker, and J. 

 Watson. 1990. Estimation of total potential fecundity and 

 atresia in the western mackerel stock, 1989. ICES Council 

 Meeting H:31. 22 p. 



