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



states 2 and 3 appeared gradually in increas- 

 ing numbers. The high percentage of atretic- 

 state 2 females was evident near the end of the 

 season, whereas at the end, females with yolked 

 oocytes in the ovaries were not present, and 

 atretic-state 3 females were predominant. 



In the ovaries sectioned from September to 

 November 1990, we observed only primary 

 growth oocytes, and the onset of vitellogenesis 

 was not detected until December 1990. During 

 this second season, atretic patterns did not fol- 

 low the trends observed in the first season (Fig. 

 3B). During March 1991 (mid-season), high 

 numbers of postspawning females ( atretic states 

 2 and 3) were noted, followed by an increase in 

 the females classified in atretic state 0, which 

 suggests a hiatus in spawning during a period 

 considered to be the middle of the spawning 

 season. 



Spawning and postspawning females during 

 the season In the first reproductive season, 

 the sample of 28 December was the first to con- 

 tain spawning females with evidence of past 

 spawning (Table 1). Since the previous sample 

 taken (20 November) had no spawning females 

 but rather females with advanced yolked oo- 

 cytes, 9 December (the middle of these dates) 

 was taken as an approximate start of the spawn- 

 ing period. The fractions of spawning and 

 postspawning females were plotted as a func- 

 tion of time elapsed since 9 December (Fig. 4). 



A sharp increase occurred in spawning rate 

 between January and February, indicating a 

 change in the spawning frequency, and by the 

 end of March all active females were spawning 

 (Fig. 4). The rate at which females passed from 

 the active to the inactive stage (postspawning 

 rate) accelerated in April. Thus, the period from 

 the end of March to the beginning of April was 

 considered to be the peak of spawning in 1990 

 on the basis of histological samples. 



During the spawning season, the sex ratio 

 (females/total) from all fish sampled remained 

 constant (mean=0.57, SD=0.035), indicating 

 that no sex-based migration probably occurred 

 in and out of the sampled area. The number of 

 sections were insufficient to detect whether the 

 incidence of atresia was age specific. However, 

 it is known that all individuals of European 

 horse mackerel do not have the same spawning 

 season (Arruda, 1984), as is the case for north- 

 ern anchovy ( Hunter and Macewicz, 1985b). For 

 this reason, we estimated the approximate 

 spawning season for an average female by sub- 





: : . 



« •- 





o fl ■».<*:■ I 



sum 



Figure 2 



Atresia stages of European horse mackerel, Trachurus trachurus, 

 oocytes. (A) or-stage atresia of two yolked oocytes. The zona ra- 

 diata has been dissolved. The hypertrophied granulosa cells (g) 

 have invaded the yolk. The thecal cell layer (t) with blood capil- 

 laries (b) is seen. The amorphous particle (ap) may be the rem- 

 nant of the zona radiata or yolk. (B) /3-stage atresia. The yolk 

 spherules show degeneration. Granulosa cells (g) and many vacu- 

 oles (v) are present. The thecal cell layer (t) with blood capillaries 

 (b) is seen. (C) 5-stage atresia. Few granulosa cells (g) in the ova- 

 rian tissue stroma. The thecal cell layer and the blood capillaries 

 no longer encompass the follicle. Harris haematoxylin and eosin 

 stains were used. Magnification: 137x. 



