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



69 



croscope, and the percentage (< or >50%>) of oocytes 

 in a-stage atresia was noted. 



Females with ovaries containing yolked oocytes 

 were classified as active, i.e. capable of spawning in 

 the current spawning season. Active females were 

 separated into spawning and nonspawning groups. 

 Spawning females contained ovaries with histologi- 

 cal characteristics of past spawning (postovulatory 

 follicles ) or imminent spawning ( hydrated oocytes or 

 migratory-nucleus-stage oocytes). The nonspawning 

 females that displayed no such characteristics were 

 assumed to be capable of spawning in the near fu- 

 ture. Females with yolked oocytes where 50% or more 

 were in a-stage atresia, or had no yolked oocytes but 

 only postovulatory or atretic follicles, or both, were 

 classified as postspawning or inactive in the current 

 season. The active and the postspawning females 

 were both considered mature. Females that showed 

 no histological evidence of past, imminent, or future 

 reproductive activity were classified as immature. 



Reproductive cycle 



Atretic states Description of atresia stages (a, /3, y, 

 and 8) follows Bretschneider and Duyvene de Wit 

 (1947), Hunter and Macewicz (1985, a and b) and 

 Macewicz and Hunter ( 1993). These stages are com- 

 mon among several species studied including the jack 

 mackerel, Trachurus symmetricus (Macewicz and 

 Hunter, 1993). 



Classification of ovaries based on oocyte and atre- 

 sia stages follows Hunter and Macewicz ( 1985b) and 

 is briefly summarized as follows: 



Atretic state No a atresia of yolked oocytes. 

 Atretic state 1 <50%<(1 oocyte to 49%) of yolked oo- 

 cytes were in a atresia. 



Atretic state 2 >50% of yolked oocytes were in a 

 atresia. 



Atretic state 3 No remaining yolked oocytes, but p 

 atresia or later atresia stages were present. 



Atretic state 3 was used to separate immature fe- 

 males from postspawners, because advanced atresia 

 stages indicate the possibility of past spawning. The 

 late postspawning ovary was also characterized by 

 disorganized septa, a condition which indicated that 

 this particular female might have spawned during 

 the latest reproductive season. 



We defined the reproductive season for European 

 horse mackerel as the period from the date of first 

 active females with yolked oocytes, not necessarily 

 spawning, until the last active females sampled. The 

 fraction of active females classified as spawning was 

 used to estimate the spawning rate index, while the 

 fraction of the postspawning females was used as an 



index of the rate at which females passed from the 

 active to the inactive state. These data were fitted to 

 logistic curves (y=l/(l+e a ~ bx ), where y is the fraction 

 of the females used and x is the number of days 

 elapsed since the initiation of spawning; Hunter et 

 al., 1992). We calculated the approximate spawning 

 season for individual females by subtracting the days 

 from the 50% spawning point to the 50% post- 

 spawning point. 



In some females, multiple spawning stages (mi- 

 gratory-nucleus oocytes, hydrated oocytes, 1-d-old 

 postovulatory follicles, 2-d-old postovulatory follicles) 

 occurred within the same ovary indicating different 

 potential or past spawnings. Under the assumption 

 that the stages of advanced yolked oocytes and par- 

 tially yolked oocytes could represent two potential 

 spawnings, as many as six different spawning stages 

 could be distinguished histologically in the same 

 ovary. Hence, the mean number of stages per sam- 

 pling date was estimated, providing information 

 about seasonal changes of spawning frequency 

 (Hunter et al., 1992). 



Gonadosomatic and hepatosomatic index We also 

 followed the seasonal changes in ovarian develop- 

 ment by calculating the mean monthly gonado- 

 somatic (GSI) and hepatosomatic (HSI) indices. The 

 formulae used were the following: GSI= 100 (ovary 

 weight)/(body weight-ovary weight) and HSI= 100 

 (liver weight)/* body weight-liver weight). 



Estimation of length at 50% maturity 



Total lengths of all histologically identified females 

 were used to estimate the length at 50% maturity 

 (ML 50 ) of European horse mackerel during the 

 spawning season. We assumed that all females with 

 vitellogenic ovaries would reach sexual maturity, and 

 therefore such females were included in the propor- 

 tions of mature females. The proportions were esti- 

 mated at length classes of 10 mm, and the data fit- 

 ted to a logistic curve. Lengths of females with ova- 

 ries classified macroscopically were similarly fitted 

 to a logistic function for comparison of the ML 50 

 value. Macroscopically mature females possessed 

 ovaries that were classified to maturity stages above 

 stage II. 



Results 



Oogenesis 



In the ovaries of European horse mackerel, oocyte 

 development is divided into a primary growth phase 



