74 



Fishery Bulletin 94(1). 1996 



2 



ra 

 E 



S 

 efl 



Q. 



20 40 60 80 100 120 140 160 180 200 220 



Days elapsed since 9 December 



i 



Dec 



1 1 



Feb Mar Apr 



May 



Figure 5 



Mean number of potential and past-spawning stages per 

 European horse mackerel, Trachurus trachurus, female 

 during the period from 9 December to 5 July 1 1989-90); 

 bars are two standard errors of the mean. 



in temperature, one of the main factors influencing 

 maturation (Billard et al., 1981), could not have af- 

 fected these second season changes because tem- 

 perature remained almost constant in the study 

 area. Another factor known to stimulate ovary re- 

 sorption is an imbalance in the sex ratio (Trippel 

 and Harvey, 1990). However, in the present work 

 this ratio did not differ between the two reproduc- 

 tive seasons and remained constant. Although a re- 

 activation of regressing ovaries has been postulated 

 as a possible event (Hunter and Macewicz, 1985b), 

 it is not known whether postspawning European 

 horse mackerel females can be reactivated during a 

 spawning season. The high levels of atretic states 2 

 and 3 for European horse mackerel in the middle of 

 the season may not necessarily indicate cessation 

 of spawning but merely the end of a spawning cycle 

 within the spawning season. This implies that Eu- 

 ropean horse mackerel seems to be very flexible in 

 adjusting its spawning time to different conditions 

 and that its spawning cycle might be more variable 

 than previously assumed. Such a flexibility may be 



very important for the survival of the species in highly 

 oligotrophic waters, such as the Greek and northeast 

 Mediterranean waters in general (Stergiou, 1993). 



The index of spawning rate provided information 

 about the spawning behavior of active European 

 horse mackerel females during the first reproduc- 

 tive season. The increase of this rate, beginning very 

 early in the season, indicated an increase in spawn- 

 ing frequency in the same period! Hunter etal., 1992). 

 This increase was supported by an increase in the 

 mean number of potential or past-spawning stages. 

 The decrease in this number toward the end of the 

 season likely indicated a decrease of spawning fre- 

 quency, although all the remaining active females 

 were spawning. 



We noticed in using both histological and macro- 

 scopic data that the spawning season of European 

 horse mackerel lasts about six months. This season 

 ranges from 3 to 6 months for northern to southern 

 areas of the eastern Atlantic, respectively (Arruda, 

 1984; Eaton, 1989; Eltink 7 ). However, we believe the 

 duration of the season, during which spawning was 

 observed, differs from the spawning duration for the 

 average female, which may be very representative 

 of the population. Smaller northern anchovy females 

 have a shorter spawning season because of high lev- 

 els of atresia (Hunter and Macewicz, 1985b). The 

 same may be true for European horse mackerel be- 

 cause all females did not start and finish spawning 

 on the same day. The estimated spawning season for 

 the average European horse mackerel female (94 

 days) is higher than that for the same species in the 

 eastern Atlantic which is assumed to be 65 days 

 (Eltink 2 ). 



The GSI, as an index of ovarian changes during 

 the spawning season, is of lesser importance than 

 histological data for multiple spawners. For instance, 

 this index does not distinguish partly spent (active 

 females with postovulatory follicles) and spent (in- 

 active females with regressing ovaries) females. The 

 peak GSI value may not indicate the peak spawning 

 time but rather a later developing stage for most fe- 

 males (Arruda, 1984). However, high values of this 

 index can be used to determine the period when hy- 

 drated oocytes can be identified (Hunter and 

 Macewicz, 1985a). This is very useful for batch fe- 

 cundity estimates, where hydrated oocytes need to 

 be counted. GSI calculated over the entire year indi- 

 cated the approximate reproductive period, and the 

 low GSI values near the end of the season consti- 

 tuted a helpful guide to validate the end of the sea- 



Eltink.A. 1991. Horse mackerel egg production and spawn- 

 ing stock size in the North Sea in 1990. ICES Council Meet- 

 ing H:27, 14 p. (Mimeo I 



