Mackie et al.: Variability in reproductive development of Spanish mackerel (.Scomberomorus commerson) 



351 



years. Data for the Kimberley and West Coast regions 

 were limited but concurred with gonad staging data 

 and also confirmed the low reproductive status of S. 

 commerson within the West Coast region. 



Spawning 



Evidence of spawning was found in 237 of the histologi- 

 cally processed ovaries. Thirty-eight percent (;? = 90) of 

 these were about to spawn when captured (stage 5a), 

 62% (n=147) had recently spawned (stage 5c), and one 

 was running, ripe (stage 5b). The ovaries of only two 

 macroscopically staged females were also running, ripe. 

 Most of these spawning fish (n=219) were captured in 

 the north Kimberley region (eighteen from the Pilbara 

 region). The most southern location from which a spawn- 

 ing female was obtained was Exmouth (one recently 

 spawned fish), and no females captured in the West 

 Coast or more southern regions showed histological (or 

 macroscopic) evidence of spawning. 



Spawning females collected during 1999 and 2000 

 were either prespawning (stage 5a) and caught in the 

 morning, or had recently spawned (stage 5c) and were 

 caught in the afternoon (Table 1). The absence of hy- 

 drated oocytes in the afternoon and new POFs in the 

 morning showed that the entire cycle of oocyte matu- 

 ration, ovulation, and spawning is completed within 

 a 24-hour period. Because no new POFs were present 

 in ovaries sampled during the morning the transition 

 from new to old POFs occurs during the night, within 

 about 12 hours of spawning. The lack of evidence to 

 show that females spawned on more than two consecu- 

 tive days indicates that old POFs are unrecognizable 

 after 24 hours. 



Spawning fraction was estimated by using data ob- 

 tained in the Kimberley region during September 1999 

 when 95% (n=344) of ovaries were examined by using 

 histological methods. Analyses were based on the num- 

 ber of prespawning (stage 5a) ovaries sampled during 

 the morning (usually between 0600-0900 h). Afternoon 

 samples (usually 1500-1800 h) were not used because 



the number of spawning fish was likely to be under- 

 estimated because of the low catchability of running, 

 ripe (stage 5b) females. Thirty-five percent (n = 59) of 

 mature females in the morning samples were about to 

 spawn (stage 5a). Spawning frequency was therefore 2.9 

 days. Comparison of spawning fractions in samples of 

 at least ten females showed higher spawning fractions 

 (33-56%) for the Kimberley region compared with the 

 Pilbara region (4-28%). 



Spawning fraction was also estimated for the morn- 

 ing samples as the proportion of macroscopically staged 

 mature ovaries that contained hydrated oocytes. Thirty- 

 one of the 180 mature females were identified as such, 

 providing an estimated spawning fraction of 17.2%, and 

 a spawning frequency of 5.8 days. 



Thirty-six percent (n=54) of spawning females (stages 

 5, a-c) had spawned on two consecutive days. For exam- 

 ple, 39 ovaries contained oocytes in the MNS or hydrat- 

 ed stage of development (i.e., spawning was imminent 

 when fish were captured) and also contained old POFs. 

 Another 15 ovaries had both old and new POFs. 



Discussion 



Scomberomorus commerson has a gonochoristic life his- 

 tory in which the gonad differentiates into an ovary or 

 testis at around 300-400 mm FL. Males differentiate 

 and reach sexual maturity at a smaller body size than 

 females, as is the case with the congeneric species S. 

 maculatus and S. cavalla (Beamariage, 1973; Schmidt 

 et al., 1993). Consequently, more than 90% are sexually 

 mature by the time the minimum legal length of 900 

 mm TL is reached in the fishery. In contrast only 50%> of 

 females are mature at 898 mm TL. Although mortality 

 of released undersize fish may be high because of dif- 

 ficulties in removing fishing hooks, this size limit deters 

 fishermen from targeting small fish and relatively few 

 are captured (Mackie et al. 3 ). 



Biases in sex ratios have been observed in several 

 species of Scomberomorus (e.g. Trent et al., 1981; Sturm 



