Dentzau and Chittenden Population dynamics of Polydactylus octonemus in the Gulf of Mexico 



447 



,.. 



95\Conlit]ence Limits 

 AbouHheMean 



Range 



SEP OCT DEC APR JUN JUL AUG SEP OCT NOV JUN JUL AUG SEP OCT MAY JUN JUL AUG 



Figure 8 



Monthly mean gonadosomatic indices, 

 ranges, and 95% confidence limits about 

 means for female Polydactylus octonemus, 

 September 1978-AugTast 1981 . 



est November-December (Fig. 8), except in August 

 1980 and 1981, when the few late-developing fish great- 

 ly skewed the means, and in mid-April 1979, when one 

 age-I fish was gravid. The latter fish suggests some 

 spawning in April and possibly May. 



The apparent overall spawning period spans 45-120 

 days. Calculated overall spawning periods were 144 

 and 80 days in 1978, 53 and 80 days in 1979, 46 and 

 103 days in 1980, and 64 and 121 days in 1981 based 

 on respective observed and predicted growth/day 

 values. The broad 144-day interval for 1978 was based 

 on only 9 fish and may be unreliable. Ignoring that 

 value, spawning encompassed a 45-120 day interval 

 which brackets the 90 day duration length frequencies 

 indicated. 



A large fraction of the successful spawning may oc- 

 cur in a period much shorter than the calculated overall 

 spawning period duration. Using mean 90% instead of 

 99% confidence limits for observations, spawning 

 periods were 78 and 43 days in 1978, 33 and 50 days 

 in 1979, 27 and 60 days in 1980, and 40 and 76 days 

 in 1981, based on respective observed and predicted 

 growth/day values. On average, the duration estimated 

 using 90% confidence limits was 59% of that using 99% 

 limits. This suggests most spawning occurs in a rela- 

 tively small part of the overall spawning period. 



Cohorts are apparently produced by fish that first 

 spawn when 10-14 months old, assuming the hatching 

 dates and sizes at early age noted previously. Spawn- 

 ing at 10-14 months of age is supported by (1) the 

 disappearance from collections of all but one fish 

 sometime during late October-late December when 

 9-11 months old (Fig. 6), (2) the occurrence of large 

 mean and maximum GSI values in October-December 

 (Fig. 8), and (3) the collection of early developing and 

 more sexually mature fish during September-Decem- 

 ber (Fig. 7). ' 



Polydactylus octonemus exhibit a sex ratio of 1.00 

 male to 1.78 females. This ratio was observed in 800 

 fish and differed significantly from 1:1 (x" = 64.98, 

 a = 0.05, df= 1). 



Bathymetric distribution, recruitment, 

 and movements 



Polydactylus octonemus in the northwestern Gulf oc- 

 cur from <5 m depths to at least 27 m off Freeport. 

 Only one specimen was collected deeper than 27 m, a 

 75-mm fish at 86 m in April, which may represent a 

 pelagic young captured as the net was being retrieved. 

 Greatest abundance generally occurred at 5 m, the 

 shallowest depth occupied (Fig. 9), except in 1978 when 

 this depth was occupied only in December, a time when 



