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



441 



fish were age 0, we forced through the origin a regres- 

 sion to approximate age in days at observed sizes; this 

 procedure is detailed under the methods used to 

 calculate hatching dates. Cohorts (year classes) were 

 specified on length-frequency figures by the season and 

 year when they were hatched, e.g., winter 1981. Arith- 

 metic means were used to describe central tendencies 

 of length for each cruise, because length frequencies 

 within each cohort were generally normally distributed. 

 Descriptions of spawning periodicity (beginnings and 

 ends) using length frequencies assumed the following 

 size and age combinations predicted from regressions 

 of total length on age (Fig. 2): 25-40 mm TL at 1 



Figure 2 



Mean observed and predicted sizes at age (days) for Polydactylus 

 octonemus cohorts with a summary of the iterative process used to 

 calculate final hatching dates (H.D.) and set time-scales for final 

 growth equations (F.E.). Initial growth equations (I.E.) were scaled 

 to a 1 February hatching date. All regressions are significant at 

 o = 0.005. The first two and three collections of the 1980 and 1981 

 cohorts, respectively, were deleted from calculations; see text on 

 "Maturation and Spawning Periodicity" for explanation. 



E 150 



E 



A Winler 78 



SS^a Conlidence Interval \ 

 About Observed Mean I 



Range 



l.E.:v= 19.44t0 89526«-0 00114 k' 

 x-Int = 21 



F.E. :v=0 14»0 94298X-0 00TI4«' 



r' =0.96 



H.0.= 11 JAN 78 



SO 120 180 240 300 360 420 460 



AGE (DAYS) 



~F M A M J J A S 6 N D J F M A M 

 1976 1979 



COLLECTION DATE 



B Winter 79 



I E v = -21 62»1 41077X -0 00239«' 

 «-lnt = 16 



F.E. :v=0 34+1 33423«-0 00239«' 



r'=0.97 



H.D = 16 FEB 79 



) 60 120 180 240 300 360 420 480 



AGE (DAYS) 



MAMJ J ASONDJ FMAMJ 

 1979 1980 



COLLECTION DATE 



C Winter 80 



LE.:y=4.8V0. 785241 

 X -lnt.= 6 



F.E.:v=0 10+0.78524X 



t'=0 95 



H O = 26 JAN 80 



60 120 180 240 300 360 420 480 



AGE (DAYS) 



F M A M J J A S O N 5 J F M A M ' 

 1980 1981 



COLLECTION DATE 



D Winter 81 



I 150 



/ 



.^ 



/' 



I.E.:v=-5.22t0.926S3x 

 x-lnl,= 6 



F E v=0 34*0 92653X 



r' =0 94 



HO =7 FEB 81 



60 120 180 240 300 360 420 480 



AGE (DAYS) 



~fi A M J J A S O i5 5 J F M A M T 



1981 1982 



COLLECTION DATE 



