Dentzau and Chittenden: Population dynamics of Polydactylus octonemus in the Gulf of Mexico 449 



Figure 10 



Monthly length frequencies oiPolydnctylii,s octonemus in Galveston 

 Bay. Texas. 1968 and 1969. Adapted from seine and trawl data of 

 Gallaway and Strawn (1974); length frequencies are not moving 

 averages of three. 



(Fig. 12). Usually no P. octonemus were captured 

 December-February. 



Age determination and growth 



Only one cohort of P. octonemus usually occurs at any 

 one time in the northwestern Gulf. The only exceptions 

 to this we found were in April 1979 when two cohorts 

 appeared, albeit on separate cruises, and possibly in 

 November of 1977 and 1980 (Fig. 6), when the second 

 cohort was represented by only one individual. 



Polydactylus octonemus is not abundant in the demer- 

 sal phase in the northwestern Gulf after 8-11 months 

 of age and apparently reaches only 15 months. Cohorts 

 were present only until their first November or Decem- 

 ber after which they were not captured again (Fig. 6), 

 except for the 194-mm fish in April 1979 that was the 

 only fish older than 12 months. 



Apparent growth of P. octonemus varied between 

 cohorts, but observed mean sizes, and von Bertalanffy 

 and quadratic regression predictions, averaged 

 135-165 mm at 6 months, 165-215 mm at 9 months, 

 and 180-205 mm extrapolated to 12 months (Table 4). 

 Regardless of the growth model, for a given cohort, 

 predicted sizes agreed within 6 mm or less at 6 and 9 

 months and within 11 mm at 12 months. 



Fitted von Bertalanffy equations were: 



(1978):^, = l95.2[l-e-«oo™9"-3'''»0)]; 



annual K = 2.92; annual ^, = 0.0833 



(1979):/, = 230.4[l-e-"005955(f-ii.69)]. 



annual K = 2.17; annual /,, = -0.0320 

 where /< = TL in millimeters at time t in days. 



Maximum size, life span, and mortality 



Polydactylus octonemus in the demersal phase ap- 

 parently reach a maximum size of about 230 mm in the 

 northwestern Gulf but more typically only 200-205 

 mm. The largest of 4324 specimens collected was 229 

 mm; 99% were <202 mm and 99.5% were <206 mm 

 (Fig. 3). The latter two sizes estimate an apparent li. 

 The apparent typical maximum life span in the 

 demersal phase of P. octonemus in the northwestern 

 Gulf is only 1 year. A value of /^ = 1 seems reasonable 

 because (1) /^ values of 202 and 206 mm can be 

 substituted into hatching-date regressions and von Ber- 

 talanffy equations to predict values of 0.69-1.00 year; 

 (2) observed and predicted mean sizes at 9 months are 

 165-215 mm and predicted sizes at 12 months are 

 180-205 mm; (3) the largest specimen was about 11 

 months old when collected in December 1978; and 



