Jones et a I.: Population dynamics of Pogonias cromis 
457 
Mortality 
Mean instantaneous total mortality rates, Z, ranged 
from 0.08 to 0.13. Estimates obtained from a maxi- 
mum observed age of 59 years, and for age truncated 
at the 95 th percentile — 48 years, were 0.08 (A= 8%) 
and 0.09 (A=10%) with Hoenigs (1983) method, and 
0.08 (A=8%) and 0.10 (A=10%) with Royce’s (1972) 
method. A regression estimate obtained from the 
slope of a catch curve truncated at older ages (Fig. 9) 
was 0.12 (A=13%) with 95% confidence intervals of 
0.11 (A=12%) and 0.13 (A= 14%). This regression line 
did not deviate significantly from linearity ( ANOVA; 
F=1.18; P> 0.05). A regression estimate obtained from 
the slope of the full catch curve, i.e. with all older 
cohorts even when n< 5, was 0.09 (A=9%) with 95% 
confidence intervals of 0.08 (A=8%) and 0.09 (A= 
10%). This regression line, too, did not deviate sig- 
nificantly from linearity (ANOVA; F=1.29; P> 0.05). 
Discussion 
Age determination methods 
We believe otoliths are the preferred, most reliable 
hard part to use for ageing black drum. Reasons for 
this include high precision and readability of otoliths, 
their continued growth with increase in fish size and 
age, the increase in the number of annuli with size, 
and validation over most of the life span. Otolith 
annuli are extremely clear and easy to read, even 
out to 59 annuli, and agreement between readings 
was absolute, 100%. In contrast, fin rays and spines 
often produced unreadable sections, and fewer bands 
were counted than on otoliths, especially at older 
ages. 
We have not yet been able to validate black drum 
otolith ages completely in the Chesapeake Bay re- 
gion with marginal increments or other analyses. 
However, evidence from other regions indicate that 
black drum otoliths are valid throughout much of 
their life. For example, Fitzhugh and Beckman, 1 and 
Beckman et al. (1990) used marginal increment 
analysis to validate otolith annuli formation in black 
drum to age 43 from Louisiana. However, because 
most of their fish were age 5 to 27, they had to group 
the few fish at older ages. Our putative ages extend 
an additional 15 years beyond the range these au- 
thors described. Other evidence indicates members 
of the family (Sciaenidae) consistently produce an- 
nuli throughout life. Beckman et al. (1989) used 
marginal increment analysis to confirm annulus for- 
mation to age 37 in red drum, Sciaenops ocellatus. 
Ross et al. ( 1995) confirmed annuli formation in two 
red drum aged 38 and 40 through oxytetracycline 
marking of otoliths. Although we have not yet fully 
