Jones et al.: Population dynamics of Pogonias cromis 
459 
plained by the differential seasonal migration north- 
ward of older, larger fish from a population centered 
farther south, as was first postulated by Welsh and 
Breder (1923). Finally, two fish tagged in northeast 
Florida were captured about four months later at the 
mouth of Chesapeake Bay; thus long-range migra- 
tions do occur (Murphy 3 ). 
In summary, migration and gear selectivity are 
likely explanations of the age structure of the Chesa- 
peake Bay fishery and the apparent absence of age 
1-5 fish in this region. However, given our data, we 
cannot rule out local recruitment failure. Movement 
and exchange is supported by similar sizes-at-age in 
fish from Florida and Chesapeake Bay (Table 2): 
mean maximum length is 117.2 cm TL for Florida, 
117.3 cm TL for Virginia; maximum ages along the 
east coast are 58 for Florida (Murphy and Taylor, 
1989), 46 for Georgia (Music and Pafford, 1984), and 
59 for Virginia (this study). 
Stock unity 
Several lines of evidence suggest that black drum on 
the U.S. east coast are from a common stock. Fish 
throughout the area appear to have similar growth. 
Von Bertalanffy growth function parameters that we 
estimated for the Chesapeake Bay region (1/^= 117.3 cm; 
7f=0. 105/'yr; t Q =- 2.3 yr) were similar to those that 
Murphy and Taylor (1989) found in northeast Florida 
(L m =ll7 .2 cm; if=0.124/yr; t 0 =- 1.29 yr). In contrast, 
black drum from the Gulf of Mexico grow more 
quickly, are smaller at age, and have a smaller maxi- 
mum size (Table 2). Mitochondrial DNA evidence also 
suggests a common stock in the western North At- 
lantic Ocean. No significant differences in frequency 
3 Murphy, M. D. 1995. Florida Marine Research Institute, 
Department of Environmental Protection, 100 Eighth Ave. S.E., 
St. Petersburg, FL 33701. Personal commun. 
of mtDNA haplotypes were found in fish taken from 
Virginia and the east coast of Florida (Gold 4 ). How- 
ever, Atlantic east coast fish differed from those 
sampled in the northern Gulf of Mexico (Gold et al., 
1995). Finally, limited tagging data directly suggest 
black drum move between Chesapeake Bay and 
Florida (as noted previously). 
Implications of mortality estimates 
The long life we found in black drum indicates a low 
mortality rate for larger fish and a stock that cannot 
support heavy fishing pressure. Our greatest esti- 
mate of instantaneous total mortality, Z, converts to 
an annual total mortality (A) of less than 13%. As Z 
= F + M, natural mortality must also be less than 
13%. Because black drum do not completely recruit 
to the fishery until age 21 in the Chesapeake Bay 
region, our estimates of total mortality apply to the 
period of 21 years ago and earlier. For our estimates 
to be valid today, fishing mortality on young fish must 
still be low throughout the stock’s range. Values of Z 
have important implications for management. Stocks 
with high M generally can withstand the highest fish- 
ing mortality because fishing simply takes fish that 
would otherwise die from natural causes. In contrast, 
stocks with low M (like black drum) do not have a 
potential for such “excess” natural mortality that can 
be diverted into fishing mortality (Gulland, 1983; 
Murphy and Taylor, 1989). 
Life history strategy 
Black drum have an unusual life history for a long- 
lived fish. They achieve a large size quickly — 84% of 
4 Gold, J. R. 1995. Center for Biosystematics and Biodiversity, 
Department of Wildlife and Fisheries Sciences, Texas A&M 
University, College Station, TX 77843. Personal commun. 
Table 2 
Estimates of von Bertalanffy growth function parameters from various studies of black drum. Standard errors in parentheses 
(when available). 
Growth parameters 
Sample 
size 
Total length 
range (cm) 
Area and study 
L^lcm) 
K 
*0 
Atlantic coast 
Murphy and Taylor (1989) 
117.2 (0.9) 
0.124(0.003) 
-1.29 (0.08) 
397 
20.2-127.5 
Northeast Florida 
Present study 
117.3 (0.4) 
0.105(0.003) 
-2.3 (0.2) 
871 
22.9-130.2 
Gulf of Mexico 
Doerzbacher et al. (1988), Texas 
Beckman et al. (1990), Louisiana 
79.8(4.2) 
110.0 
0.219 (0.027) 
0.038 
-16.42 
383 
1072 
20.3-99.1 
