Winner et al.: Age and growth of Archosargus probatocephalus in Tampa Bay, Florida 
163 
Table 3 
Estimates of the von Bertalanffy growth parameters, by sex, and with sexes 
combined: asymptotic length (L„ ), growth coefficient (k), and hypothetical 
age at size zero «o) for sheepshead {Archosargus probatocephalus) col¬ 
lected in Tampa Bay, Florida, 1993-2009. Sample sizes in) and asymptotic 
standard errors (in parentheses) are listed. Combined includes female and 
male sheepshead, as well as sheepshead for which sex was not determined 
(n=169). 
Females 
Males 
Combined 
Loo (mm) 
K 
^0 
n 
419.1 (7.206) 
0.272 (0.019) 
-1.099 (0.162) 
1429 
422.5 (9.948) 
0.255 (0.023) 
-1.115 (0.205) 
797 
418.7 (5.309) 
0.273 (0.014) 
-0.981 (0.107) 
2395 
better fit for sheepshead from Louisiana waters than 
a model in which sexes were combined. Predicted siz¬ 
es at age for Louisiana sheepshead were similar for 
both sexes through age 6, with a mean difference in 
size at age of 11.6 mm FL between the sexes (Table 
4), but this difference in size was greater at older ages 
(25.8 mm difference in FL for ages 7-20). Despite find¬ 
ing no significant difference in sex-specific growth for 
sheepshead from northwest Florida, both sex-specific 
and combined-sex growth models were presented by 
Dutka-Gianelli and Murie (2001); mean differences 
in predicted size at age between sexes (11.2 mm FL) 
were larger than the differences we found (mean differ¬ 
ences of only 3.3 mm FL; Table 4). Therefore, evidence 
of growth differences between males and females has 
often been varied among previous studies. For recent 
stock assessments of sheepshead in Florida, growth 
was assumed to be similar for males and females, but 
coast-specific growth parameters were used because 
growth varied significantly between the two regions 
(Munyandorero et al.^). 
Our estimates of and the (Table 3) were with¬ 
in 2 standard errors of those estimated for fish from 
Louisiana waters (males: L„,=419, to= -0.901; females: 
L„=4A1, tQ= -1.025; Beckman et ah, 1991; Table 4). Es¬ 
timated for sheepshead from South Carolina (505.0 
mm FL) and northwest Florida (490.4 mm FL) were 
greater than those for sheepshead from either Tampa 
Bay or Louisiana. For sheepshead from South Caro¬ 
lina and northwest Florida and in our study, values 
of k were similar but smaller than those reported for 
sheepshead in Louisiana, indicating that Louisiana 
sheepshead reach more rapidly. Beckman et al. 
(1991) found predicted lengths for age-3 Louisiana male 
and female sheepshead were 80% and 77% of their L^, 
respectively. We found that sheepshead in Tampa Bay 
are not predicted to reach 80% of until age 5 for 
females (81%) and age 6 for males (83.8%). Similarly, 
in studies conducted in South Carolina (Wenner^) and 
northwest Florida (Dutka-Gianelli and Murie, 2001), 
sheepshead were predicted to reach 80% of by age 
5 and age 6, respectively. The differ¬ 
ences in growth parameters between 
these latter 2 studies may be attrib¬ 
uted to differences in sampling meth¬ 
ods, ontogenetic habitat shifts, or es¬ 
tuarine-specific differences in growth 
or mortality. Furthermore, the varia¬ 
tion in growth parameters between 
these studies may also be affected 
by variability in genetic composition 
(subspecies) among the regions where 
these studies were conducted. 
Reliance on fishery-dependent 
samples can introduce size- and age- 
related biases that can result in mis¬ 
leading interpretations of fish growth 
and size distribution, and age struc¬ 
ture of a population (Tangier, 1978; 
Miranda et ah, 1987; Hilborn and Wal¬ 
ters, 1992). All the sheepshead analyzed in a Louisiana 
study were collected from commercial and recreational 
catches. Almost 60% of their fish came from catches 
with gill nets, which tend to be size selective, often re¬ 
sulting in a narrow size range of collected fish (Pope et 
ah, 1975). Beckman et al. (1991) indicated that their 
sample of sheepshead was probably not representative 
of the Louisiana population because of gear selectiv¬ 
ity and sorting of catches before sampling. In a South 
Carolina study, most sheepshead larger than 300 mm 
FL were caught in recreational fishing tournaments 
and probably represented a greater percentage of larg¬ 
er fish than that of the overall population (Wenner^). 
Consequently, the sampling methods of these studies 
could have introduced sufficient size-at-age bias to the 
effect that the sampled fish did not represent the popu¬ 
lation as a whole. Our fisheries-independent sampling 
design increased the likelihood that our data would 
approximately represent the size and age structure of 
the population of sheepshead in Tampa Bay. Specimens 
were collected by using a variety of gear types (a ma¬ 
jority of specimens with the nonselective large haul 
seine [77%]), and at randomly selected sites (68%) that 
represented a variety of habitats. 
Adult sheepshead have been reported to occur over 
hard structure (reefs, jetties, and piers) in both estua¬ 
rine (Johnson, 1978; Ogburn, 1984) and offshore (Sed- 
berry and van Dolah, 1984) waters. Sheepshead also 
have been reported to undergo an ontogenetic shift 
in habitat as juveniles (Hildebrand and Cable, 1938; 
Johnson, 1978), moving from shallow nursery habitats, 
which often include sea grasses, to hard-structure 
habitats of adults. Sheepshead in our study were col¬ 
lected from relatively shallow waters (mean depth: 1.2 
m [standard error 0.02]) in the Tampa Bay estuary, 
whereas portions of sheepshead in the Louisiana and 
northwest Florida studies came from deeper, offshore 
areas. Render and Wilson (1992) found no significant 
differences in size or age between the sheepshead col¬ 
lected in offshore and inshore waters of Louisiana, but 
sample size and gear selectivity (i.e., similar among 
