160 
Fishery Bulletin 115(2) 
Month 
Figure 3 
Median monthly percent marginal increment (indicated by black points) for otoliths of 
sheepshead {Archosargus probatocephalus), all ages combined, collected in Tampa Bay, 
Florida, 1995—1998. Vertical lines indicate the interquartile ranges. Numbers above 
monthly percent marginal increments indicate sample sizes. Months of minimal mar¬ 
ginal increment (indicating annuli deposition) are highlighted in gray boxes. 
Age and growth 
Sheepshead ranged from <1 year to 15 years of age, 
and the mean ages of males (3.67 years) and females 
(3.73 years) were similar (Fig. 5). The overall age-fre¬ 
quency distributions of males and females (Fig. 5) did 
not differ significantly (KS test: 0.032, P>0.05). Sheeps¬ 
head of ages 2-4 accounted for more than half of the 
individuals collected (62.9%), but sheepshead aged 7 or 
older were relatively rare (7.9%). The oldest fish (sex 
not determined: 524 mm FL, 14.7 years; male: 404 mm 
FL, 14.9 years; and female: 345 mm FL; 15.2 years) 
were collected in a large haul seine. 
Observed length at age was variable for both sexes 
(Fig. 6). Growth was relatively rapid for both males 
and females. By age 1, sheepshead, regardless of sex, 
reached a size predicted to be more than 40% of L„, 
and, by age 6, they had reached sizes greater than 
80% of L^. Growth rates of both sexes slowed after age 
6. Males achieved a slightly greater L„ than females 
(3.4 mm FL greater), but females grew at a slightly 
higher rate (as measured by K; Table 3, Fig. 6) than 
males. The von Bertalanffy growth models for males 
and females (approximate randomization test: P<0.01) 
were significantly different. Although predicted size at 
age was greater for females than for males in all age 
classes from ages 1 through 10 (Table 4), the difference 
between predicted size at age between sexes was mini¬ 
mal, 7 mm FL or less (mean difference of 3.3 mm FL) 
across all age classes. 
Discussion 
Age determination and validation 
Sheepshead age and growth has been studied by us¬ 
ing both scales and sagittal otoliths. Although scales 
have been used to age sheepshead (Music and Pafford^; 
Schwartz, 1990; and Wenner^), validation of annuli on 
scales of sheepshead has indicated that scales are not 
as reliable as otoliths for aging this species. Music and 
Pafford^ could validate scale annuli only in sheepshead 
younger than age 5, and annuli in scales of sheepshead 
older than age 2 have been reported to be unreadable 
(Schwartz, 1990; Wenner^). Age has been underestimat¬ 
ed in sheepshead and other fish species when scales 
were used, and age estimates from the use of scales 
have been lower than those derived from otolith sec¬ 
tions (Beamish and McFarlane, 1983; Carlander, 1987; 
Lowerre-Barbieri et aL, 1994; Dutka-Gianelli, 1999). In 
our discussion, the only studies considered in growth 
comparisons are those in which ages were estimated on 
the basis of validated otolith annuli. 
We used marginal-increment analysis, which has 
been used to validate annulus deposition in the sag- 
