Beckman et al : Age and growth estimation of Archosargus probatocephalus in Louisiana waters 



o 



z 



rx 

 O 



250 

 200 



-r FEMALE 

 — MALE 



10 12 14 16 18 20 22 

 AGE (years) 



Figure 3 



Von Bertalanffy growth models by length for male and female 

 sheepshead. M = male; F = female. 



Due to the large variability in age at a given body 

 size, size does not accurately estimate age for sheeps- 

 head, especially beyond 2-3 years of age. For exam- 

 ple, a given sheepshead greater than 400 mm or 1.5 

 kg could be of any age from 3 to 20 years. 



Otoliths ranged in weight from 28.3 mg (for a 229- 

 mm, 312-g, 2-year-old fish) to 323.5 mg (for a 450-mm, 

 2410-g, 18-year-old fish). Age-otolith weight (OW) re- 

 gressions (Fig. 5) were significantly different for males 

 and females (P< 0.0001 for slopes, P = 0.0006 for inter- 

 cepts); therefore multiple-regression models were fit 

 separately by sex. Dependent variables included in 

 multiple regression models at the 0.10 level of signif- 

 icance were otolith weight and total weight for males, 

 and otolith weight and fork length for females. The ad- 

 dition of any other variables did not significantly im- 

 prove the fit of the models. The model statistics are 

 presented in Table 2. 



Since otolith weight accounted for more of the 

 variability in age (83-85% vs. <60% for fish length or 

 weight), it was the best estimator of age of all mor- 

 phometric variables considered. However, there was 

 still considerable variability in otolith weight within 

 each age class. Although some of the remaining vari- 

 ability (1-2%) was accounted for by considering fish 

 length or weight in addition to otolith weight, the unex- 

 plained variability was great enough that these models 

 could not be used for precise estimation of sheepshead 

 age, such as is needed to determine population year- 

 class structure. However, they could be used to approx- 

 imate age distribution patterns in the population. In 

 order to obtain precise age estimates of individual fish, 



4000 

 3600 

 3200 

 2S00 

 2400 

 2000 

 1600 

 1200 

 800 

 400 



10 12 14 

 AGE (years) 



Figure 4 



Von Bertalanffy growth models by weight for male and female 

 sheepshead. M = male; F = female. 



otolith annulus counts are necessary. 



The linear relationship between otolith weight and 

 age indicates that otolith growth is continuous for 

 sheepshead, whereas fish size (length and weight) 

 asymptotes at intermediate ages and, therefore, is not 

 continuous. This suggests that fast-growing (younger) 

 sheepshead have lighter otoliths than equal-sized slow- 

 growing (older) sheepshead, i.e., otolith growth con- 

 tinues with age, independent of fish growth. This may 

 be a general characteristic of fish growth, as similar 

 observations have been made for other fish species 

 (Templeman and Squires 1956, Beamish 1979, Wilson 



