Barbien et al Age, growth, and mortality of Micropogonias undulatus 



ment values progressively rise to a somewhat stable 

 maximum from October through March or April, 

 indicating a period of little or no otolith growth. 

 Because only two age-8 fish were collected, it was 

 not possible to validate annuli beyond age 7. 



To confirm our interpretation that the blurred 

 opaque band around the otolith core represents the 

 first annulus, (i.e., that fish hatched in the fall form 

 a mark during their first spring), otolith sections of 

 young of the year (94-114 mm) collected during the 

 period March-June were examined. All those col- 

 lected in March-April were developing fine opaque 

 marks around the core, and all those in May-June 

 had an opaque mark already formed (Fig. 3). 



Otolith age readings were very precise, both 

 within and between readers. Percent agreement was 



Figure 3 



Transverse otolith section of a young-of-the-year Atlan- 

 tic croaker ( 114 mm TL) collected in June 1990 in Chesa- 

 peake Bay. The arrow indicates the outer edge of the first 

 annulus formed during the period April-May. SG=sulcal 

 groove; Ve=ventral; Pr=proximal; a=artifact of preparation. 



99.5% for reader 1, 99.3% for reader 2, and 99.2% 

 between readers. In all cases of disagreement, the 

 difference never exceeded 1 year. Only one of the 

 1,967 left otoliths sectioned was crystallized and 

 could not be read. In that case, the right otolith was 

 read. Difficulty in ageing Atlantic croaker from 

 otolith sections did not increase with increasing age. 

 However, proper identification of the first annulus 

 was very important. All disagreements, independent 

 of age, were due to problems in identifying the first 

 annulus. 



Otolith size relative to fish size and age 



Changes in otolith size relative to fish size were not 

 constant along all axes (Fig. 4). Otolith maximum 

 length was the only axis that showed a linear, 

 isometric increase with fish length. Otolith ra- 

 dius, the axis along which annuli were read in 

 transverse sections, showed a non-linear rela- 

 tionship with fish length, and had the small- 

 est r 2 of all variables (Fig. 4). The curvilinear 

 relationship suggests that otolith growth rela- 

 tive to fish growth slows down along this axis 

 as fish get bigger. 



Despite its poor relationship with fish length, 

 otolith radius showed a very strong linear re- 

 lationship with fish age. An ANCOVA model 

 showing length, age, and their interaction ex- 

 plained 97% of the variation in otolith radius 

 (Table 1). All factors in the model were highly 

 significant (P<0.01). Similar models for otolith 

 maximum length, maximum thickness, and 

 weight were also highly significant and had 

 high coefficients of determination (r 2 >0.85). 

 However, significance for these models was due 

 to fish length only, neither age nor the inter- 

 action factor was significant. 



Growth 



Observed lengths varied greatly within ages 

 (Fig. 5). Atlantic croaker showed a rapid in- 

 crease in size during the first year, but annual 

 growth rate greatly decreased during the sec- 

 ond year, remaining comparatively low there- 

 after (Fig. 5). On average, 64% of the cumula- 

 tive total observed growth in length occurred 

 in the first year and 84% was completed after 

 two years. 



No differences in mean lengths at age were 

 found between sexes (Mest at each age; P>0.05 

 for all ages). Mean observed total lengths for 

 pooled sexes were 201, 263, 274, 285, 290, 307, 

 309, and 313 mm, for ages 1-8, respectively. 



