Barbieri et al.: Age, growth, and mortality of Micropogonias undulatus 



Proximal 



Ventra 



Figure 1 



Transverse otolith section of an 8-year-old Atlantic croaker caught in Sep- 

 tember 1988 in Chesapeake Bay. Arrows indicate annuli. The translu- 

 cent zone beyond the last annulus represents additional growth after the 

 annulus was formed during April-May. SG =sulcal groove, a = artifact 

 of preparation. Ventral and proximal indicate axes of orientation. 



data for all recruited ages having five or more fish 

 (Chapman and Robson, 1960). To avoid sampling 

 bias associated with individual gears, we considered 

 the age-frequency distribution obtained from data 

 from combined gears as the best estimate of Atlan- 

 tic croaker age composition in Chesapeake Bay 

 (Ricker, 19751. Commercial trawl collections were 

 not used in this analysis because they had different 

 length compositions than the other gears and could 

 be biased towards small fish. Because in catch curve 

 analysis the age group represented by the apex of 

 the catch curve may or may not be fully recruited 

 to the gears (Everhart and Youngs, 1981), mortal- 

 ity estimates were based on ages 3-7 only. Data 

 from 1988 to 1991 were combined to minimize the 

 effect of variation in year-class strength (Robson and 

 Chapman, 1961). The right tail of the catch curve 

 (Ricker, 1975) was tested for deviation from linear- 

 ity by analysis of variance (ANOVA). Values of Z 

 were converted to total annual mortality rates, A, 

 by using the relationship A = 1 - e - z { Ricker, 1975). 

 All statistical analyses were performed by using 

 the Statistical Analysis System (SAS, 1988). Rejec- 

 tion of the null hypothesis in statistical tests was 

 based on a=0.05. F-tests in ANCOVA were based on 

 Type III sums of squares (Freund and Littell, 1986). 



Assumptions of linear models were checked by re- 

 sidual plots as described in Draper and Smith 

 ( 1981). For the OL-TL, OW-TL, and TW-TL relation- 

 ships, and for all ANCOVA and ANOVA analyses, 

 data were log 10 -transformed to correct for non-lin- 

 earity and heterogeneous variances. For the catch 

 curve analysis, log e -transformed numbers at age 

 were regressed on age. Unless otherwise indicated, 

 back-transformed data and regression equations are 

 presented in the results. 



Results 



Age determination 



Transverse otolith sections of Atlantic croaker show 

 very clear, easily identified marks that can be used 

 for ageing. Typical sections have an opaque core 

 surrounded by a blurred opaque band composed of 

 fine opaque and translucent zones (Fig. 1). This 

 band represents the first annulus. The width of this 

 annulus varies among fish, from a very narrow band 

 that is almost continuous with the core, to a wide, 

 well-defined band clearly separated from the core. 

 Because of this variation in width and proximity to 



