Fishery Bulletin 92(1). 1994 



peake Bay region using a validated otolith-ageing 

 method. We also evaluate the relationship between 

 otolith size and fish size and age, and discuss the 

 implications of using otoliths for ageing Atlantic 

 croaker. Finally, based on current information on 

 growth, and size and age compositions in Chesa- 

 peake Bay, we discuss the hypothesis of White and 

 Chittenden (1977) and Ross (1988) regarding the 

 existence of a basically different population dynam- 

 ics pattern for Atlantic croaker north and south of 

 Cape Hatteras, North Carolina. 



Methods 



Atlantic croaker were collected between June 1988 

 and June 1991 from commercial pound-net, haul- 

 seine, and gillnet fisheries which operate from early 

 spring to early fall in Chesapeake Bay. Local fish 

 processing houses and seafood dealers were con- 

 tacted weekly or fortnightly, and one 22.7-kg (50-lb) 

 box of fish of each available market grade (small, 

 medium, or large) was purchased. Although boxes 

 of fish were not randomly selected, Chittenden 

 (1989) found only minor among-box differences in 

 Atlantic croaker length compositions in pound-net 

 and haul-seine catches. Because nearly all variation 

 in size compositions was captured by the within-box 

 variation, box selection did not present a problem. 



Since Atlantic croaker migrate from Chesapeake 

 Bay in early fall to overwinter offshore (Haven, 

 1959), samples for the period November-March 

 were obtained from commercial trawlers which op- 

 erate in Virginia and North Carolina shelf waters. 

 Young of the year (90-114 mm total length, TL) used 

 to validate the first annulus on otoliths were ob- 

 tained from the Virginia Institute of Marine Science 

 juvenile bottom trawl survey. 



Fish were measured for total length (TL, ±1.0 

 mm), weighed for total weight (TW, ±1.0 g), sexed, 

 and both sagittal otoliths removed and stored dry. 

 The left otolith was transversely sectioned through 

 the core with the diamond blade of a Buehler low- 

 speed Isomet saw. Sections 350-500 urn thick were 

 mounted on glass slides with Flo-texx clear mount- 

 ing medium and read under a dissecting microscope 

 (6-12x) with transmitted light and bright field, with 

 the exception of samples from the period April-May, 

 when sections were also read with reflected light 

 and dark field to help identify the last annulus. 



Ages were assigned based on annulus counts; 

 January 1 was taken as an arbitrary average 

 birthdate when fish from one age class were as- 

 signed to the next oldest (Jearld, 1983). Although 

 the average spawning date (average biological 



birthdate) of Atlantic croaker in the Chesapeake Bay 

 region occurs in September (Barbieri et al., unpubl. 

 ms.), we chose, for ageing purposes, to use January 

 1 as the average birthdate because annuli are 

 formed during the period April-May (see Age deter- 

 mination below). To assess ageing precision, all 

 otolith sections (n- 1,967) were read twice by each 

 of two readers, and agreement between readings and 

 readers evaluated by percent agreement. All dis- 

 agreements were resolved by a third reading with 

 both readers. 



Annuli were validated by the marginal increment 

 method (Bagenal and Tesch, 1978). For each age, the 

 translucent margin outside the proximal end of the 

 last annulus was measured along the ventral side 

 of the otolith sulcal groove (Fig. 1). Measurements 

 (±0.02 mm) were taken with an ocular micrometer 

 at 25x. 



To evaluate growth, observed lengths at ages 

 1-7 were fit to the von Bertalanffy model (Ricker, 

 1975) by using nonlinear regression (Marquardt 

 method). Model parameters were the following: L m , 

 the mean asymptotic length; K, the Brody growth 

 coefficient; and t () , the hypothetical age at which a 

 fish would have zero length (Ricker, 1975). To cor- 

 rect for growth after the time of annulus formation, 

 only data for September, the peak spawning and 

 thus average biological birthdate for Atlantic 

 croaker in the Chesapeake Bay region (Barbieri et 

 al., unpubl. ms.), were used for growth analysis. 



To evaluate changes in otolith size relative to fish 

 length and age, 30 randomly selected otoliths per 

 age, for ages 1-7 ( 198^100 mm TL), were measured 

 for maximum length (OL, ±0.05 mm) and maximum 

 thickness (OT, ±0.05 mm), and weighed (OW, 

 ± 0.001 g). After sectioning, otoliths were measured 

 for otolith radius (OR, ±0.02 mm), defined as the dis- 

 tance between the center of the core and the otolith 

 outer edge along the ventral side of the sulcal groove 

 (Fig. 1). Relationships between otolith measure- 

 ments and fish TL were evaluated by regression 

 analysis. The effect offish age on these relationships 

 was evaluated by analysis of covariance (ANCOVA). 



Linear regression was used to determine a length- 

 weight relationship for fish ranging from 152 to 400 

 mm TL (36.3 to 967.0 g TW). Difference between 

 sexes was tested by ANCOVA. The hypothesis of 

 isometric growth (Ricker, 1975) was tested by t-test. 



Instantaneous total annual mortality rates, Z, 

 were estimated from maximum age by using 

 Hoenig's pooled regression equation (Hoenig, 1983), 

 by calculating a theoretical total mortality for the 

 entire lifespan following the reasoning of Royce 

 (1972), and by the regression method with a catch 

 curve of combined pound-net, haul-seine, and gillnet. 



