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



Table 1 



Summary of ANCOVA to evaluate the effect of 

 Atlantic croaker (Micropogonias undulatus) total 

 length (TL) and age on otolith maximum thickness 

 (OT), maximum length (OL), weight (OW), and ra- 

 dius (OR), n = 210 for each analysis; a = 0.05. 



Otolith 

 relation 



Source of 

 variation 



P-value 



OT 



OL 



OW 



OR 



age 



model 

 TL 

 age 

 TL x 



model 

 TL 

 age 

 TL x age 



model 

 TL 

 age 

 TL x age 



model 

 TL 

 age 

 TL x age 



0.85 



I) s.s 



0.90 



0.97 



0.0001 

 0.0001 

 0.3263 

 0.6214 



0.0001 

 0.0001 

 0.9780 

 0.7907 



0.0001 

 0.0001 

 0.0863 

 0.1402 



0.0001 

 0.0001 

 0.0001 

 0.0008 



tured by each gear was approximately 200 mm, al- 

 though these data represent only market foodfish 

 grades (small, medium, or large) and did not include 

 smaller fish sold as scrap. The maximum length 

 recorded was 400 mm, from a pound-net catch in 

 1988. However, for all gears 99% of the Atlantic 

 croaker collected were <345 mm. 



Age compositions from different gears were not as 

 similar as length frequencies suggest (Fig. 7). Haul- 

 seines, gill nets, and commercial trawls caught a 

 large proportion offish at ages 1 and 2, and had age 

 2 as the first age fully recruited. Pound nets cap- 

 tured a comparatively larger proportion of fish at 

 ages 4-7, and had age 3 as the first age fully re- 

 cruited. Age-1 fish were not fully recruited to any 

 of the gears sampled, but this may reflect, in part, 

 the exclusion of scrap fish from our collections. 



The maximum age sampled was 8 years. Despite 

 the large sample size and the variety of gears used, 

 only two eight-year-old fish were collected, one from 

 a pound net in September 1988 (334 mm) and one 

 from a gill net in September 1990 (293 mm). 



Mortality 



Instantaneous total annual mortality rates (Z) 

 ranged from 0.55 to 0.63. Estimates obtained for a 



maximum age of 8 years were 0.55 (A=42%) by us- 

 ing Hoenig's (1983) method, and 0.58 (A=43%) by 

 using Royce's ( 1972) method. A regression estimate 

 obtained from the slope of the catch curve (Fig. 8) 

 was 0.63 (A=47%); confidence intervals were 0.36 

 (A=30%) and 0.90 (A=59%). The regression line did 

 not deviate significantly from linearity (ANOVA; 

 ^=1.15: P=0.40). 



Discussion 



Age determination 



Our criteria for ageing Atlantic croaker from otolith 

 sections differ from those of Barger (1985), in that 

 we considered the first annulus to be the blurred 

 opaque band surrounding the otolith core. However, 

 evidence from both studies seems to support our 

 interpretation. Barger (1985) reported 58% of the 

 otoliths in his samples had marks that were too thin 

 or discontinuous and too close to the core to be con- 

 sidered annuli. By examining otoliths of young of the 

 year during the period of annulus formation, we 

 were able to validate this mark as the first annu- 

 lus, formed during their first spring in the estuary. 

 Because spawning of Atlantic croaker in the Chesa- 

 peake Bay area extends from late July to Decem- 

 ber (Barbieri et al., unpubl. ms.) and the first an- 

 nulus is formed during their first spring after hatch- 

 ing, fish forming the first annulus could range from 

 5 to 10 months of age. As marginal increment plots 

 indicated, all subsequent annuli formed at yearly 

 intervals. 



Variation in the width of the first annulus also 

 seems to reflect the protracted spawning period of 

 Atlantic croaker. Early hatched fish (July-August) 

 would probably be large enough by April or May to 

 have this annulus close to, but not continuous with, 

 the otolith core. In contrast, late-hatched fish (No- 

 vember-December) would be small in the spring and 



