WHITE and CHITTENDEN: AGE DETERMINATION OF ATLANTIC CROAKER 



30  

 25 - 

 20 - 

 15 - 

 10  

 5 







I 



< 



5 - 



10 - 



z 

 uu 



a. 15 



UJ 

 Q. 



20 



25 H 

 + 30 

 35 - 

 40 - 

 45- 



50 



May 



March 



Novemb 



September 

 August 



August 



September 



50 



200 

 TOTAL LENGTH (MM I 



—I 

 300 



FIGURE 4. — Monthly somatic weight changes in Atlantic 

 croaker. Percentage changes are in comparison to somatic 

 weights in June. The lengths of the curves represent observed 

 size ranges. 



October just prior to the time of peak spawning. 

 However, greatest somatic weight loss was ob- 

 served in March when individuals of 170-250 mm 

 had lost 20-24% of the June weight. The ob- 

 served somatic weight-length relationships and 

 apparent weight changes in November may be 

 anomalous. Absolute somatic weight decreased in 

 fish smaller than 140 mm, but the percentage 

 weight loss in fish greater than 160 mm was about 

 5%. Croaker mature at about 140-160 mm, and 

 most fish were small and immature in November. 

 These smaller fish may have just begun to mature 

 for spawning, and their inclusion in the data may 

 have biased the observed pattern in November. 

 This interpretation is supported by the regression 

 coefficients of X and X 2 which were markedly 

 smaller during November than during other 

 months in the August-March period (Table 2). 



Somatic weight changes have not been reported 

 for croaker. Additional data, especially from the 

 post-peak spawning period December to February, 

 are needed to fully understand their annual cycle 

 of somatic weight change. Possibly, the percen- 

 tage of somatic weight loss may be greater in late 

 fall and winter than we observed in March. 



AGE DETERMINATION AND GROWTH 



General Basis for the Method of 

 Age Determination 



Scale marks similar to annuli were distin- 

 guished by standard criteria, especially cutting 

 over and differential spacing of circuli. Croaker 

 appear to form two marks on their scales each year 

 except that no mark is formed during their first 

 winter. Some fish form no mark during their first 

 year if 1 5 October is defined as the hatching date of 

 croaker. Even-numbered marks (cold-period 

 marks) form from about December to March, and 

 odd-numbered marks (warm-period marks) form 

 from about May to November. Fish that do not 

 form a mark in their first year would not have 

 mark numbering that corresponds to the typical 

 odd and even system. Cold-period marks were 

 most distinct and were used as "year" marks, 

 although they represent 1-1 V2 yr of growth. 

 Recognition of the first cold-period mark is the 

 basis for this method. Subsequent marks, espe- 

 cially cold-period marks, seem to be easily 

 identified. 



Age determination was validated by: 1) es- 

 tablishing the time of year when each mark forms, 

 2) establishing age through analysis of length 

 frequencies, and 3) showing that modes of back- 

 calculated and observed lengths at each age agree 

 with age determination by length frequencies. 



Repeated reading suggests this method of age 

 determination is consistent. We found 91% 

 agreement between the first reading of scales from 

 200 fish (112 age and 88 age I) and a second 

 reading 3 mo later. 



We have suggested 15 October as a defined 

 hatching date for croaker. Definition of a hatching 

 date is essential in age and growth studies, so that 

 year classes and age groups can be referenced. In 

 the northern hemisphere 1 January is a standard 

 defined hatching date. That date is convenient and 

 has biological reality, especially for species that 

 spawn in the spring and summer of one year. In 

 more northerly waters, furthermore, growth 

 seasons tend to be short; and spawning tends to be 

 restricted in time and often occurs about when the 

 annulus forms. Croaker of the Carolinian Pro- 

 vince, in contrast, have a long, possibly year- 

 round, growing season; and their spawning 

 "season" is so long that it takes place over much of 

 two calendar years. Therefore, it seems more 

 convenient and biologically sound to select their 



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