WHITE and CHITTENDEN: AGE DETERMINATION OF ATLANTIC CROAKER 



Age Determination and Growth by 

 the Length-Frequency Method 



Our length-frequency distributions suggest two 

 croaker year classes occurred off Texas. One age 

 group greatly predominated in the length fre- 

 quencies of trawl-caught fish from the bay and 

 Gulf during June (Figure 1). The size range of that 

 age group was primarily about 100-150 mm in the 

 bay and about 120-160 mm in the Gulf. Young-of- 

 the-year first appear in Texas bays about 

 November and increase in size from about 10-50 

 mm during January to 30-85 mm in March, 40-100 

 mm during May, and 70-130 mm in June (Gunter 

 1945; Parker 1971; Gallaway and Strawn 1974). 

 Therefore, the fish we captured by trawling during 

 June must be young-of-the-year. These young-of- 

 the-year fish grew to about 1 10-170 mm in August, 

 120-175 mm in September, and 140-180 mm in 

 October when they reached age I. Similar sizes in 

 October have been recorded by Gunter (1945), 

 Parker (1971), and Gallaway and Strawn (1974). 

 The fish that became age I in October were about 

 130-190 mm in November, and fish captured in 

 March were about 165-220 mm. The large fish 

 caught in June by angling near the oyster reef 

 were about 190-270 mm and apparently were 

 survivors of the year class that became age I on the 

 preceding 15 October. These age 1+ fish were 

 about 200-310 mm in September when they 

 approached age II. This agrees with Gunter's 

 (1945) size estimates for age II croakers off 

 Texas. 



With minor differences, length frequencies 

 reported throughout the Carolinian Province by 

 many workers, including Hildebrand and Cable 

 (1930), Gunter (1945), Suttkus (1955), Bearden 

 (1964), Hansen (1969), Christmas and Waller 

 (1973), Hoese (1973), and Gallaway and Strawn 

 (1974), show growth and age composition similar 

 to our findings. Growth north of Cape Hatteras 

 seems similar to that in the Carolinian Province. 

 Haven (1957) presented monthly length fre- 

 quencies of fish he considered young-of-the-year. 

 His fish ranged from about 150 to 220 mm in 

 September, but the mode was about 175-180 mm. 



Agreement of Observed and Back-Calculated 

 Lengths with Length-Frequencies 



Observed sizes at ages 0, I, and II agree closely 

 with ages determined by length frequencies 

 (Figure 8). Only age fish were captured in May 



and age I fish in July, so that graphs are not 

 presented for these months. The frequencies show 

 overlap in size between the various ages each 

 month. This is to be expected, especially in a 

 species having a prolonged spawning season, and 

 makes it impossible to use the length-frequency 

 method to assign age confidently where sizes at 

 age overlap. The observed lengths of age fish in 

 September were primarily 130-170 mm (mean = 

 151 mm), but they ranged from about 110 to 220 

 mm. This age group was about 140-220 mm (mean 

 = 158 mm) during October when they became age 

 I and about 130-220 mm (mean = 172 mm) during 

 November. The observed lengths of age I fish in 

 September were about 200-340 mm with the mean 

 being 253 mm. This age group was about 190-360 

 mm (mean = 274 mm) in October when they 

 became age II. 



Lengths back-calculated to cold-period marks 

 reasonably agree with the sizes at age I estimated 

 by length frequencies in October (Figure 9). 

 However, cold-period marks apparently begin to 

 form generally after October; so that the back- 

 calculated lengths should be larger than the 

 observed lengths in October. The similarity 

 suggests Lee's phenomenon, possibly due to 

 selective mortality favoring survival of smaller 

 croaker. Back-calculated lengths were somewhat 

 smaller than the sizes at age 1+ in March, as 

 would be expected. Back-calculated lengths from 

 age 1+ fish were primarily 110-210 mm at age I 

 with a mean length of 165 mm. In agreement, 

 back-calculated lengths from six age 11+ fish had a 

 mean of 181 mm at age I and 270 mm at age II. The 

 body-scale regression equation used to back- 

 calculate length was: 



Y = 2.6000 + 4.6389Z - 0.0122X 2 



where Y represents total length in millimeters, 

 andX represents the scale radius (millimeters x 

 42). The sample size was 1,123, and the total 

 length range was 90-360 mm. About 88% of the 

 variation in total length was associated with 

 variation in scale radius. 



Growth estimates based upon the length- 

 frequency method and from observed and back- 

 calculated estimates using the scale method show 

 very close agreement. Mean lengths in October 

 were about 155-165 mm at age I and 270-280 mm 

 at age II depending upon how age was determined. 

 The wide back-calculated and observed size 

 ranges found at age may be due to the long 



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