ROE, CUMMINS, and BULLIS : CALICO SCALLOP OFF FLORIDA 



AGE AND GROWTH 



Monthly size frequencies are shown in Fig- 

 ure 2. From June until September the distri- 

 bution is bimodal, but in October the lower mode 

 disappears and a single mode exists until April. 

 This is clearly illustrated in the 1968 data be- 

 ginning in June when modes occur at 11.9 and 

 62.1 mm. In August modes are present at 26.0 

 and 60.6 mm and in October at 34.0 and 61.5 

 mm. Only one mode at 51.0 mm occurs in De- 

 cember. 



The lower mode value gradually increases 

 during this period while the upper mode de- 

 creases. These changes reflect a melding of year 

 classes caused by a more rapid growth rate in 

 the younger year class than in the older one. In 

 December the two year classes are indistinguish- 

 able. Since year classes cannot be separated 

 in the winter data, it is difficult to determine 

 when the older class disappears. Presumably 

 this occurs some time between December and 

 April. 



Growth rates could not be accurately deter- 

 mined from the data because of gear selectivity 

 for the smaller sizes and because year classes 

 were often inseparable in the size-frequency dis- 

 tribution. A growth estimate was derived for 

 a selected bed located on transect C in 22 to 27 

 fm. The size-frequency data indicate two year 

 classes were present most of the survey period 

 ( Table 1 ) . These are more distinct in the sep- 

 arated 1968 data (Table 2). 



The mean size increases linearly by about 

 1.0 to 2.0 mm per month during August to Feb- 

 ruary (Table 1). From April to December the 



Table 2. — Monthly size frequency data for transect C, 

 22 to 27 fm, divided by year class. Classes are 10 mm. 



mean gradually decreases because of recruit- 

 ment from the spring spawn. 



Since the age groups interact the means given 

 in Table 1 actually represent a combined year 

 class average. Therefore, the June to December 

 1968 data have been separated into two year 

 classes based on size (Table 2). Year class A 

 was spawned in the spring of 1968 and year 

 class B, representing the larger sizes, was 

 spawned in the spring of 1967. The monthly 

 mean for year class A increased curvilinearly 

 24 mm from June to December. Year class B 

 mean increased only 2.0 mm from June to Oc- 

 tober then decreased slightly in December. The 

 increase appeared linear. 



Both year classes show evidence of a sigmoid 

 growth curve. Gibson (1956) gives a sigmoid 

 curve for Pecten maximus where time is mea- 

 sured in years rather than months as in calico 

 scallops. 



MORTALITY 



Calico scallops on the Ca]3e Kennedy grounds 

 e.xperienced light commercial exploitation prior 



Table 1. — Monthly size frequencies for transect C, 22 to 27 fm, by 10-mm size classes, 



1967-68. 



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