RAFT CULTURE OF OYSTERS IN MASSACHUSETTS 



489 



ill mean heiglit for the ruft-<ri-owii Wiirehain 

 Kiver oysters at tlie end of tlie first and second 

 year was 38 nun., and 2:5 mm., respectively, com- 

 pared to 39 nnii.. and 21 mm. for the raft-grown 

 Mill Creek oysters (fi<r. '.)). From tiiese oh.serva- 



90 



80 



s 

 s 



2 30 



I I I I I I I I I I I I I I I I I I I I I I I I I I I I 



/ 

 / 



I I I I I 'I 



JASONOJFMAMJjaSONDJFMAMJJASOND 



ISETTING YEARI FIRST YEAH I SECOND YEAR | 



FniUKE !). — Growth curve.sof raft-Krowu Wareliiiiu River 

 and Mill Creelv oysters. 



tions it appears that the Mill Creek oysters do not 

 grow faster, hut had an earlier start. 



Because of rapid growl li of the oysters attached 

 to the raft, tiieir siiells were thin and fragile; tiie 

 average thickness along the principal axis of the 

 right valve was only 1.4 nnn. On Xovemher 13. 

 1958, the raft oysters were planted on tlie bottom 

 in order to let their shells thicken. After a year 

 on the hottoni. tlie shells of these oysters thickened 

 more than thieefold, and averaged 4.() mm., along 

 the principal axis. The thickening of shell on the 

 bottom is apparently tlie result of a slower rate 

 of growth in height and length, consi'ciueiitly, 

 largei- amounts of calcium carbonate are dei)osited 

 on the existing shell. 



Figure 9 shows tlie growth curve of Mill Creek 

 oysters from 19r)7 through 19r)9. By the end of 

 19o9, following 14 months of attachment to the 

 raft and 1 year on the bottom, the oysters averaged 

 88 mm., or ^y, inches in height. Market-size for 

 oysters in Massachusetts is :'. inches or greater; 

 therefore, the Mill Creek oystei-s had attained 

 market-size in 21/2 years instead of 4 to 5 years, 

 which elapse when oysters are grown on the bottom 

 in Cape Cod waters. 



An actual comparison of raft-grown and bottom- 

 grown Wareiiani" Ki\er oysters was made from 

 September 1956 to November 1958. As shown in 

 figure 10 no difference in growth was obser\ed at 

 the end of 1956. By the end of 1957 the raft. 

 oysters measured 49 mm. in mean height, whereas 

 those on the bottom were only 36 mm., a difference 

 of 13 mm. The difference increased to 20 mm. by 

 the end of 1958, the raft oysters measuring 72 mm. 

 ill height, and the bottom oysters 52 mm. It is of 

 interest to note (fig. 10) that the bottom-grown 

 oysters at the end of ^i^ years averaged only 3 

 mm. larger than the size obtained by the raft 

 oysters in li/^ yeare. 



70- 



60- 



50 



40 



X 



o 



30 



UJ 



S 



20 



I 



1 — r — I — I — I — 1 — 1 — r 



DIFFERENCE OF 

 20.48 mm 



r 



BOTTOM GROWN 



II 



SONDJFHAUJJ 



I I I I t I 



S N J F M 



I 1956 I 



1957 



UJJASONO 



1958 



I 



FiGUHK 10. — (irowth curves of riift-Krowii iiiul liottom- 

 firown Wiiroliaiii liiver oysters. Symbols imliciite when 

 shell (jrowth stopped for the year. 



