RAFT CULTURE OF OYSTERS IN MASSACHUSETTS 



491 



tivcly: whereas the clean oystei-s were 68.6 mm. 

 ill lu'ijrht and 47.5 mm. in leii<rtli. Since little 

 (litl'erence was found between the two groups it 

 appears that periodic removal of fouling material 

 had no apparent effect on shell growtli. 



A year after the oysters were planted on the 

 bottom a sample was collected to determine the 

 extent of fouling. The oyster shells were almost 

 barren of fouling organisms except for some 

 colonies of Microriorui fttollfeni and Didemniim 

 sp., some Bryozoans, and algae. Most of the 

 original fouling plants and animals had either 

 died or been slouglied off. Noticeably absent was 

 the algae Gracilarui confervoidefi which in other 

 yeai-s was one of the principal fouling organisms 

 on tiie iiottom grown oysters. 



Spawning 



Observations by Loosanoff and Xomejko ( 1949) 

 and Walne (1958) have shown that the growth 

 of the Eastern oyster in Long Island Sound and 

 the European oyster, Ontrea edidh, is not inter- 

 rvipted during the spawning season. In the years 

 1957 and 1958 the raft oysters were ripe during 

 July. ^Examination made by the end of August 

 showed that the majority of them had spawned 

 although the actual act of spawning was not ob- 

 served. Since nearly 60 percent of the total an- 

 nual sliell increment occurred during July and 

 August (see: figs. 11 and 12), it is apparent that 

 shell growth was not inhibited during this period. 



MORTALITY OF RAFT OYSTERS 



The greatest losses among the suspended oysters 

 occurred when the raft was blown ashore during 

 a winter storm in the early part of 1958. As 

 mentioned, all i)ut two strings of the Wareham 

 River oysters were destroyed, although we were 

 able to save almost all of the strings that con- 

 tained Mill Creek oystei-s. In 1958. the mortality 

 among the remaining Mill ("reek oysters was 

 determined. By the eiul of the year 17.7 percent 

 were dead. The mnnber of monthly deaths on 

 one string was recorded from April to December 

 1958. The deaths per month ranged from to 4 

 oysters, a total of 14 of the original 79 oystei-s 

 (table 2). Since there is no known disease in 

 these waters, it is unlikely th;it the deaths were 

 the result of an ejiizootic. 



T.vBi.E 2. — Ciiitiulative ilcatlm on one Hiring of raft onxtern 

 from April to December, i.9.58 



Date 



April 17 



M;iy2 



June 2- 



July 3 



Augusts 



September 5 

 October 3... 

 November 5 

 December 2. 



.Number on 

 one string 



Monthly 

 deaths 



Sum of 

 dead 



Percentage 

 dead 



1.3 

 5.1 

 6.3 

 11.4 

 11.4 

 16.5 

 17.7 



In the winter of 1958 the raft-grown oystei's 

 were transplanted on the bottom. A sample was 

 collected a year later to determine the mortality 

 that had occurred since planting. Of 87 oystere 

 examined, 64 were alive and 23 were dead, an 

 annual mortality rate of 26.4 percent. The total 

 natural mortality rate for the years 1958 and 

 1959 was 39.4 percent .'' 



The mortality rate after two years is much 

 greater when oysters are kept on bottom all the 

 time. From a planting of 400 bushels of seed 

 oysters, by a local fisherman in 1956, approxi- 

 mately 90 pei-cent were dead by the fall of 1958. 

 The poor survival of bottom grown oysters is 

 believed to result from losses inflicted b}' preda- 

 tory whelks and drills, which invade the oyster 

 beds in large numbei-s. It appears that by sus- 

 pending oysters from a raft for one year the 

 number surviving is increased sixfold. 



PRACTICAL ASPECTS OF RAFT CULTURE 



One of the purposes of this experiment was 

 to determine the commercial feasibility of raft 

 culture in Cape Cod waters. The total cost of 

 materials and labor to construct the 2-log raft, 

 to prepare and attach the strings, and finally to 

 market the oysters was estimated at $248 (p. 492). 

 After one year on the raft and one year on the 

 I)ottom 25 bushels of raft-grown oysters (averag- 

 ing 250 to a bushel) were .sold and 7 Ijushels were 

 replanted. The wholesale value of these ;'.2 bushels 

 in 19.")9 at tlie Chatham price of $11.50 a bushel 

 was $368, yielding a gross profit of $120 or $3.75 

 a l)ushel for this small-scale operation. 



•• Based im a progressive death rate ; if there are 1,000 oysters 

 and 17.7 percent die In the first year. 823 oysters are still alive. 

 If 26.4 percent of the remaining oysters die during the second 

 year, there are still 606 of the original 1.000 o.vsters alive, or a 

 niiirtality rate for the two years of 39,4 percent. 



