oyster drills were killed on this lot by the second 

 treatment and not by the first in 1966, I believe 

 that treatments in early May are much more effec- 

 tive tlian those made later in the summer. 



On lot 49 the treatment killed 86 percent of the 

 oyster drills (84.6 percent of the thick-lipped 

 drills and apparently all of the Atlantic oyster 

 drills) and reduced their numbers from 6.3 to 0.9 

 per square meter (table 4) . 



On lot 50 the treatment killed 84 percent of the 

 oyster drills (86.4 percent of the thick-lipped drills 

 and 55.6 percent of the Atlantic oyster drills) and 

 reduced their numbers from 13.9 to 2.8 per square 

 meter (table 4). 



Lot 19, which served as a control, was sampled 

 at the same time as the other lots. The density of 

 oyster drills per square meter was about the same 

 on each date; on May 10 it was 19.5, and on June 

 30 it was 21 .2 (table 4). 



E-ffect on associated animals. — Divei-s did not ex- 

 amine these lots closely during or immediately 

 after treatment. At intervals during the summer 

 of 1967, however, they observed that healthy 

 flounders, young starfish, mud crabs, and other 

 animals were numerous on the beds. They saw no 

 affected animals. In fact, most animals were more 

 numerous on treated lots than on areas barren of 

 oysters nearby. The divers did not count the young 

 starfish on unplanted areas, but on October 6 they 

 counted 8.8 young-of-year starfish per square meter 

 on lot 18, and 35.3 per square meter on lot 40. 



Effect on predation. — I carefully recorded mor- 

 talities of oysters on these lots from the time they 

 were planted through November when oyster drills 

 became dormant. In May 1967, 1- and 2-year-old 

 oystei-s were planted on lot 40 and 1-year-old 

 oysters were planted on lots 42, 49, and 50; and 

 from June through early September, 1967-year- 

 class hatchery-reared seed oysters about 5 nnn. in 

 length were planted on lot 18. Losses of oystei-s 

 because of predation by oyster drills did not ex- 

 ceed 1.5 percent on any of these lots by late Novem- 

 ber (table 5). Because enough drills were present 

 on some lots to cause higher mortalities — lot 40, 

 for instance, had 3.8 oyster drills per square meter, 

 and lot 50 had 2.3 i^er square meter — most live 

 oyster drills must have been sufficiently "stunned"' 

 by the Polystream to prevent their feeding. This 

 apparent "stunning" effect was also evident on lot 

 42 in 1966. 



CONTROL OF OYSTER DRILLS WITH POLYSTREAM 



Oysters planted on these lots freshly treated 

 with Polystream grew normally. For example, the 

 1-year-old oysters on lot 50 increased in volume 

 from an average of less than 1 cc. to about 15 cc. 

 each during the 1967 growing season. My determi- 

 nations of growth of oysters planted on untreated 

 bottoms show that this amount of growth is about 

 normal. 



Area 9: Foster's Cove, R.I., 1967 



Foster's Cove on tlie south shore of Rhode Is- 

 land, about 4 ha. in area, is a tidal pond connected 

 to Charlestowa Pond by a narrow inlet. Depth of 

 water over the oysters ranges from to 2 m. at low 

 tide. There is little exchange of water between the 

 two areas; thus, the principal water currents in 

 the cove are caused by winds. Examination of three 

 sections of Foster's Cove on November 10, 1966, 

 indicated that oyster drills had killed about 75 

 percent of the oysters. 



On May 31, 1967, two areas totaling 0.8 hectare 

 were treated with Polystream (Granular) . 



Effect on gastropods. — Divei-s made no observa- 

 tions during or immediately after treatment. My 

 later observations showed that the treatment killed 

 all Atlantic oyster drills (no thick-lipped drills 

 were present) in both areas and reduced their 

 numbers from 9.5 and 3.6 to 0.0 per square meter 

 (table 4). 



Effect on associated aninuds. — On June 8, 8 days 

 after the treatment, I examined the areas by walk- 

 ing along the shores and divers also examined 

 them. Along the north shore, perhaps 15 m. from 

 one of the treated areas, there were 4 dead toad- 

 fisli, Opsanus tau; 50 dead silversides, Menidia 

 menidia; 500 to 1,000 dead niunimichogs; 4 dead 

 blue crabs, Callinectcs sapidus; 50 dead shrimp; 



Table 5. — Percentage of oysters killed hii oijs'er drills and 

 starfish in center areas of lots in Norwalk, Conn., 1967 



lAccumulatcd niontlily totals '] 



Lot number May June July Aug. Sept. Oct. Nov. 



Perunt 



182 0.0 0.0 O.n 0.6 0.4 1.0 0.2 



40< .0 0.0 1.0 0.0 0.0 .5 



42' .0 0.0 0.5 I.O 0.0 .5 



4(1' .0 0.0 0.0 0.3 1.5 .0 



50> .2 1.0 0.0 1.0 0.7 .0 



' Sampling errors account for slight variation In numbers. 



• Oysteis niised in hatcheries in 1967. 

 ' l-year-old oyslcrs. 



• Mixture 0(1065 and l'J66oystor set (1 and 2 years old). 



295 



