predation. Some observations on clam setting and survival also are included. 

 Chemical control of predators led to increases in density of clams per unit 

 area up to 8-fold. Mechanical control of predators might work, for oyster 

 drills, mud crabs, and sea stars remain on the surface of the substrate, 

 whereas moon snails, whelks and rock crabs, which often are buried. in the 

 bottom by day, frequently emerge at night. Principal predators of hard clam 

 were gastropods and crabs, which take mostly small clams. Strength of clam 

 year classes is determined by predation as well as by success of setting. 

 Identity of predators could be determined from dead shells by the nature of 

 damage, e.g., small, parallel-sided holes were made by drills; larger, 

 sharply bevelled holes were by moon snails; whelks made small chips along 

 shell margins; and fragmented shells were caused by mud crab, rock crab, 

 blue crab, and other crab species. - J.L.M. 



1193 



MacKenzie, Clyde L. , Jr. 1979. 



Relation of biological and environmental factors to soft-shell and hard- 

 shell clam management. In Proc . Northeast Clam Industries: Management 

 for the Future. Ext. Sea Grant Advisory Program, U. Mass. and MIT Sea 

 Grant Program, SP-112: 67-78. 



Managing objectives for hard clam (Mereenaria mereenaria) include increasing: 



1) abundance and production of clams to improve economic prosperity of the 

 fishing community; and 2) supplies of quality clams at moderate prices to 

 consumers. At present, little is known about: 1) predators of larvae; 



2) effect of bottom conditions, particularly associated biota, on setting 

 density; 3) numerical aspects of setting; and 4) predators of seed clams 

 during the first few weeks after setting. Percentages of larvae and seed 

 which attain harvestable size have not been determined, but predators take 

 most. Hard clams have large enough biotic potential to stock beds with 

 dense clam populations, yet abundance is usually too low to support fish- 

 eries. Environmental factors usually are not optimal in terms of predator 

 density and perhaps setting surfaces. Hard clam occurs in salinities from 

 about 15 to 35°/ 00 , at depths from low tide mark to more than 7 meters, in 

 sand, sand-gravel-stone, mud, and their combinations. Summer temps must 

 rise above 15°C but remain below 33°C. They require a bed containing few 

 predators, or some cover if predators are abundant. Hard clam larvae do 

 not set as densely in mud as in sand. Effects of presence of macroscopic 

 plants and animals on setting density has not been studied. Identified 

 predators include moon snails, oyster drills, whelks, blue crab, green, 

 rock, and mud crabs, starfish, and fishes, including rays, summer flounder, 

 tautog, and puffers. As a rule, new generations of predators appear 

 simultaneously with new generations of prey. Most adult predators also 

 select small-sized prey. Hard clam is scarce where predators are abundant 

 and little cover is available. Most predators remain on the surface of the 

 bottom and therefore can be removed without destroying clams . The abundance 

 of clams can probably be increased several times on most beds by developing 

 a predator removal machine and taking other measures not specified. 



- J.L.M. 



1194 



MacKenzie, Clyde L., Jr. 1979. 



Management for increasing clam abundance. Mar. Fish. Rev. 41(10): 10-22. 



Low clam abundance does not stem from limited biotic potential, but from 

 environmental constraints, such as predation. General awareness that clam 

 (Mereenaria mereenaria and Mya arenaria) abundance can be increased by 

 environmental improvement has been absent. Clam setting is favored by the 

 fact that predators do not consume all young clams and cannot kill clams 

 above certain lengths; fishermen do not take undersized clams, many of which 

 are already mature; fishermen also do not take all legal-sized clams on a 

 bed; the spawning season lasts a few months, and in some part of the season 

 environmental factors are suitable; fishermen do not degrade the bottom, 

 so that setting is not affected. A major opportunity to increase clam 

 abundance is to control predation of clam spat and juveniles. This has been 



334 



