natural selection states that natural selection 

 will favor genotypes which increase the popula- 

 tion growth rate, A . Since the A for a population 

 is based on the life history parameters of age- 

 specific fecundity and survivorship, the greater 

 the sensitivity of A to changes in a particular life 

 history value, the greater the potential for effect- 

 ing evolutionary change through that parameter. 

 Existing evidence indicates that population 

 growth rate is more sensitive to changes in sur- 

 vival rates than to changes in reproductive out- 

 put. Cole (1954) reached this conclusion when he 

 suggested that in species with repeated repro- 

 duction and relatively large litter size, there is 

 little selection pressure favoring increased fe- 

 cundity. Similarly, Caswell (1978) using Harts- 

 horn's (1975) data for Pentaclethra macroloba, a 

 tropical rain forest tree, illustrates by the use of 

 models, that, for this species, population growth 

 rate is more sensitive to changes in growth and 

 survival than to changes in fecundity. The data 

 reported here for Mya arenaria follow the same 

 pattern; A is relatively insensitive to changes in 

 egg production. The most interesting results of 

 the sensitivity analyses are produced by changes 

 in the survivorship parameters. 



SURVIVORSHIP 



Based on our analyses, two important general- 

 izations can be made regarding the sensitivity of 

 A. First, whenever r s < bi < bi, the population 

 growth rate will be more sensitive to changes in 

 r s than to changes in any of the other survivor- 

 ship parameters. Second, in growing populations, 

 i.e., where A > 1, A is always most sensitive to 

 changes in the settlement rate. In terms of Dee- 

 vey's (1947) categorization of generalized survi- 

 vorship curves, the relationship r s < 6i < bi is 

 probably operative in most Type III curves, which 

 are characterized by extremely heavy mortality 

 early in life. Consequently, these generalizations 

 are of interest since the types of life history 

 features exhibited by Mya arenaria are likely to 

 be common to other species of marine organisms, 

 many of which are also commercially important. 



On a more practical level, the ability to identify 

 those life history stages to which the population 

 growth rate is most sensitive may serve as a use- 

 ful tool in directing the efforts of those interested 

 in shellfish management. For instance, the mod- 

 els described here indicate that larval settlement 

 is the most critical stage in Mya arenaria's life 

 history. Developing a better understanding of the 

 factors surrounding metamorphosis and settle- 

 ment and implementing a method for inducing 

 spatfall would probably be the single most effec- 

 tive way to increase clam yields. 



Another area for consideration centers around 

 the survivorship of the first year class. Since the 

 population growth rate of Mya arenaria is also 

 very sensitive to changes in the b\ parameter, 

 a second way to increase clam productivity is 

 to improve the survivorship of clams 2 mo to 1 

 yr of age (ca. 2-25 mm shell length). This age 

 class corresponds to that postlarval stage in Mya 

 arenaria which is the most vulnerable to both 

 biotic (predation) and abiotic (wash-out, temper- 

 ature and salinity fluctuations) factors in the 

 environment. 



The practice of transplanting juvenile Mya 

 arenaria from one flat to another has been used 

 by managers since the turn of the century (Bel- 

 ding 1930) in efforts to 1) replenish depleted clam 

 beds or 2) reduce densities in "overcrowded" beds. 

 Currently, there is a renewed interest in this 

 procedure 1 even though in the past, these efforts 

 have met with varying degrees of success (Bel- 

 ding 1930; Turner 1951; Smith et al. 1955). The 



FIGURE 2.— Sensitivity of a range of X's (0.25-2.0) to changes 

 in the survivorship (ft,- ) of Mya arenaria in each age class. 



'D. E. Wallace, Director, Department of Marine Resources, 

 State of Maine, Boothbay Harbor, ME 04538, pers. commun. 

 April 1983. 



540 



