BROUSSEAU and BAGLIVO: FIELD POPULATIONS OF MYA ARENARIA 



total number of eggs produced by a female of age 

 X which may be considered to be "female eggs". 

 Because the sex ratio of both populations is 1:1 

 (Brousseau 1987a), only half of the oocytes produced 

 will eventually become females. Accordingly, each 

 rrijc is one-half the total annual fecundity per female 

 of age X. 



Mortality 



To determine size-specific l-yr survival rates of 

 adult clams (>1 year of age), M. arenaria from 

 Stonington and Westport were collected, individual- 

 ly tagged, and returned to their original sites (Ston- 

 ington: 31 May-1 June 1985; Westport (Plot A): 1 

 May 1984 and Westport (Plot B): 22 May 1984). 

 Clams were measured to the nearest 0.1 mm antero- 

 posteriorly and marked for identification using a 

 method described previously (Brousseau 1979). 

 Tagged clams were replanted in parallel furrows ex- 

 cavated to a depth of 20-30 cm. All plots were 0.3 m 

 apart and located at midtide level (-I-3.0 m). At the 

 end of the test period, the clams were collected. All 

 numbered individuals, both alive and dead, were 

 returned to the laboratory for measurement. A total 

 of 1,049 clams from the Stonington population were 

 tagged, of which 78% were recovered. One-year 

 recovery rate at the Westport site was 40% of the 

 1,977 clams initially planted. Mortality rates were 

 determined on the basis of the number of clams 

 recovered. This method gives a more accurate 

 estimate of mortality since it does not consider clams 

 that were not recaptured in these estimates. 



If, on recapture, a dead clam showed no evidence 

 of growth, either death had occurred naturally in 

 a slow-growing individual or premature death had 

 occurred as a result of trauma due to the marking 

 procedure. In order to correct for premature mor- 

 tality caused by handling, the proportion of live 

 clams which showed an increase in shell length over 

 the year was calculated (Table 1). For the Westport 

 population all live clams <40 mm shell length grew, 

 while 94.7% of live clams over 40 mm grew. For the 

 Stonington population all live clams <50 mm shell 

 length had grown during the year, while 91.9% of 



Table 1 . — Distribution of Mya arenaria recovered alive showing 

 growth and no growth during the test period. 



Westport 

 (clams >40 mm) 



Stonington 

 (clams >50 mm) 



live clams over 50 mm grew. In order to estimate 

 size-specific survivorship then, the following rules 

 were applied: 



Westport: 

 Classes <40 mm: P(surviving one year) (2) 



= NJiN^ + Nog) 

 Classes >40 mm: F(surviving one year) 

 = NJ{N^ + Nog/OMI) 



Stonington: 

 Classes <50 mm: ^(surviving one year) (3) 



= NJiN^ + Nog) 

 Classes >50 mm: P(surviving one year) 

 = NJiN^ + Ar^G/0.919) 



where A^^ is the number alive and Nog is the num- 

 ber dead with growth. In the smaller classes (West- 

 port, <40 mm; Stonington, <50 mm), the sum A^^ 

 + Nog represents all recovered individuals, while 

 in the larger classes, the adjustment factor allows 

 us to add a number of dead without growth back 

 into the total recovered in the same proportion as 

 the live with no growth. 



Age-specific survival rates, P_j, were based on the 

 same age-size information used to determine age- 

 specific fecundity rates. These rates were calculated 

 using the formula: 



Px = -t.pj P(size-class i \ a;-yr-old) 



(4) 



Number 

 Percent 



Growth No growth Growth No growth 



196 11 440 39 



94.7 5.3 91.9 8.1 



where p^ is the probability that a clam in size-class 

 i survives one year, P(size-class i \ x-yr-old) is the 

 conditional probability that an a;-yr-old clam is in 

 size-class i, and the sum is taken over 10 mm size 

 classes i. 



The probability of surviving from settlement to 

 one year (a period of about 10 months) is calculated 

 from the estimates of the density of the population 

 in 2 consecutive years. Both populations were sam- 

 pled during periods of maximum settlement in 1985. 

 This period occurred approximately 3 weeks earlier 

 in the Westport population. At each sampling, 18 

 to 35 samples (0.1 m^ x 30 cm deep) were taken 

 along transects running from mean low water shore- 

 ward to the mean high water mark. Samples were 

 wet-seived in the laboratory (2 mm mesh) and the 

 shell lengths of the clams retained by the seive were 

 measured. Clams of size 2-20 mm in the first year 

 of sampling are assumed to be the spat; those of size 

 20-50 mm the following year are assumed to have 

 come proportionately from the settlement of the 

 previous year in the same ratios as the empirical 

 probability distributions (Appendix Tables 1, 2). 



569 



