FISHERY BULLETIN: VOL. 81. NO. 1 



75°W 



70° W 



65 8 W 



45° N - 



40° N - 



- 45°N 



- 40° N 



75°W 



70°W 



65°W 



Figure 1.— Location of sampling sites. Site codes are given in Table 1. 



Salinity, at low tide, was measured by a refrac- 

 tometer; tidal position was estimated on a scale of 

 0-1, where = subtidal and 1 = full exposure. 

 Estimates of the average annual temperature 

 near each site were obtained from various litera- 

 ture sources, and estimates of the tidal range 

 were obtained from National Ocean Survey 

 (1978). 



Sediment samples (composites of two surface 

 cores 21 cm 2 X 8 cm depth) were collected and 

 analyzed to determine grain size distribution 

 and organic content. The sand fraction was ana- 

 lyzed by dry sieving; silt-clay by the hydrometer 

 method (American Society for Testing Materials 

 1963). The particle size distributions obtained 

 from the two analyses were pooled, and the cumu- 

 lative frequency versus grain size (<J>) was plotted 

 for each sample. From the graphs the following 

 summary statistics were obtained: Median grain 

 size (Md4>), quartile deviation (QD</>), and skew- 

 ness (Skq</>) (Buchanan 1971). The results were 

 reported in phi notation rather than millimeters 

 [(f) = — log2(mm)], as this scale is commonly used 

 to describe grain size characteristics and be- 

 cause it allows for greater discrimination in the 

 silt-clay range which may be more meaningful 

 biologically. 



The percent organic matter was determined 

 by measuring the percent weight loss of a small 

 aliquot upon ignition at 550°C for 4 h (Buchanan 

 1971). Estimates for total sedimentary hydrocar- 

 bons through infrared analysis were obtained 

 from C. Brown. 2 The sites and their environmen- 

 tal parameters are given in Table 1 along with 

 their dates of collection, latitude, and code. 



Clams were sampled from one or more trenches 

 dug by a standard clam hoe or shovel, with the 

 exception of the Chesapeake Bay sites where a 

 commercial hydraulic escalator dredge was used. 

 All clams excavated were retained for analysis. 

 For each individual, shell length (maximum shell 

 dimension) was measured by vernier calipers to 

 the nearest millimeter. 



Age structure was determined via length-fre- 

 quency analysis for 19 of the populations. Simi- 

 lar information for six of the sites (BN, WF, SP, 

 GC, PY, JL) was available from Appeldoorn 

 (1981), though only the West Falmouth (WF) 

 growth data were used in subsequent analyses 

 since major growth interruptions resulting from 

 pollution events occurred at the other sites. 



2 C. W. Brown, Professor, Department of Chemistry, Univer- 

 sity of Rhode Island, Kingston, R.I., pers. commun. May 1979. 



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