PETERSON ETAL.: MARK-RECAPTIRK TEST OF HARD CLAMS 



The sampling site was in shallow subtidal waters just 

 outside the area legally open to "clam kicking," a 

 form of mechanical clam harvesting practiced by 

 local commercial clammers. Our sampling site con- 

 tained no bottom ruts or other disturbance features 

 commonly left by mechanical clam harvesters. 



On 16 January 1980, we collected 73 clams using a 

 hydraulic suction dredge to sample haphazardly 

 chosen locations within Johnson Creek. On 15 Feb- 

 ruary 1980, another 51 clams were collected by 

 excavating haphazardly located 0.25 m 2 sampling 

 frames either using the hydraulic dredge (24 sam- 

 ples) or hand digging and sieving through 3 mm mesh 

 (10 samples). 



The hydraulic dredge consisted of a 3 hp gasoline 

 engine attached to a pump which generated a water 

 flow of ~5 1/s through a 0.8 cm diameter metal tube. 

 The tube penetrated at an angle into the side of a 12.7 

 cm diameter pipe. When water was forced into the 

 pipe, suction was created at one end. To collect hard 

 clams, the suction end of the pipe was swept slowly 

 and systematically across the bottom such that it 

 vacuumed up the top 15 cm of sediments and their 

 living contents. All of this material was deposited into 

 a 3 mm mesh nylon bag to permit sorting of clams 

 from sediments and debris. This technique was 

 nearly 100% efficient and was not size selective for 

 M. mercenaria >5 mm long (tests given in 

 Appendix). 



All M. mercenaria collected from Johnson Creek 

 were returned to the laboratory live, held overnight at 

 4°C, killed by steaming, and measured. One valve 

 from each clam was then sectioned and aged by the 

 techniques that we had tested earlier. From these 

 measurements of length and estimates of age, we 

 estimated the size (length)-frequency distribution, 

 age-frequency distribution, and growth rate of M. 

 mercenaria in the Johnson Creek area of Core 

 Sound. 



RESULTS 



Tests of Aging Methodology 



1-Year Class and Older 



In total we recovered, sectioned, and analyzed 

 marginal shell growth in 152 individual M. mer- 

 cenaria, all initially planted on 21-22 June 1978. 

 These clams were retrieved in approximately equal 

 numbers on each of four dates from each of the three 

 caging treatments (Fig 2). Macroscopic inspection of 

 polished and sectioned shells revealed repeating 

 features that could conceivably serve as annual 



markers. These features (analogous to those de- 

 scribed by Jones (1980) for other species) appeared 

 as bands in the outer and middle layers (following the 

 terminology of Pannella and MacClintock 1968) that 

 differed in appearance from the surrounding shell 

 structure (Fig. 3). Bands near the umbo tended to be 

 lighter in color and more translucent in appearance 

 than the surrounding shell matrix, whereas bands 

 toward the shell margin tended to be darker than the 

 surrounding shell matrix and were usually purple in 

 color. The first band deposited (nearest the umbo) 

 differed consistently from all subsequent bands. It 

 appeared more diffuse and was often united with the 

 second band in the middle shell layer without an 

 obvious termination at the shell surface. Although all 

 bands extended to the external shell surface and 

 were present in the outer layer, they did not always 

 retain the coloration and textural distinctions out- 

 side of the middle shell layer. Attempts to relate sur- 

 face growth breaks to the presence and absence of 

 internal bands failed on most clams. The outer shell 

 surface contained many more lines suggesting 

 growth changes or interruptions, thus making an 

 unambiguous matching with internal bands im- 

 possible. 



Figure 2 presents the numbers of bands counted in 

 the marginal growth increment of each hard clam re- 

 trieved from our field plots as a function of caging 

 treatment and time of retrieval. For 91% of all clams, 

 three independent observers counted an identical 

 number of growth bands. In the rare cases of dis- 

 agreement (listed by date on the legend to Figure 2), 

 the majority vote was plotted. All 152 clams without 

 exception showed a growth band having just begun at 

 the time of initial planting (21-22 June 1978). For 

 Figure 2 we chose to count that band and all suc- 

 cessive ones. The numbers of bands added did not 

 differ as a function of caging treatment on any of the 

 four retrieval dates (Fig. 2). Thus, although our cag- 

 ing treatments may have altered the hydrodynamic 

 regime and thereby the local growth environment for 

 the test clams, the aging technique was consistent. 

 This result extends the scope and generality of our 

 test of whether growth bands are predictably repeat- 

 ing annual markers in a southeastern population of 

 M. mercenaria. 



At each retrieval date, there was little variance 

 among clams in the number of bands added since 

 planting (Fig. 2). In the October 1979 collection, 38 of 

 39 clams had deposited exactly 2 additional bands; in 

 the October 1980 collection, 35 of 38 clams had 

 deposited 3 additional growth bands; and in the 

 October 1981 collection, 42 of 46 clams had 4 

 additional growth bands. This represents a 93% level 



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