FISHERY BULLETIN: VOL. 77. NO. 4 



Summer Sampling in 1977 



To follow the growth of the 1976 fall settlement 

 for 1 yr, three random 0.25 m^, 3 cm deep gravel 

 samples were taken from near Plot A in early 

 August 1977, and again in the middle of Sep- 

 tember. No treatment effect determinations were 

 possible due to a lack of sufficient study plots. 

 Material was preserved by freezing, instead of by 

 a formaldehyde solution. In the laboratory the 

 material was thawed and washed through Tyler 

 sieves (1.190 mm minimum mesh size), and then 

 the residue on the screens was placed in a large 

 cake pan. The clams were sorted by eye, and a 

 height and length measurement was taiken on all 

 clams. 



Core samples taken from Plots R, S, and T, in 

 the summer and fall of 1977 to test for larval 

 settlement were used as the basis for determining 

 the grovrth of the clam spat that settled in July 

 1977. 



RESULTS 

 Growth 



To test for possible changes in the height- 

 length ratio due to growth, a linear regression 

 was run on logj^-transformed height versus 

 length measurements for clams from settling size 

 through 1 yr. Correlation was high (r = 0.997) 

 and thus only length measurements were used to 

 express results. A plot of clam lengths deter- 

 mined from samples taken after the initial set- 

 tlement in September 1976 through the following 

 12 mo is shown in Figure 2. 



The average length of newly settled clams was 

 0.206 mm (N = 129; SE = 0.01). The clam spat 

 that settled in September 1976 grew about 2.5 

 times their settling length in 2 mo. Little growth 

 occurred between November and January; 

 growth commenced again by the middle of March. 

 By June the spat had attained a length of 2.17-2.7 

 mm. 



In contrast to the growth of the fall settlement, 

 the clams that settled in July 1977 attained an 

 average length of 2.82 mm (N = 47; SE = 0.12) by 

 the middle of September. This was a growth of 

 13.5 times their size at settling in 2 mo and was 

 slightly larger than the size reached in 9 mo by 

 clams from the fall settlement. 



Growth rings were laid down in October. Clams 

 that settled in July attained an average length of 



iQi : 



B  



-■ 8 • 



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I 



l + t 



+ t 



N 



1976 



J F M 

 MONTH 



Figure 2. — Plot of individual and mean length measurements of 

 Manila clams recovered during sampling periods. Sample sizes 

 from September 1976 to September 1977 were 129. 136. 97, 72, 

 67, 94,61, and 110. 



6.16 mm (N = 42; SE = 0.51) by this time. Clams 

 that settled in the fall also formed rings their first 

 October, but since they were so small, the mark 

 was not discernable by the following summer. By 

 September 1977 the clams from the fall settle- 

 ment had attained an average length of 14.93 mm 

 {N = 110; SE = 0.26). Approximately 0.5 mm 

 growth occurred before the end of October, at 

 which point their first growth ring was visible. 

 Clams that settled in July were about 23 mm long 

 1 yr later. 



No differential growth was detectable until 

 June 1977 between clams that settled in fall 1976 

 into the different experimental treatment sub- 

 strates. A Kruskal-Wallis rank sums test on 

 clams sampled in June detected a significant dif- 

 ference tP<0.001) between those sampled from 

 substrates with no adult clams and those from 

 substrates with adult clams. The treatments were 

 ranked by the average length of clams that each 

 contained (Treatment 2 had the largest clams; 

 Treatment 3 had the smallest). A series of one- 

 tailed Mann-Whitney pairwise comparisons 

 (pairwise P<0.05) were performed to test which 

 treatments differed significantly (Hollander and 

 Wolfe 1973). The following were the results (no 

 significant difference between pairs underlined in 

 common): 



2 1 



4 3 



894 



