WILLIAMS: GROWTH AND SURVIVAL OF MANILA CLAM SPAT 



cidental ingestion of newly settled young (of some 

 molluscs by adults of the same or different species; 

 f) physical damage or disturbance; and g) preda- 

 tion. Most of the above factors did not appear to be 

 significant in this study. 



a) Yoshida (1953) experimented with 1.0-3.8 

 mm Manila clams and found a survival of 90% or 

 greater in water temperatures averaging 7° C. 

 The water temperatures at Little Skookum aver- 

 aged 8. 9°-12. 7° C during winter and probably were 

 an insignificant cause of mortality, except during 

 the coldest part of January (Table 1). Low water 

 temperature probably attributed to the slight in- 

 crease in clam spat mortality observed at this 

 time. 



b) When Cahn (1951) surveyed the Japanese 

 Manila clam fishery, he cited starvation under 

 overcrowded conditions of newly settled spat as a 

 probable cause of death. The survival rate in this 

 study, however, remained fairly equal between 

 treatments with no adult clams and those with 

 high adult clam densities. Thus, the expected 

 cropping of the food supply to spat that settled 

 among the adults did not appear to affect the 

 spat's survival. In addition, almost nothing is 

 known about the food requirements of newly set- 

 tled spat, so adequacy of food supply as a control- 

 ling factor would be difficult to determine. 



c) Lack of sufficient space for growth between 

 two consecutive year classes was proposed by 

 Hancock ( 1973) as one of the largest factors con- 

 trolling survival of newly settled spat in C. edule. 

 He proposed that space requirements for growth 

 of shells would conflict in two adjacent year class- 

 es, with the smaller cockles being forced from 

 the substrate as they grew. In nonadjacent year 

 classes, 0-group cockles could maintain their po- 

 sition between shells of older adult clams. This 

 hypothesis was based on clams first observed 

 when 10 mm long. Space limitation did affect 

 survival of the spat in this study, because their 

 size was quite small compared with the space 

 available for growth. 



d) There was no evidence of parasites and/or 

 disease in either the adult stocks or the newly 

 settled spat. 



e) Under experimental conditions, Kristensen 

 (1957) found that inhalation by adult cockles of 

 newly settled spat could cause death of the small 

 spat, even when the larvae were discharged soon 

 afterward. Hancock (1973), however, did not feel 

 that the presence of adults adversely affected 



survival of settled young in his studies at Burry 

 Inlet, but he felt that mortalities were related to 

 oyster catcher, Haematopus oachmani , preda- 

 tion. Since he did not look at the spat until 5 mo 

 after they had settled, it would be difficult to de- 

 termine at which stage in their early life history 

 the largest mortality occurred. In the present 

 study, the largest initial settlement occurred in 

 Treatment 1, but by April this treatment had not 

 only the lowest survival but also the lowest abso- 

 lute density of the four treatments. If mortality 

 was caused by ingestion by adults, then a higher 

 density would have been expected for Treatment 

 1, which contained no adults. 



f) Shellboume (1957) studied small oysters and 

 found that shifting surface semds subjected newly 

 settled spat and juveniles to increased mortality 

 due to abrasion. Quayle (1952) felt the largest 

 cause of mortality for the spat of Venerupis pul- 

 lastra was the unsuitability of the substrate. 

 Glock (1978) planted small (2-4 mm) hatchery- 

 reared Manila clams on a southern Puget Sound 

 beach and then covered some of the area with 

 protective mesh covering. He had a much higher 

 survival rate under the areas with plastic mesh, 

 and attributed this in part to stabilization of the 

 sediment. He also found predation rates to be low. 

 In this study, the Treatment 1 areas in Plots A, II, 

 and III were readily observable through the third 

 month after settling, due to a slight difference in 

 color of the gravel brought down from the high 

 intertidal area. This observation indicated that 

 any movement of the surface gravel must have 

 been slight in order not to mask the visible differ- 

 ences of this treatment. In the laboratory, I sub- 

 jected the spat to considerable mechanical agita- 

 tion during the process of washing and sieving; 

 however, only a few of the thousands of spat that I 

 observed had damaged shells. I also found some 

 unbroken, empty clam shells that were equal in 

 size to live clams sampled, but never a number of 

 shells equivalent to the mortality observed. I con- 

 cluded that abrasion did not cause substantial 

 mortalities in this study. 



g) Thorson (1966) felt that the biological factor 

 with the greatest effect on survival of newly set- 

 tled larvae was predation. Muus (1973) came to 

 the conclusion after a very complete study on the 

 early life history of newly settled bivalves in Den- 

 mark. Since the largest mortality in this study 

 occurred when the spat were quite small, it 

 seemed most probable that if predation were the 

 cause, then the majority of the loss would be to 



897 



