CHAPTER 11, PART 1 



Tabi E 1 1.2-1 — Survival of surf clams at different dtssolved oxygen concentrations at 10° C 



8 weeks at 2.1 ml'l followed bv 8 weeks at 0.7 ml/1. 



sealed jars. The first clam died on day 8; all clams were 

 dead by day 30. 



The mean oxygen consumption value for clams held in 

 2.1 ml/1 D.O. water was 9.91 ^.lOj/h/g, with a standard 

 error of ± 0.45. The mean value for clams held in 1.4 ml/ 

 1 D.O, water was 14.58 p-lO^/h/g, with a standard error 

 of ± 0.33. Student's "t"-test indicates that the difference 

 between these two mean values of 100 readings each is 

 highly significant (P < 0.001). Clams held in 0.7 ml/1 water 

 consumed all the detectable ogygen in 24 hours; thus they 

 had the lowest calculated rate of oxygen consumption. 

 The background level of oxygen consumption by other 

 components in the seawater "blanks" was 0.29 ml/1 per 

 day. Hourly rate studies were not made, thus the point 

 at which oxygen was depleted in the 24-hour cycle was not 

 determined. 



DISCUSSION 



Survival of surf clams in this study was clearly related 

 to the amount of oxygen available in the water. Levels 

 below 1.4 ml/1 were nearly always fatal. Surf clams were 

 able to live for various prolonged periods of time at 1 .4 

 ml/1 D.O. or above, similar to conditions in the low oxygen 

 zone in New Jersey. 



In our studies, no deaths were recorded at 2.1 ml/I D.O. 

 and clams placed in water at 0.7 ml/1 after previous ex- 

 posure to 2.1 ml/1 survived for 8 weeks, an indication that 

 a gradual shift to anaerobic pathways is possibly advan- 

 tageous. 



Studies with flowing-water systems indicated that pro- 

 longed survival was possible at or near 1.4 ml/1. Flowing 

 water helped eliminate a possible buildup of toxic mate- 

 rials and prevented temporary low D.O. pockets from 

 forming around clams, as might occur in static water. Even 

 with water movement, however, a prolonged or repeated 



low oxygen condition causes mortality of clams and may 

 lead to the eventual elimination of a viable population. 



Size apparently plays a role in surf clam survival under 

 low D,0, conditions. A direct correlation was noted be- 

 tween size and survival; large clams withstood low D.O. 

 better than small ones. The one exception in this study 

 was a group of 1.8 to 2.2 cm clams that survived 0.7 ml/ 

 1 D.O. for 58 days. We believe this to be an anomalous 

 condition perhaps due to some experimental error. An- 

 other group of researchers at this laboratory, working on 

 aquacultural aspects of surf clam biology, found a massive 

 mortality of juvenile clams when the D.O. level acciden- 

 tally dropped below 2.1 ml/1 in an outdoor tank system. 

 A careful examination showed that the survival rate in- 

 creased with size (R. Goldberg, National Marine Fisheries 

 Service, Milford Laboratory, personal communication.) 



Hydrogen sulfide probably contributes an additive toxic 

 element to the low D.O. problem (Theede et al. 1969). 

 Our preliminary tests with hydrogen sulfide-laden sedi- 

 ments were inconclusive, however, as were some tests 

 performed with sodium sulfide solutions at the NMFS 

 Sandy Hook Laboratory (R, K, Tucker, personal com- 

 munication). Several technical problems complicated the 

 measurement of hydrogen sulfide levels. 



Although temperature has been reported as a critical 

 factor in survival of marine animals under low D.O. con- 

 ditions (Theede et al. 1969; Vernberg 1972), we found no 

 appreciable differences in survival among clams held at 

 10° and 20° C. Future studies might include a broader 

 range of temperatures. 



Oxygen consumption values for 24-hour periods were 

 calculated for clams held at three D.O. levels, at 10° C. 

 The lowest values were obtained from clams held in 0.7 

 ml/1 D.O. water, not an unexpected observation, because 

 the limited amount of detectable oxygen was completely 

 consumed during this period. Clams held at 1.4 ml/1 con- 

 sumed oxygen over 24 hours at a rate 50 percent higher 



279 



