CHAPTER 11, PART 1 



Table 11.1-5. — Estimated hiomass loss of surf clams off New Jersey, 



1976-77 



Table 11.1-6. — Estimated hiomass loss of ocean qiiahogs off New 

 Jersey. 1976-77 



DISCUSSION 



The low dissolved oxygen (D.O.) water mass associated 

 with early mortalities (June and July) of finfish and in- 

 vertebrates included an extensive area off central New 

 Jersey in early August and September. Finfish were vir- 

 tually absent from the region, but the much less mobile 

 benthic invertebrates could not escape the anoxic water. 

 With each mortality in the affected area, the demand on 

 the available oxygen increased, because of the natural 

 decay processes. The condition persisted for all least 13 

 weeks, July 1 to September 30, and probably had devel- 

 oped to some degree before July. 



Mortalities of marine organisms off the New Jersey 

 coast have been reported in the past. Most recently 

 (Young 1973), scuba divers saw lobster (Homariis ainer- 

 icanus) and rock crab {Cancer irroratiis) during October 

 1971, but did not mention clams. Although accurate di- 

 agnosis was not possible, low D.O., high temperature, 

 and flocculated material in the water were suspected to 

 have affected the lobsters and crabs. Ogren and Chess 

 (1969) listed numerous observations around shipwrecks, 

 reefs, and bottom communities during September and 

 October 1968; Ogren (1969) summarized the event. Sev- 

 eral species of finfish and invertebrates were seen living, 

 dying, and dead. Surf clams (Spisula solidissima) were 

 found lying on the bottom. Water temperatures taken at 

 one site (the Delaware wreck) were considered normal for 

 the season. Levels of D.O., however, were abnormally 

 low (range: 0.34 ml/1 to 0.72 ml/1) and were believed re- 

 sponsible for the unusual behavior and mortalities of the 

 finfish and invertebrates. The D.O. levels recorded during 

 1976 were as low and lower than normal. 



The effect of D.O. thresholds in producing some phys- 

 iological, behavioral, or other response in marine inver- 

 tebrates is poorly known, and determinations are com- 

 plicated by the ability of many marine invertebrates to 

 survive anaerobically (Davis 1975). Some invertebrates 



can tolerate very low levels of oxygen and even show an 

 independence above a low critical tension. Davis included 

 the soft clam (Mya arenaria) as an oxygen-independent 

 species with critical oxygen tension values of 40 to 50 mm 

 Hg. Although the experimental conditions under which 

 the values for soft clams were obtained may not be strictly 

 comparable. Savage (1976) found that the median bur- 

 rowing time of the surf clam was substantially slower at 

 a concentration of 1.0 ml OJ\ (ca. 123 mm/Hg) and at 

 ir C. Surf clams ceased burrowing after 3 days at 15.4° 

 to 15.8° C and 0.6 ml 0,/l (ca. 68-77 mm Hg). In one 

 experiment in which the ambient temperature was in- 

 creased about 6° C and during experiments at seasonally 

 high temperatures of 20. 8° and 21.0° C in July and August, 

 clams also died in water of the lowest oxygen content (0.62 

 ml OJ\ or ca. 92-93 mm Hg). Oxygen depletion, then, 

 had the greatest effect on burrowing, and, coupled with 

 high temperatures, on survival too. It is not known 

 whether surf clams exhibit independence, as soft clams 

 do; a comparison of oxygen tension values may not be 

 justified. The generally higher values for surf clams may 

 be the result of experimental or special specific differ- 

 ences. 



Studies have related the reactions of ocean quahogs to 

 low D.O. levels. Brand and Taylor (1974) reported on 

 pumping behavior. In well-oxygenated water (160 mm 

 Hg), active pumping of currents in and out of ocean qua- 

 hogs' siphons to fulfill respiratory and digestive require- 

 ments alternated with periods of inactivity. The pumping 

 activity was independent of shell movements. Complete 

 closure of the shell often resulted in quiescence for several 

 hours. For ocean quahogs, and some other subtidal spe- 

 cies, periods of pumping varied, but amounted to 40 to 

 60 percent of the observation time. In water with low 

 D.O. (30-50 mm Hg), pumping time increased to over 95 

 percent; at lower oxygen tensions, it stopped and the shells 

 closed. Taylor and Brand (1975) investigated the effect 

 of hypoxia on the rate of oxygen consumption in ocean 



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