NO A A PROFESSIONAL PAPER 11 



quahogs held in water at 10° C and 34%f salinity. Oxygen 

 consumption by large ocean quahogs (2.9-16 g dry weight) 

 was more or less constant to levels of 40 to 50 mm Hg, 

 which are values considered to be critical oxygen tensions 

 for the species. Above critical levels, the clams exhibited 

 respiratory independence, but small-sized clams (1 g dry 

 weight) showed respiratory dependence under hypoxic 

 conditions. The differences in response to oxygen were 

 also believed to be modified by temperature and the phys- 

 iological condition of the clam, other factors that compli- 

 cate the identification of a species as an oxygen regulator 

 or oxygen conformer. The critical oxygen tension (Pc) 

 values for ocean quahogs compare with those for soft 

 clams (Davis 1975) and are lower than those for surf clams 

 (Savage 1976). As reported by Brand and Taylor (1974), 

 ocean quahogs can compensate for oxygen levels lower 

 than the Pc values by greatly increasing pumping activity 

 or closing their shells. Both are reactions to stress. 



The larger capacity of ocean quahogs to function under 

 low oxygen conditions appears to be species specific. 

 Theede et al. (1969) compared the resistance of a number 

 of marine bottom invertebrates to oxygen deficiency and 

 hydrogen sulfide. Spisiila solida (a European relative of 

 the surf clam) and the ocean quahog were among several 

 bivalves used in the experiments. Resistance was meas- 

 ured in hours over which 50 percent survived experimental 

 conditions (LD-50). All ocean quahogs survived for 55 

 days in water of 0.15 ml 0,/l (10° C, 15%c) and for 33 to 

 41 days in water of similar oxygen deficiency, but treated 

 to create a hydrogen sulfide (H^S) condition. The survival 

 times were the longest for any of the invertebrates tested, 

 but 5. solida was not among the species listed. 



In experiments with isolated gill pieces in water of a 

 similar oxygen deficiency and H,S level but higher salinity 

 (30%'c), the survival of S. solida tissues was affected 3 to 

 7 days earlier than those of Mytilus ediilis. Modiolus mo- 

 diolus, and soft clams. For S. solida, ciliary activity ceased 

 and cell damage was irreversible after 24 hours in oxygen- 

 deficient water treated to create H,S. The tissue pieces 

 survived the experimental conditions for 3 to 4 days. 

 Under similar oxygen and H,S conditions, but lower sal- 

 inity (15%(i), isolated gill tissue pieces of ocean quahogs 

 survived for 8 days, although ciliary activity ceased and 

 was irreversible after 8 to 24 hours. Lower experimental 

 temperatures and higher salinity greatly increased survival 

 and recovery of ciliary activity for ocean quahogs. Ocean 

 quahogs were considered to be highly resistant to oxygen 

 deficiency and hydrogen sulfide. 



Off New Jersey, the bottom temperature increased 

 sharply in early October, after the low D.O. levels had 

 been affecting the benthic fauna for several weeks. Scallop 

 mortalities, although less intense than surf clam mortali- 

 ties, were found in the shoreward portion of the resource 

 and in the area of low D.O. levels. Temperature increases 



may have been only an added stress on individuals near 

 death from the effects of low D.O. Thermal stress may 

 have delayed death. Waugh (1975) observed mortalities 

 for intertidal bivalves [the ribbed mussel. Modiolus de- 

 missus ( = Geukensia demissa), the blue mussel, Mytilus qj 

 edulis, and the soft clam, Mya areuaria] after removal 

 from experimental lethal stress conditions and return to 

 normal conditions. Delayed mortalities were higher and 

 occurred earlier in short-term experiments than in long- 

 term experiments. 



As Davis (1975) pointed out, many invertebrates can 

 tolerate low levels of oxygen, but intolerant or less tolerant 

 species may be lost from communities. As a result, a new 

 species may invade the commmunity or some species al- 

 ready present may increase. Relative to the surf clam fish- 

 ery, which is currently faced with a low supply and a low 

 recommended annual yield (Brown et al. 1977), the loss 

 of a substantial quantity of clams and uncertain recruit- 

 ment in the affected area have a serious socioeconomic 

 impact. Relocation of vessels to fish the remaining stock 

 increases the effort on the already low supply. Some ves- 

 sels may not be able to fish stocks at greater depths and 

 farther from shore. The biological implication is that a 

 sizable brood stock has been lost. 



The surf clam, ocean quahog, and sea scallop were each 

 affected in different degree by the oxygen depletion event 

 off New Jersey. The surf clam is highly susceptible to low 

 D.O. levels and H.S, and most of the clams were killed 

 within the mortality area; thus it was the most severely 

 affected of the three bivalves. The ocean quahog was 

 mostly offshore of the affected area and was little affected. 

 The sea scallop also was outside the affected area and was 

 little affected. 



SUMMARY 



For surf clams, a 6,750-km- area of mortality was de- 

 limited off New Jersey. The area extended from imme- 

 diately north of Manasquan Inlet to immediately south of 

 Atlantic City, and seaward to about 37 m. An almost 

 complete kill (92%) took place in the mortality area, but 

 was least intense in the 3- to 15-km-wide beach zone. An 

 estimated 61.5 percent of the total surf clam biomass was 

 lost off New Jersey. Surf clam landings were substantially 

 lower (31%) in New Jersey during 1976 than during 1975. 



The principal ocean quahog resource occurs deeper than 

 37 m and thus only the shoreward margin of the population 

 was affected. The mortality area was 9,105 km-, and 25.4 

 percent of the quahog biomass within it was lost. Of the 

 entire New Jersey ocean quahog resource, 6.3 percent of 

 the biomass was lost. New Jersey vessels began fishing 

 ocean quahogs in 1976 and landed 71.7 percent of the U.S. 

 total. 



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