oxygen saturation levels were reduced. Menhaden 

 in the thermal plume still displayed external symp- 

 toms of gas bubble disease, but very little mortality 

 was observed. 



Field observations of the menhaden continued 

 on April 23 and 24, 1973, and although the majority 

 of fish still displayed symptoms of gas bubble dis- 

 ease, they appeared to be more responsive and in 

 somewhat better condition. 



The area around and under the surface thermal 

 plume was examined on April 23, 1973, utilizing 

 SCUBA. A large school of pollock (Pollachius virens) 

 was observed along the interface between the 

 ambient water and the thermal effluent. Tempera- 

 tures where the pollock concentrated ranged 

 between 44° and 48°F. The pollock appeared in 

 good condition, were actively feeding, and showed 

 no external symptoms of gas bubble disease. Other 

 fishes observed and showing no sign of gas bubble 

 disease were striped bass {Morone saxatilis), short- 

 horn sculpin (Myoxocepha/us scorp/'us), and sea 

 raven (Hemitripterus americanus). No lobster 

 (Homarus americanus) were sighted but cancer 

 crabs (Cancer spp.) and green crabs (Carcinus 

 maenas) were common and feeding upon decom- 

 posing menhaden on the ocean bottom. 



Attempts to remove the school of menhaden 

 from the discharge area by commercial fisherman 

 using purse seines were unsuccessful due to the 

 presence of several large boulders in the discharge 

 area and also the high velocity of the cooling water 

 discharge which interferred with normal methods 

 of purse seining. The school of menhaden was last 

 observed in the thermal plume area on April 27, 

 1973. 



Another large school of adult menhaden (esti- 

 mated 200,000 to 300,000 fish)' was observed in the 

 Pilgrim Station thermal plume in early July 1973. 

 The menhaden remained in the vicinity of the sta- 

 tion discharge for about 1 day. During the 1-day 

 observation, no menhaden mortality was noted, 

 although a few menhaden were observed propelling 

 themselves on their sides on the water surface and 

 at times jumping clear of the water. None of the 

 several menhaden examined showed external signs 

 of gas bubble disease. The menhaden were con- 

 centrated in the thermal plume where water tem- 

 peratures ranged from 68°F to 82°F. Dissolved oxy- 

 gen concentrations and percent saturation at the 

 periphery of the school of fish ranged from 9.6 to 

 9.9 rng/S and 124% to 146%, respectively. 



As of the end of September 1974, no large 

 schools of adult menhaden had been sighted in the 

 immediate vicinity of the station discharge. It 

 should also be noted, however, that from October 

 1973 until August 1974, Pilgrim Station has been at 

 reduced power or not operating due to refueling. 



maintenance and contested licensing hearings 

 regarding a change in the fuel design. 



DISCUSSION 



Since the Pilgrim Station menhaden mortality, 

 concern over the potential problem of gas bubble 

 disease at coastal electric generating stations has 

 increased. Recognizing that fishes are attracted to 

 power plant thermal discharges which may be 

 supersaturated with dissolved gases, it has become 

 obvious that the tolerance of important species to 

 gas supersaturation needs to be determined. 



Such studies on the tolerance of menhaden to 

 supersaturation at varying water temperatures are 

 currently being conducted (Clay et al., 1974). Sub- 

 sequent experiments will attempt to determine 

 the recovery rates of menhaden after exposure to 

 supersaturation as well as determining the impor- 

 tance of other factors (e.g. physiological stress due 

 to extreme activity levels) in modifying the toler- 

 ance of menhaden to gas supersaturation. 



Other considerations include the ability of some 

 fishes to detect and avoid water supersaturated 

 with dissolved gases. Meldrim, Gift and Petrosky 

 (1973a and 1973b) found that the behavioral 

 responses of some freshwater fish to supersaturated 

 conditions varied with the species. They noted that 

 the yellow perch (Perca flavescens) showed no 

 definitive response to supersaturated water while 

 the silvery minnow (Hybognathus nuchalis), golden 

 shiner (Notemigonus crysoleucas), and the satinfin 

 shiner (Notropis analostanus) were very responsive 

 to supersaturated conditions. Stickney (1968) 

 observed that the Atlantic herring (a species related 

 to menhaden) definitely tended to avoid gas-super- 

 saturated water, but only when saturation levels 

 were high enough to produce gas bubble disease. 

 It is possible that menhaden are also capable 

 of detecting and avoiding supersaturated 

 environments. 



The ability of fish to detect and avoid super- 

 saturated environments may be altered by other 

 factors, however. Meldrim et al. (1973b) have 

 reported that the behavioral response of the golden 

 shiner to supersaturated conditions may change 

 depending on the water temperature associated 

 with the exposure. They noted that the golden 

 shiner usually avoided gas supersaturations that 

 exceeded 110%. However, when temperature 

 increases of 5°C and 10°C are associated with the 

 supersaturated conditions, temperature preference 

 of golden shiner overrides avoidance of the super- 

 saturation. It seems likely that similar behavior 

 occurred during the menhaden incident described 

 in this paper, i.e., the preference for above ambi- 

 ent temperatures within the discharge channel and 



Menhaden Mortality at a Power Plant 79 



