and test tanks. Dissolved gas pressure was measured 

 with a Weiss saturometer made by Eco Enterprises, 

 and the nitrogen saturation and total dissolved gas 

 saturation calculated according to the equations: 



Total dissolved gas % saturation = 



P a tm + A p 

 p atm 



where P a tm = atmospheric pressure at time and 



and 



altitude of saturometer reading 

 AP = saturometer reading 



% N 2 sat = ^(P atm + AP) 

 • (0.76)rl 



l b; /22.41 ml/m moleX 

 feO' 2 ^32.00 mg/m mole/ 



n/ 



00 mg/m mol 

 Patm - Ph 2 0> (0-7902) 



where 2 = oxygen concentration in mg/? 



B0 2 = Bunsen solubility coefficient for 

 oxygen 



P|_l q = partial pressure of water vapor as 

 a function of temperature 



T = observed temperature in °C. 



As the test fish died, necropsies were performed 

 as soon as possible. Weight, fork length, sex and 

 sexual maturation were determined and age esti- 

 mated using a modification of a graph by C. E. 

 Richards (1969) (Fig. 3). Of the 77 which have died 



AGE, YR 



FIG. 3 Menhaden age determination. 



since capture, fish length ranged from 225 to 

 310 mm, the average weight was 304 g, 46% were 

 males and 54% were females. The majority of fish, 

 60%, were age two, 20% age three, 17.3% age four, 

 and 2.7% age five. 



RESULTS 



The necropsies of control specimens showed 

 no apparent external or internal symptoms of gas 

 bubble disease. However, as can be seen in Table 1, 

 increased nitrogen saturation increased the evi- 

 dence of gas emboli at 22°C. At 105% saturation 

 nitrogen, all test specimens lived for 96 hr with no 

 pathological evidence of gas bubble disease. How- 

 ever, observations conducted during the course of 

 the experiment showed definite behavioral changes 

 in the test fish in comparison with the control fish. 

 The test fish were excreting mucus, swimming 

 erratically, and more excitable than the control fish. 

 Body color change was also apparent on some test 

 fish. 



TABLE 1 Etiology of Gas Bubble Disease in Menhaden 

 at 22°C 



% Saturation 



Observations 



Pathology 



N 2 Total O2 



100 95 70 None apparent. 



105 95 75 Some mucus, erratic 

 swimming, body color 

 change noted in 1 fish, 

 no deaths in 96 hr. 



110 107 102 Some mucus, erratic 

 swimming, body color 

 change noted in 2 fish, 

 2 died within 96 hr. 



120 110 85 Mucus, erratic 



swimming, death 

 in 24 hr. 



Appearance of gas 

 bubbles in: 



None apparent. 



None apparent. 



Eyes, intestines, 

 pyloric caeca, and 

 mesentery. 



Eyes, intestines, 

 pyloric caeca, roof 

 of mouth, bulbus 

 arteriosis, fins, 

 operculum, gill 

 arterioles. 



At 110% saturation nitrogen, behavioral changes 

 similar to those observed at 105% were noted. 

 Necropsies showed some evidence of emboli in the 

 intestines, the pyloric caeca, the operculum, and 

 the eye. Two fish died within 96 hr, one during the 

 3rd day and one between the 3rd and 4th days. 



At 120% nitrogen saturation, death occurred 

 within 24 hr with classic symptoms of gas bubble 

 disease displayed. Evidence of exophthalmia was 



Gas Embolism in Adult Menhaden 83 



