Test II, the fish may have adjusted to the cage. The 

 Test III population was also reduced by about 50% 

 by the time of the highest supersaturation, enabling 

 the population to occupy a smaller space, perhaps 

 near the cage bottom. 



Depth Distribution in to 4 Meter Cage 



It was hoped that some additional information 

 on the depth distribution of juvenile salmonids 

 could be gained by recording the depths occupied 

 by the fish in the to 4 m volition cage. However, 

 ihe depth distribution of the fish in the to 4 m 

 cage could not be determined by either electronic 

 means or by direct observation. The depth distri- 

 bution can be estimated by comparing the cumula- 

 tive mortalities and the percent of survivors with 

 signs of GBD in this volition cage to the mortali- 

 ties and survivors in the fixed-depth and intermit- 

 tent-exposure cages. The comparisons of mortali- 

 ties strongly suggest that the fish in the to 4 m 

 cage remained somewhat below a depth of 1 m 

 most of the time during all three tests. In the third 

 test, both the mortalities and the percent of sur- 

 vivors with GBD in the 1 to 2 m cage were greater 

 than those in the to 4 m cage. Both mortalities 

 and the percentages of survivors with signs of GBD 

 were greater in all of the intermittent-exposure 

 cages than in the to 4 m cage during Tests II and 

 III. These data indicate that the fish in the to 4 m 

 volition cage spent something less than 8 con- 

 tinuous hr/day above a depth of 1 m. 



Gas Bubble Disease Lesions 



An attempt was made to correlate the obvious 

 external GBD lesions (symptoms, signs) with the 

 severity of the disease in each of the three tests. 

 The appearance of bubbles along the lateral line 

 was not used due to the difficulty of detecting this 

 lesion in the field. In most fish the gas bubbles 

 almost always appeared first between the rays of 

 the caudal fin (Fig. 8). Fish with bubbles in this fin 

 only were considered to have slight GBD lesions. 

 Bubbles appeared next most frequently in the dor- 

 sal fin, followed by the anal (Fig. 9) and paired fins. 

 Bubbles were also, in several instances, observed 

 in the adipose fin. Fish with bubbles in several fins 

 and/or a few bubbles on the head, or with a mild 

 case of exophthalmia were considered to have 

 moderate GBD lesions. Bubbles on the head 

 (Fig. 10) generally occurred only after several of the 

 fins were involved. In the area of the head, bubbles 

 appeared on the opercles, jaws, around the eyes 

 and inside the mouth. In very severe cases, the 

 head appeared to be a foamy mass due to the bub- 

 bles covering all surfaces (Fig. 11). 



Exophthalmia has often been considered to be 

 a classic indication of GBD. In these live cage tests, 



FIC. 8 Bubbles in the caudal fin of a juvenile chinook. This is 

 usually the first indication of gas bubble disease. 



FIG. 9 Numerous bubbles in the anal fin of a juvenile chinook. 



FIG. 10 The head of a juvenile chinook showing numerous 

 bubbles around the eyes, mouth, and the opercle. 



34 Weitkamp 



