120 



100 



BO 



s 



Ml 



40 



20 



O PARTIAL OCCLUSION 

 • TOTAL OCCLUSION 





4 8 12 16 20 



NUMBER OF ACTIVE SCALES IN SENSORY UNIT 



FIG. 4 Juvenile steelhead trout trunk lateral line showing 

 complete gas bubble occlusion. A. Anterior suprascalar pore. 

 B. Posterior infrascalar pore. 



FIG. 3 Spontaneous activity of single lateral line nerve fibers 

 from gas stressed fish and corresponding number of scales 

 eliciting neural activity. Regression line represents spontaneous 

 activity of fibers from control fish at 13°C to 15°C and is the 

 same as shown in Fig. 2. 



Twenty-two fibers were isolated and charac- 

 terized under conditions of partial bubble formation. 

 Fifty percent responded normally in all respects, 

 and upon localization of the sensory units no 

 emboli were visible. The remaining fibers behaved 

 as did those fibers isolated when emboli formation 

 was complete. In addition to the inability to match 

 spontaneous activity to the number of scales in the 

 sensory unit, the limited number of scales that did 

 respond to firm pressure either contained or were 

 adjacent to scales with gas emboli. 



We additionally isolated and characterized 

 nerve fibers from three fish that were maintained 

 3 consecutive days in the test apparatus. On days 1 

 and 3 support was with equilibrated water and day 2 

 was with gas supersaturated water. This procedure 

 provided for an evaluation of any residual effects 

 the gas emboli may have on the functioning of the 

 sensory units. Results showed a return to normal 

 response patterns as the emboli disappeared 16 to 

 20 hr after return to equilibrated water. 



In fish showing signs of gas bubble disease, 

 the reduced ability of the trunk lateral line sensory 

 units to respond to stimuli appears to be mechanical 

 in nature. Fig. 4 is a photomicrograph of a section 

 of the trunk lateral line of a juvenile steelhead trout 

 that exhibits complete occlusion with gas emboli. 

 A longitudinal section through this area would 

 appear as in Fig. 5. The physical presence of the 

 gas emboli probably acts directly on the cupula 

 rendering it immobile. This cupular immobility pre- 



ANTERI0R SUPRASCALAR PORE 



CUPULA 



POSTERIOR INFRASCALAR PORE 



FIG. 5 Longitudinal section through a totally occluded lateral 

 line. 



vents the shearing displacement action necessary to 

 stimulate the hair cell receptors. Although we never 

 monitored nerve fibers from the numerous cephalic 

 branches of the lateral line, morphological and 

 visual observations indicate that these receptors are 

 equally affected. This being the case, the ability of 

 an affected fish to detect and localize predators, 

 or stationary objects may be impaired. This would 

 have a definite adverse affect on survival. 



From mark and recovery experiments, Ray- 

 mond (1970) estimated that survival of downstream 

 migrating salmonids in the Snake River basin 

 between 1964-68 was nearly 100%. Following dam 

 construction and resultant high concentrations of 

 dissolved gas (during periods of heavy spilling at 

 dams) in the water of the lower Snake River, sur- 

 vival of downstream migrants dropped to 30%. 

 Considering factors of fish tolerance, behavior, and 

 gas concentration, Ebel (1973) estimated that 60% 

 of the total 1970 mortality was due to direct effects 

 of gas bubble disease. Predation and mortality 

 associated with dam passage accounted for the 

 remaining 40% of the loss. We believe that a large 

 proportion of the mortality attributed to these non- 

 specific causes was probably an indirect effect of 

 gas bubble disease through impairment of the 

 lateral line sensory system. 



Lateral Line Function 97 



