FIG. 1. Four-£ pressure chamber used to test the responses 

 of salmonids to various supersatu ration conditions. Chamber 

 is mounted on tilting table which facilitates handling of fish. 



gauges and flowmeters for monitoring and regulating 

 the conditions to which the fish are exposed. The 

 chamber is mounted on a tilting table which facili- 

 tates the transfer of fish in and out of the chamber. 



Test Evaluation 



The procedure to estimate time to complete 

 saturation of critical tissues and organ systems 

 (tissues and organs that are of vital importance to 

 the fish, e.g. heart) has been an acute bioassay tech- 

 nique where coho salmon (60 to 100 mm) were 

 exposed to a certain supersaturation condition and 

 a dive score taken of the response. The dive score 

 was calculated as follows: 2 points for a dead fish, 

 1 point for loss of equilibrium and for neither of 

 these. Signs of distress such as increased irritability, 

 excitement, and rapid ventilation were hot counted. 

 Most of the fish responded to the given stress either 

 within 30 min or at some time much later (hrs). 

 Therefore, dive scores were recorded at 15 and 30 

 min after any particular exposure, and an average 

 of the two scores was used for data. 



A gross autopsy of mortalities was conducted 

 to locate the bubbles within the fish tissues to deter- 

 mine if there were differences in the pathologies 

 caused by the different procedures used to impose 

 supersaturations in the fish. 



Internal Supersaturation 



To create internal saturation, the fish were 

 placed in the chamber and saturated at various 

 depths (by bubbling gases under pressure through 

 the chamber at 2 V/min).* After various lengths of 

 time (exposure) at depth, the fish were rapidly de- 

 compressed to the surface at 100 ft/min, removed 



from the chamber, placed in water containing gas 

 at 1 atm, and a dive score recorded. 



External Supersaturation 



In this series of tests, coho (at surface satura- 

 tion) were placed directly i,nto supersaturated water 

 and dive scores recorded. Initial saturation levels 

 ranged from 200 to 700% of one surface value, but 

 due to mechanical manipulation of the chamber 

 and oxygen consumption by the fish, these levels 

 decreased during the test. In these preliminary tests, 

 no additional gas was added to compensate for the 

 decrease. 



Combined External and Internal 



Supersaturation 



During the internal supersaturation tests, the 

 gas in the fish was allowed to diffuse outward into 

 fresh water when the fish was removed from the 

 chamber. If this outward diffusion was eliminated 

 for short periods of time, more gas should remain 

 in the fish and the severity of bubble formation and 

 its effects should increase. To minimize this out- 

 ward movement of gases, fish were saturated at 

 depth (100 ft only in these tests) as in the internal 

 supersaturation tests, but instead of placing the fish 

 in water containing gases at 1 atm, the fish were 

 held in the chamber (in the supersaturated water) 

 for various lengths of time (up to 15 min) and a dive 

 score recorded. 



RESULTS AND DISCUSSION 

 Internal Supersaturation 



As the pressure of saturation and resulting 

 supersaturation after decompression were increased, 

 the response increased as reflected by the dive scores 

 (Fig. 2). No signs of bubble disease were noted after 

 decompressions from 66 ft (300% saturation). At 

 100 ft no scores were recorded; however, these fish 

 were on the threshold of distress because any 

 additional stress such as an induced fright 

 response applied to the fish after decompression 

 resulted in signs of bubbles. At pressures greater 

 than 100 ft, dive scores increased at each level until 

 100% mortality was reached at decompressions 

 from 200 ft (700% saturation). From 60 to 90 mtn 

 exposure was required to obtain maximum 

 lethality from decompression from any one depth 

 (e.g., no significantly greater increase in percent 



•As pressure is increased, water and tissues can keep greater 

 amounts of gas in solution (according to C (concentration) 

 = P (pressure) x oc (solubility), if gas is bubbled through fresh 

 water at 34 ft, the water will take up approximately twice 

 the concentration of gas found in water at the surface). 



48 Beyer, D'Aoust, Smith 



