100 



> 60- 



40 



20- 



200 FT(700% SAT. I 



30 60 90 



EXPOSURE TIME (MINUTES) 



FIG. 2 Responses of coho salmon to decompressions (internal 

 supersaturations) from various depths. Exposure time indicates 

 the length of time the fish were maintained at any particular 

 depth before decompression. The initial saturation is indicated 

 in parentheses. N = 15 fish (3 tests x 5 fish per test), ranges 

 indicated by vertical broken lines. 



dive score was observed after 60 to 90 min at 133 ft; 

 a stress producing a 35% dive score). Theoreti- 

 cally, it takes the same time period for a tissue 

 to reach equilibrium at 1 atm (total gas pressure) 

 as it does at 5 atm. This arises from the fact that 

 although a greater amount of gas must go into 

 solution in the fish's tissues, the rate increases 

 proportionally to the gradient and thus the total 

 saturation time remains the same. Therefore, 

 assuming maximum lethality is associated with 

 maximum gas absorbed for any given gas pres- 

 sure, it was concluded that saturation of critical 

 tissues was completed within 60 to 90 min for 

 this size fish. Since this time of equilibration, as 

 indicated above, should apply to any saturation 

 level, the well-documented lethal times of 1 day 

 or more for a saturation of 122% (Meekin and 

 Turner, 1974) indicates a time lag in achieving 

 maximum effect which cannot be related to gas 

 saturation or desaturation rates per se, but also 

 involves the amount of gas transfer. In other words, 

 the critical tissues of fish that die from long-term 

 chronic bioassays (greater than 10 hr) are satu- 

 rated after only 60 to 90 min of exposure. There- 

 fore, other factors related to gas bubble formation, 

 growth, and effects within the tissues are causing 

 the mortalities. Other potential mechanisms are 

 unclear at this time but additional studies by our 

 group (Casillas, Smith, and D'Aoust, 1975) are 

 investigating possibilities such as alterations in 

 the blood clotting mechanism and/or dissemi- 

 nated intravascular coagulation which has been 

 recently implicated in the sequelae of decompres- 

 sion sickness in man (DCIEM Conference, 1973). 



Bubbles were found in relation to the degree 

 of perfusion of a particular tissue, indicating the 

 importance of total gas transport. The frequency of 

 occurrence was much higher in the blood than in 

 the white muscle and fat. As the depth of exposure 

 was increased, there was obvious increase in fre- 

 quency of bubble formation in all tissues. Bubbles 

 in the blood were probably the most critical to the 

 fish because these bubbles cut off circulation to 

 other organs and systems, occluded the gills, and 

 frequently filled the chambers of the heart. 



External Supersaturation 



Fish were more susceptible to external than 

 internal supersaturation (Table 1) with fish dying 

 at 250% saturation (a concentration approximately 

 equal to water saturated at 50 ft) in 30 min. Most 

 of our results with this method were preliminary, 

 but it appears that a slightly different mechanism 

 related to the direction of the supersaturation 

 gradient and the total volume of gas available 

 accounts for this increased response. Also, in con- 

 trast to the internal supersaturations, bubbles 

 were primarily found in the dorsal aorta, coronary 

 artery, and in the heart. Very few bubbles were 

 found within the organs and tissues, mainly because 

 the bubbles in the circulatory system were effective 

 in causing death long before any bubbles were able 

 to form in the other tissues. 



TABLE 1 Responses of Coho Salmon to Acute External 



Supersaturations. N = 20 fish (10 fish x 2 tests at each 



saturation). 



Initial level of supersaturation 

 200% 250% 400% 700% 



%Dive Score 66% 100% 100% 



External and Internal Supersaturation 



When the supersaturation gradient between 

 the fish and the water was eliminated by simul- 

 taneously internally and externally supersaturating 

 the fish, the response was increased (Fig. 3). These 

 results indicate that the longer the fish are held in 

 supersaturated water after decompression, the 

 greater the dive score. The occurrence of bubbles in 

 this series of tests was similar to the internal super- 

 saturation with bubbles found throughout the tisssues. 



COMMENTS AND SUMMARY 



From the results of these acute tests, it is clear 

 that bubble formation can be induced by two 



Response of Coho to Supersaturation 49 



