D. Beyer 



B. G. D'Aoust 



L. Smith 



Responses of I 



Coho Salmon 

 (Oncorhynchus kisutch) to 

 Supersaturation at One Atmosphere 



ABSTRACT 



While saturation limits of 110-120% have been established as 

 minimum lethal levels, mortalities may not be entirely related 

 to gas saturation or desaturation rates. To compare maximum 

 saturation times and capacities for inert gas with bubble forma- 

 tion, groups of small coho salmon (60-100 mm) were exposed 

 to supersaturations that were induced either internally (by 

 decompression) or externally (by placing fish in supersaturated 

 water). A minimum of 1 hr at depth was required to obtain maxi- 

 mum lethality from decompression. Assuming maximum lethality 

 was associated with maximum gas absorbed for any one decom- 

 pression, it was concluded that for any given gas pressure, satu- 

 ration was completed within 60 to 90 min. Thus, the well- 

 documented lethal times of 24 hr or more for an over-saturation 

 of 122% of 1 atm (Meekin and Turner, 1974) indicate a time lag 

 in achieving maximum effect which cannot be related to gas satu- 

 ration or desaturation rates. The external supersaturations 

 showed that 250 to 500% total gas pressure would result in com- 

 plete mortality in 10 to 30 min. 



Since Marsh and Gorham (1905) implicated excess 

 dissolved gas as the causative agent of gas bubble 

 disease in fish, most studies of the problem have 

 centered around: 1) defining the critical levels at 

 which problems due to supersaturation arise, and 

 2) describing occurrences of and solutions to 

 specific outbreaks of the disease. It seems, how- 

 ever, that some very interesting and important 

 studies may have been overlooked. These other 

 studies involve the actual dynamics of gas bubble 

 formation within fish tissues and the relationship 

 of bubbles to gas uptake and elimination rates. One 

 only needs to look at the example of fish dying due 

 to supersaturations of around 110 to 120% total gas 

 pressure. Such saturations in human divers are 

 readily tolerated and according to current practice 

 (U.S. Navy, 1973), staged decompression from satu- 

 rations of less than 200% of 1 atm (dives of less than 

 30 ft)* is not needed. Therefore, the sensitivity of 

 fish to gas bubble disease appears to be somewhat 

 anomalous to the response of air breathing animals 

 (including man) to gas supersaturation imposed 

 by decompression, primarily because the fish are 

 affected at such low levels. 



In our studies, we have been investigating the 

 responses of salmonids to acute supersaturations 

 (greater than 150% saturation) to determine the 

 relationships among supersaturation, gas uptake, 

 and death due to bubble formation. 



To study these relationships, we have exposed 

 fish to three separate types of supersaturation 

 conditions (internal, external, and a combination 

 of both). The direction of net gas movement (air in 

 these tests) in and out of the fish and the initial 

 site of supersaturation imposed on the fish were 

 varied in each condition. Internal supersaturations 

 were produced by decompression from saturation 

 where a net outward movement of gas occurred as 

 the fish was desaturated. In the external tests, fish 

 were placed in supersaturated water, and thus the 

 net movement of gas was inward. When the inter- 

 nal and external conditions were combined, the 

 supersaturation occurred both inside and outside 

 the fish and there was initially no significant net 

 movement of gas except that allowed by bubble 

 formation in the pressure chamber. Our pre- 

 liminary studies have centered on the differences 

 in the occurrence of bubble formation among these 

 three types of supersaturations ("treatments"). 



MATERIALS AND METHODS 

 Pressure Chamber 



A 4-? stainless steel pressure chamber, capable 

 of withstanding 10 atm test levels, was used for 

 our tests (Fig. 1). The chamber has viewing end- 

 plates, sampling ports, and attached pressure 



Beyer, Smith: University of Washington, and D'Aoust: Virginia 

 Mason Research Center, Seattle, Washington. 



Contribution Number 409, College of Fisheries, Seattle, Washing- 

 ton. 



•In these studies, the terms depth, atmosphere (atm), and pres- 

 sure are interrelated as follows: 1 atm = 34 ft of fresh water 

 = 14.7 lb/in 2 . 



47 



