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Pressure in the early life history of sockeye salmon. Harvey, Harold Henry


Young sockeye salmon (Oncorhynchus nerka) may occupy the epilimnion, thermocline or hypolimnion during lacustrine residence and may make extensive vertical migrations. Residence and migration over a range of pressure presents fish with certain physiological problems. Sockeye salmon meet these problems by adaptation, compromise and fortuity. Sockeye evidenced a tolerance to pressure in excess of 20 atmospheres, equivalent to a depth of water of 680 feet. Sockeye fry showed no behavioral response to pressure prior to initial filling of the swimbladder, but thereafter pressure induced compensatory swimming. Young sockeye proved to be dependent on atmospheric air for inflation of the swimbladder. The restrictions to vertical movement imposed by the swimbladder are minimized in sockeye by a relatively small bladder volume, little excess pressure within the bladder, the bladder being thin-walled and extensible and the inability of these fish to secrete gas into the bladder. When frightened, young sockeye sounded and expelled gas from the swimbladder. Gas expulsion was found to be under adrenergic control and retention of gas in the swimbladder under cholinergic control. During decompression with upward movement through thermally stratified water, gas disease or the "bends" is obviated by the rapid clearance of dissolved nitrogen from the blood stream. Young sockeye showed a tolerance to rapid decompression except under conditions permitting swimbladder gas to appear as emboli in the blood stream.

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