Hydrostatic Pressure Tolerance
of Salmonids

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Hydrostatic Pressure Tolerance of Salmonids:

Problem:

Fish passage facilities around dams are presently limited to fish ladders or major locks that require substantial costs in design and construction. The new fish passage design developed by N. Neufeld can greatly decrease the cost of fish passage, but the fish are subjected to hydrostatic pressure equivalent to the head in the reservoir above the dam at which the device is installed. The question that was presented is what effect will high pressure have on the health of fish passing through the device.

Objective:

To examine the tolerance of salmonids to high hydrostatic pressures.

Approach:

Three rainbow trout equivalent in size and shape to 8 inch steelhead smolts were subjected to pressures between 500 psi and 1000 psi in a hydraulic pressure chamber for two minutes, withdrawn, and placed in circular fish ponds to determine the effect of the pressure on their subsequent well being over a period of a week. Time to pressurize and depressurize amounted to approximately 5 seconds before and after exposure to the water filled pressure chamber.

Results:

No adverse effects from pressure exposure was detected immediately following exposure or during the week holding period thereafter. Activity and feeding behavior were observed to be normal during the holding period. Examination of the fish upon removal from the test chamber exposed no hemorrhages or other signs that pressure was deleterious.

Discussion:

Based on Boyle's and Henry's gas laws, the amount of gas dissolved in water is determined by the partial pressure and solubility of the gas, and is independent of other gases. For practical purposes, atmospheric air is 80% nitrogen and 20% oxygen, or at a N2 to O2 ratio of 4:1. However, since O2 is twice as soluble as N2, the ratio of N2:O2 in air saturated water is 2:1, and remains at about that ratio regardless of temperature or pressure. The total amount of gas dissolved in water, however, increases as temperature decreases. At saturation under 1 atm pressure, O2 increases from 8.5 mg/1 to 14.6 mg/1 as temperatures fall from 25 degrees C to 0 degrees C. Moreover, at saturation the total amount of gas dissolved in water increases proportional to pressure. For example at 0 degrees C the amount of oxygen in water at saturation increases from 14.6 mg/1 to 29.2 mg/1 as pressure increases from 1 to 2 atm. When salmonids are exposed to deep water they will seek neutral buoyancy by filling their swim bladder with gas captured at the surface or from bubbles produced from plants or the substrate. When exposed to a sudden pressure reduction that exceeds their ability to evacuate their swim bladder, the bladder can rupture and cause mortality. Also, when fish are in deep water their serum will equilibrate with the dissolved gases in their environment. In lakes water turnover and gas recharging generally occur in winter months when temperatures are cool, and hence the ability of water to hold dissolved gas is greater. As the weather warms in the spring and summer, the dissolved gas remains in solution because of the hydrostatic head created with water depth. Fish brought from that depth faster than their ability to equilibrate with the decreasing hydrostatic pressure will suffer from the bends much the same as a deep water diver. In these cases and similar circumstances when fish are depressurized death occurs from gas expansion or from serum gas coming out of solution. The bad reputation associated with pressure and fish health is related to the above circumstances. However, if fish are at the surface and taken to high pressure and subsequently return to the surface, even rapidly, none of these effects will occur as long as the fish hasn't increased the gas in the swim bladder or equilibrated with higher levels of dissolved gas. Therefore, when fish are pressurized from the surface, no ill effects would be expected to occur, and consequently when they are subsequently depressurized in such a situation no deleterious effects will occur because they are still equilibrated with the surface environment. Since the fish exposed to Neufeld's new fish passage apparatus enter it from the surface, they will not suffer any ill effects from pressurization or even from rapid depressurization.

Conclusion:

The new fish passage system designed by N. Neufeld when employed near water surfaces for fish entrance is expected to be functionally safe for fish passing through the system and experiencing pressures in excess of 500 psi.

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