I’m trying to understand how barometric pressure affects fish behavior.
I’ve been told that for inshore species (as opposed to deep water species) that fish will move up and down in the water column to offset the air pressure. So if the pressure is falling, fish will move deeper and if the pressure is rising, fish will move higher so the pressure in their air bladders stays relatively constant despite changes in atmospheric air pressure.
This sounds plausible, but my problem is with the relative pressures involved.
STP is 14.7 psi at sea level and it only takes a downward movement of 33 feet to double that so 0.445 psi per foot.
But barometric pressure rarely fluctuates more than 2 inches of mercury which is something like 1 psi. So even the wildest barometer swing would only require a change of one foot up or down in the water column to negate it. It’s nothing.
In other words, it’s impossible for me to believe that barometric pressure is affecting swim bladders in any significant way.
Barometric pressure might affect fish in other unknown ways, but I can’t see how the “air bladder” argument makes any sense.
I think you’re referring to this (from Wikipedia - bolding mine):
It seems to me that you’re looking at this backwards. These fish use the swim bladder to sense depth. If the effect of air pressure were significant, the fish probably wouldn’t use their bladders that way, or would have evolved some means of compensating. Since the effect is not significant, it remains as a curiosity for interested humans.
All fish species with a swim bladder have a way of introducing or releasing gas from it; the gas volume is not fixed like a balloon and they don’t adjust their depth to compensate for minor changes to their buoyancy or discomfort. Depending on the species (and the depth of water it’s adapted to), they’ll either regulate the amount of air in their swim bladders slowly through taking dissolves gasses out of their blood (deeper water fish), or by burping bubbles out and taking gulps of air in from the water surface (shallow water species).
Again, depending on species, fish have varying sensitivities to barometric pressure and many species can detect very minor changes, especially those with a labyrinth organ. Some types of fish are well known to change behavior during minor barometric pressure variations, such as the aptly name “weather loach”.
Fish behavior during barometric pressure changes can also be influenced by their prey movements in relation to weather changes. So yes fish behavior is affected by barometric pressure to varying degrees, but the reason you’ve heard of isn’t a major one.
Normal atmospheric pressure at the surface changes by at most 1 part per thousand per hour during ordinary daily weather & diurnal variation.
At 3 parts per thousand per hour it starts getting characterized as rising or falling rapidly. Which doesn’t happen every day. At time of strong frontal passage or an approaching hurricane it can get up around 10 parts per thousand per hour.
All in all we’re talking about a pretty slow-moving subtle signal in the vast majority of circumstances.
Are fish “aware” of this change? It sure sounds like it. Are they ascending and descending just based on the physics of pressure deltas, not on active biological control of their swim bladders? No way.