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Old 10-31-2008, 11:04 PM   #1 (permalink)
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Default Article: Cold Front Myths -- Busted

Cold Front Myths -- Busted

Terry Battisti

There has been a lot of talk recently – both here and on other sites and magazines – about the effect cold fronts have on bass. Unfortunately, I don’t have all of the answers nor do I claim to. But, I do have a large number of articles from the past and a decent background in science that allows me to be able to look at the data and figure some stuff out. So, after looking at these posts, a recent article in In-Fisherman, and Ziert’s article on Nighswonger’s site, I’ve come up with this.

I will address each issue in its own section.

Barometer and it’s effect on Water Pressure

This is without a doubt the most misunderstood and blatantly misused variable in bass fishing. Water pressure at depth changes 1 atmosphere (1 ATM = 14.7 psi) for every 33-feet of depth. So, at the surface, there is a pressure equal to that of which the atmosphere is. Go down 33 feet and you are at twice the atmospheric pressure. Now, the BIG question now is – “How does a rapid decrease (front moving in) in pressure and/or a rapid increase (high pressure moving in) effect the fish. In other words, what does the fish have to do in order to maintain a constant internal and external pressure in itself. This is where is gets really interesting and the first Myth is busted.

People say that when a front is coming in (i.e.: the pressure is dropping) that the fish move up shallow to maintain pressure and to feed. Okay, I agree with the fact that fish seem to be more active but, move up in the water column to maintain their pressure? Nope, that is wrong. The second thing people say is that when the high-pressure moves in, the fish go deeper to maintain their internal pressure. This is wrong again folks. Look at Figures 1 and 2 to convince yourself.

Figure 1 is a Depth vs Pressure chart for varying atmospheric pressures down to 100 feet. The solid black line in the center is at standard atmospheric pressure (29.92 inches of mercury), the green line is at 28.92 inches of mercury and the solid purple line is at 30.92 inches of mercury. These three lines represent the average swing in pressure between low and high for just about any storm we experience here in earth with the exception of a hurricane. The doted lines that bound the three solid lines are the record extremes for low pressure (25.90 inches of mercury in a hurricane) and for high pressure (32.00 inches of mercury, Siberian winter high pressure) and are in the record books as such.

Now, look at Figure 2, which is a blowup of Figure 1 from the surface to 25-feet of water. Lets assume there is a fish at 5 feet and the pressure instantly goes from 29.92 to 28.92 in. Hg. Is that fish going to move up or down to maintain its pressure in it bladder? Well, the red arrow indicates that if it wants to maintain constant pressure, it actually has to move down in depth by 1.5 feet. Now, look at the fish at 10 feet. It is currently experiencing a surface pressure of 28.92 inches of Hg. Lets say that a high-pressure system instantly come in and the pressure goes to 30.92 in. Hg. Where does the fish have to move in order to maintain its internal pressure. A vast majority of people say it has to go down in depth. Wrong!!!! The fish actually has to decrease its depth by 3 feet in order to maintain its internal airbladder pressure!!! Its amazing isn’t it?

So, now that we see how air pressure effects the pressure under water, lets talk about what it does to the fish? Do you think a fish that has undergone a pressure change of 1 inch of mercury either way is going to quit eating because of this pressure change? I don’t. It’s insignificant. The fish has only to move up or down in the water column a maximum of maybe 5 feet to equalize its internal pressure. I say pressure effects on bass eating is hogwash. It is also interesting to point out that this topic was printed in In-Fisherman a couple years ago and the article was wrong with respect to how a fish would migrate the depths to maintain internal pressure.

Weather Accompanying Frontal Changes and Bass Feeding Habits

This is where it gets tricky. Most all cold fronts are accompanied by some sort of drastic weather change. The leading edge of most has a drop in pressure followed by rain or cloudy conditions. They are then followed by an increase in pressure and blue-bird skies. What do we know about fish feeding habits that can be related to why the beginning through midsection of the front is so drastically different than the far edge of that front? Well, we all know that bass are predatory fish. They like to ambush their prey and will use anything as cover. The beginning and midsections of the front offer wind, clouds and an overall decrease in light penetration under water. In other words, the bass has the cover of darkness as an aid to capture its prey.

What about after the cold front and the high-pressure moves in? Well, the fish had been on a feeding binge for a long time. They’re full and now that there are no clouds and light penetration is at a premium, they’re less apt to feed. So, what do they do? They hunker down in the nastiest cover available and digest the food in their belly. Cast a bait a foot from them they aren’t going to go for it. Cast one on top of them 5 times and they’ll eat it out of aggression.

Water Temperature, pH and other Water Chemistry Variables

Water temperature is probably the most agreed upon variable that affects bass feeding habits. Drop the temperature and you drop their metabolism and visa versa. But, to say rain will increase or decrease the water temperature is again hogwash. Water has one of the highest heat capacities of any substance we have on earth. In other words, it takes a lot to raise or lower its temperature. Most lakes have magnitudes of orders more water in them then they will get from a rain storm. Now, this is not to say that the surface temperature won’t change because it will. But, what I am saying is the temperature of the water at 5 feet isn’t going to change much unless there is a very large amount of runoff associated with storm.

Water density also has to be looked at for rain and runoff. Water is densest at 39.2oF. Below that temperature water is less dense (this is why ice floats) and above that temperature water is less dense. So, lets take a typical spring time rain as an example. The rain is more than likely going to be hotter than the lake water itself. This warm water will stratify on the surface and will not have much of an effect on the water even at 5 feet of depth. Now lets look at a storm where the rain is colder than the water temperature. This can have an effect that can cool the lake (considerably if it rains long enough) due to the fact that the rain water is denser than the lake water. But, and there are always those buts, unless there is a significant amount of water going into the lake, there will be a very small change.

Now, couple a cold rain with wind (mixing) events and you can change the water temperature drastically in a very short time.

As a rule of thumb, if you have 1 gallon of water at 65 degrees, you’ll need another gallon on water at 55 degree to drop the temperature of the original water by 5 degrees. What’s this mean for a lake? If you have a lake that has 100,000 acre feet of water between the surface and 5 feet (at 65 degrees), you’ll need another 100,000 acre-feet of water at 55 degrees to drop it to 60 degrees. That’s a lot of rain folks. Is the sanity check over?

Now lets talk about pH. Unless you fish near a superfund site where the pH of the soil is out of line, pH isn’t much of a factor. Why? The carbonic acid – calcium carbonate cycle. It is a rare circumstance where lake water is below 6.5 pH units and above 8.0 pH units. And, between those units a bass is at its best. The calcium carbonate carbonic acid cycle is a natural cycle and acts to buffer any acid or base that goes into the lake. So, place an acid in the lake and the calcium carbonate in the soil dissolves decreasing the acidity of the water. Add a base to the water and the carbonic acid neutralizes it and more is created by the absorption of CO2 into the water.

Another thing about runoff. Like I said prior, unless your lake is next to an acid pit, you really don’t have to worry about acidic runoff. But, what about clay runoff? Clay is known to have large amounts of calcium carbonate on it and thus will actually make the lake more basic in chemistry. Basic is good too. Most aquatic plants thrive in basic water and when there is a good amount of weed growth, you have good bass fishing.

Acid rain? Well, that’s an entirely different story and one which we don’t have to deal with as much in these years of the Clean Air Act.

Bass Forage and Cold Fronts

This is from the article in In-Fisherman and credit needs to be given to this article and Steve Quinn for authoring it. I am not going to copy it verbatim but will paraphrase the important parts of it. The original title is Barometric Effects on Bass – Natural Cues to Catchability. By: Steve Quinn.

It seems that the most important thing here has to do with the movement of zooplankton. Under high pressure conditions, zooplankton go deep and/or into the cover. When the zooplankton take cover, the fish that feed on them stop feeding and therefore so do the bass. Conversely, when the barometer drops, the zooplankton move up and into the open and the feeding starts.

It was also shown that zooplankton actually move up in the water column as darkness approaches. So, you have an incoming front with clouds, the zooplankton are more apt to be available and that will also trigger a feed.
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