A barometer measures atmospheric air pressure, which creates downward pressure on the water in lakes and streams. Fish are able to sense pressure differences, enabling them to come to the surface to feed when they sense impending bad weather (decreasing or low pressure). During fair weather (increasing or high pressure), fish are less apt to have a feeding frenzy, knowing they can rise and feed any time that is convenient. This effect is more pronounced in shallow bodies of water. With this in mind, barometric pressure readings can assist the fisherman in procuring his catch.
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Place the barometer in a protected outdoor area for the most accurate measurement of outdoor atmospheric pressure. It is OK to set up the barometer at home even if the fishing location is miles away, since the pressure doesn't change much in a 20- to 30-mile radius. If travelling farther than this distance, take a barometer with you.
Record the barometer reading on an hourly basis and note whether the reading is rising, falling or steady as well as whether the reading is high or low. The general guideline for normal barometric pressure in the United States is 30 inches in hectopascals (30 in Hg). For barometers using the millibar measurement system, each in Hg equals 33.864 millibars. This translates into a normal pressure at sea level of 1013 millibars and a normal pressure inland of 1016 millibars.
Determine if your barometer reading is high or low for the given location. It only takes a +/- 0.02 change to affect the behaviour of fish. You can identify the threshold of low or high pressure in your area by multiplying the average normal pressure by 0.02 both plus and minus.
Sink your fishing hook shallow to start with if the barometric pressure is low, but has only been low for a brief period of time -- generally two to four hours. If the barometric pressure has been low for a longer period than this, the fish may have already eaten, and you will want to sink your hook to a greater depth. Sink your fishing hook deep if the pressure is above normal and rising, or steadily above normal.
Tips and warnings
- Barometric pressure is only one indicator of what fish may be doing. Water temperature, sunlight levels and locations where heavy fishing has caused the fish to dive deeper can play roles in fish behaviour.
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