A fishing barometer is a device that measures atmospheric pressure. It operates like any other barometer, with the added benefit of displaying whether or not conditions are ideal for fishing.
Use of a fishing barometer is based on the theory that barometric pressure affects fish feeding activity. While a drop in air pressure brings on feeding activity, rising pressure leads to less biting. It is also believed that high pressure results in fish moving to shallower water, while low pressure leads fish to deeper water. A fisherman utilizing a fishing barometer can adjust his tactics accordingly to take advantage of these behaviors.