Model DHR70B-BRASS - Handheld Fishing Barometer
Tide, water temperature, light level and moon phase can affect fishing success. Barometric pressure also has a major influence on fish behavior. The Ambient Weather Fishing Barometer displays ranges of pressure that are great (green), good (blue) and poor (orange) for fishing conditions with the color coded scale. A fish senses pressure changes through its air bladder. Fish that have small air bladders, such as kings, Spanish mackerel, wahoo and dolphin, aren't as affected by barometric changes as those with large bladders, such as trout, redfish, tarpon, grouper and snapper.
When their bladders expand, fish become uncomfortable. They relieve their discomfort by moving lower in the water column or by absorbing extra gas in their bladders.
Because of the anatomical and physiological stresses exerted on them, they're not worried about eating.
They're more concerned with trying to find a depth where they can stabilize their bladder pressure and feel good.
The barometer must be calibrated to your local conditions.
To compare pressure conditions from one location to another, meteorologists correct the measured pressure (referred to as absolute pressure) to sea-level conditions. Because the air pressure decreases as you rise in altitude, the sea-level corrected pressure (the pressure your location would be at if located at sea-level) is higher than your measured pressure if you live above sea-level and lower than your measured pressure if you live below sea-level.
To correct the relative pressure, consult your local news source (Weather.com, Wunderground.com).