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October 26, 2001

Chilling Depths

Learn how thermoclines effect a fish's activity.

Q: There is a lot of fishing advice relating to water temperature and thermoclines, but what exactly is a thermocline and how does it affect game fish? - Rick Weisbaum, Tampa, Florida

A: Wind mixes the warm upper layer of the ocean - down to depths of a few hundred feet, depending on the strength of the winds and currents - so water temperature changes very little between the surface and deeper water.
Logically, that warm, upper layer of water is called the mixed layer. Below the mixed layer is an area called the thermocline, where temperature drops rapidly with increasing depth. In the thermocline, temperature can change as much as 2 or 3 degrees Fahrenheit for every 3 feet that depth increases. (The standard definition of a thermocline is the zone where temperature drops at least 1 degree Celsius for every meter of increased depth.) The thickness of a thermocline varies, but it is generally many feet thick. Below the thermocline, temperature once again decreases slowly with increasing depth.
Being cold-blooded, fish depend strongly on the temperature of the water they live in. Fish living in cold waters below the thermocline have slower metabolisms and live longer than fish in warmer waters. Because the water below the thermocline stays about the same temperature year-round, fish living there don't migrate seasonally like fish that live near the surface.
Some game fish that spend most of their time in the warm waters above the thermocline, such as billfish and tunas, are adapted to make temporary dives into the deeper, colder waters below the thermocline. Billfish have special, muscular organs knows as "heaters" to keep just their eyes and brains warm and functioning when they hunt below the thermocline. Tunas are somewhat warm-blooded, able to maintain their whole bodies 5 degrees or more warmer than the surrounding waters. When tunas get too warm, they might dive below the thermocline to cool off.
In effect, the thermocline acts as a soft barrier to fish. Depending on its lifestyle, a species will usually be found in its favorite niche, either above or below the thermocline.