Fish Science
Skin Senses: Sensing Temperature

Photo: Berkley Why is Doc Jones heating up the water with a flashlight?

The previous article on somatic (skin) senses in bass dealt with pH and touch. Today we look at how bass detect temperature through their skin, and how they react to it.

Like virtually all fish science articles, this information is based on the book "Knowing Bass" by Berkley's fish guru, Dr. Keith "Doc" Jones.

Temperature Detection

As mammals, our internally regulated body temperature allows us to touch something and interpret its relative warmth or coolness as compared to our own constant temperature.

In the case of a cold-blooded critter like a fish, the detection is more of a difference in temperature than a relative hot or cold sense like we feel. They are alerted to changes based on what they've just been experiencing.

Bass use the vast network of free nerve ends in their skins to detect these temperature changes. Some studies suggest a sunfish can detect a temperature change of less that one-fifth of 1 degree F, which is the middle of the road for fish in general. We don't know how they do it, but it is likely different from how we humans do it.

We can tell hot from pressure, light touch from cold, etc. But bass don't have such specialized free nerve endings. They may not distinguish among these sensations. But they may combine them with input from their other senses such as their lateral line.

Behavioral Regulation

In any case, bass detect temperature well enough to regulate their thermal environment. But they have to do this behaviorally – that is, they get up and move. If it's too hot, they seek cooler water.

However, bass are thermophilic, meaning they are highly attracted to warmer water. Until the water temperature moves into the mid- to upper 90s, bass generally seek the warmest water conveniently available – given that other water-quality parameters are acceptable: Often oxygen content or some other environmental factor becomes limiting before heat does.

Ever notice that northern largemouths seem like they just can never get warm enough in their normal habitats?

Interestingly, a bass finds "better" temperature water by swimming constantly and adjusting its frequency of turning. If the water is cold and getting colder, it turns more frequently, which increasing its chances of avoiding traveling too far into the colder water. If the water is getting warmer, it turns less, going farther into the warmer water before adjusting its path.

Take-Away Points

> Bass love to get in hot water. Especially in the north, seek the warmest water with the most bassy-looking habitat. Bass don't know the water on the other side of the lake might be warmer, so just look for the warmest area in the cove, creek or bay you're fishing in.

> Even though bass love warm water, other factors still influence their behavior and sometimes override their desire for warmth. For example, big post-spawn females are known to retreat to deeper, cooler water to recuperate instead of staying up by the warm nests.

> Bass boat thermosensors located on the hull or trolling motor measure just the surface temperature and often tell very little about the water temperature down where the bass are really living.

> Warm or cold water can be moved around. For instance, wind can drive heated surface water to the windward side of the lake, while cooler water rises to replace it on the leeward side.

Notable

> The biggest thing about water temperature is that cooler water can hold more oxygen – with two major exceptions. One is that in lakes that stratify, the cold, lower layer below the thermocline is lean on oxygen. The other is when a thick layer of ice forms and air can't mix with surface water, and a winter kill can occur.

> Doc Jones is director of research at the Berkley Fish Research Center in Spirit Lake, Iowa. His book "Knowing Bass, The Scientific Approach to Catching More Fish" is available at amazon.com, Barnes and Noble, and other booksellers.