Lakecaster Online Archives - June, 2004

Hearing Mechanisms of Largemouth Bass

By Todd Driscoll
Texas Parks and Wildlife Department
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Several months ago, the topic of this column was the visual ability of largemouth bass and how properties of water and its associated clarity affect it. Given the hundreds of artificial lure color options available, it is important that anglers understand how and to what degree bass interpret them. Similarly, sound is important while fishing, as lures either rattle, thump, knock, vibrate, or at the very least (i.e., a weightless plastic worm) make a splash and displace water as they sink. This month, we continue on the topic of sensory abilities and focus on hearing and vibration detection mechanisms of largemouth bass.

As was the case with color interpretation, characteristics of the underwater environment affect sound travel. Anyone that has spent time underwater can attest to the excellent ability of water to transmit sound. In fact, sound travels 4.5 times faster in water than in air. As a result, sound is an influential factor in the life of a bass.

Largemouth bass utilize two separate components that work together to interpret sound - the inner ear and the lateral line system. The inner ear of bass is just that - completely internal (housed within the skull) with no outside connection. It has three separate sensory patches that are lined with 20,000 or more hair-like cells. Each internal patch has an otolith, or ear bone that "floats" on top of these cells. Sound waves cause the otoliths to move, which stimulates the underlying hair cells. Different types of sounds cause the otoliths to move in various ways. The internal swim bladder, a gas-filled sac used for floatation, assists the inner ear with sound interpretation. The swim bladder membrane compresses when exposed to sound waves. These compressions are also transmitted to the inner ear, magnifying the interpretation of the sound. Through use of the inner ear, largemouth bass can generally detect the direction of sound production at a distance of up to 30 feet.

The lateral line system is comprised of a head component and the body canal that extends from the head to the tail. The head system is a series of branched canals concentrated around the eyes, top of head, and beneath the jaw. The head system is underneath the skin and not visible, but the body canal is external and consists of a series of pores that allow water to flow along the length of the canal. Any movements in water cause flow that can be detected via the lateral line system. Therefore, this system serves as "distant touch", as bass can feel an object as far away as three body lengths. The body canal is used for sense of direction, while the head system is used for up-close inspection.

Largemouth bass combine signals from the inner ear and lateral line to form an overall summary of a sound. Once alerted by sound, bass prefer to use their sense of sight to pinpoint prey. However, during low visibility periods (at night or in muddy water) feeding is almost entirely dependent on sound interpretation via the inner ear at a distance and the lateral line to isolate prey and strike. In terms of comparable hearing abilities, bass seem to be intermediate in the fish world, just like with their eyesight. In fact, minnows, common carp, and catfish all have more specialized hearing structures and can interpret sound in much more detail than bass.

The tackle industry has obviously taken advantage of the hearing abilities of bass. Almost all hard baits are designed to make noise underwater and bass are typically attracted to these noises. But is it because these noisy baits mimic natural prey? In most cases, the answer is no. When have you observed a swimming shad or minnow sounding like a Rat-L-Trap? Since they are at the top of the food chain as adults, bass have little to fear and often strike out of curiosity or sheer aggressiveness towards an unfamiliar trespasser. Soft, subtle baits (i.e., weightless flukes and wacky worms) closely mimic sounds of natural prey and are typically the bait to use in clear water situations when bass are less aggressive.

Certain types of sounds - mainly sudden, loud noises - initiate the startle reflex in bass. Things to avoid would be the sudden powering of your trolling motor, cranking your outboard motor on your fishing hole, dropping your tackle box on the boat floor, or making a large splash while casting your lure.

Hopefully, this article sheds some light on the hearing abilities of bass, but much more information can be found in "Knowing Bass: The Scientific Approach to Catching More Fish", a book by Dr. Keith Jones, lead researcher for the Berkley Fish Research Center. If you have questions or concerns about area fisheries, please stop by the Inland Fisheries office at the Jasper State Fish Hatchery or contact us by phone (409-384-9572) or email (todd.driscoll@tpwd.state.tx.us). Good luck and good fishing!