Noisy Fish

Have you ever looked into your aquarium and heard an unusual noise? Or wondered if your fish are able to communicate with each other? 

It is widely known that many fish are able to emit some sort of noise and worldwide there is currently an estimated 800 species of fish (this number is certainly underestimated due to the lack of specific research) thought to be able to produce intentional biological sounds. Intentional sounds that fish produce are usually associated with specific behaviours such as aggression, defence, territorial advertisement, courtship, and mating.

'Soniferous' fishes (those that emit a sound) don’t have voices as most animals do but are able to produce noises by the three main methods: drumming, stridulation, and hydrodynamics. These noises vary and are often referred to as either grunts, knocks, whistles, booms, growls, thumps, and even 'musical tones'. 

Drumming is done by vibrating special muscles, known as sonic muscles, on or near their swim bladder. The swim bladder (also known as gas bladder or air bladder) is an internal gas-filled organ that primarily contributes to the ability of a fish to control its buoyancy, and stability in water, but it also has other functions, as a resonating chamber to produce or receive sound. The bladder can be vibrated by rapid contraction and relaxation of these sonic muscles and this rapid change in the swim bladder volume produces a sound, just as the vibration of the head of a drum produces sound. In fact 'Drum' fish (also known as 'croakers' or 'hardheads') of the family Sciaenidae, are able to produce a noise so loud that it can be heard from 60 feet beneath the surface and increase the rate of their sound production during spawning aggregations in order to attract females. The majority of sounds produced in this way are short pulses with fundamental frequencies ranging from about 45 - 60 Hz (e.g. Goliath Grouper) to about 250 - 300 Hz (e.g. Toadfish sp.). Higher frequency harmonics produced by drumming are sometimes present above 1000 Hz (e.g. Silver Perch). Indeed the aforementioned Toadfish produce 'foghorn' sounds as a courtship signal that are so loud, they severely disturb people who reside near local spawning aggregations. And Damselfish can change the sounds they produce in response to changes in the sounds of fish in neighbouring territories, suggesting that sound production is involved in territorial defence in this species.

Stridulation is the rubbing together of body parts, usually bones. For example, Grunts (marine fishes of the family Haemulidae) grind their pharyngeal jaws (specialised gill structures) in their throat. In contrast, many catfishes have specialised pectoral fin spines that can be moved against adjacent bones to make a 'squeaking' sound (in a similar way to grasshoppers). This is especially common with catfish of the family Doradidae. It is thought that stridulation may be used intentionally to produce sound as a fright response or territorial display, and the resulting noise may be modified or amplified by the swim bladder. The component frequencies of stridulatory sounds range from 100 to 8000 Hz, while predominant frequencies are generally between 1000 and 4000 Hz and those influenced by the swim bladder have frequencies well below 1000 Hz. Other examples of fish species that produce sound by stridulation include marine catfishes and sea horses.

Hydrodynamic sound production occurs when a fish quickly changes direction and/or velocity. These sounds are extremely low in frequency and are simply a by-product of swimming. It is possible that hydrodynamic sounds may be important to 'predator-prey' interactions and communication. For example, it has been discovered that sharks can detect the low-frequency hydrodynamic sounds emitted by smaller fishes. Therefore, schooling fishes may inadvertently attract a shark simply by the sounds produced during swimming, rather than protecting them.