Unlike most peoples’ perception, fish are quite vocal and produce a variety of sounds. Scientists have found over 80 species of fish that produce sounds under different circumstances. They produce sounds when they are excited or startled (escape sounds), during reproduction or when aggressive (territorial defence). Most sounds are low in frequency (25-250 Hz) and low in intensity (ca. 90 dB). Fish can produce sounds by contracting their muscles around the swim bladder, which gives a drum like or grunting sound (described as grunts, groans, thumps and knocks). Other sounds can be produced by releasing air bubbles, grinding teeth, stridulation (rubbing) of fins and other body parts, or by contracting heavy body muscles. Cod is known to produce grunting sounds during courtship and aggressive behaviour. During my Master studies at the University of Tromsø in Norway, I have recorded intense clicking sounds from cod, while it was approached by a seal or a human diver. The purpose and sound production mechanism of these clicks is at presence unknown, but we suggested it may have anti predator functions.
The purpose of the ocean sounds project is to further investigate grunting and clicking cod around Lofoten Islands.
Sound production in fish commonly occurs during reproductive periods, during both courtship (e.g., Brawn, 1961; Myrberg, 1981; Lobel, 1992; Nordeide & Kjellsby, 1999) and agonistic behaviour (Myrberg et al., 1993), especially in connection to male territorial behaviour (Santiago & Castro, 1997). These sounds are usually pulsed, with most of their energy below 3 kHz and have been described as grunts, moans or clicks (Schneider, 1967; Hawkins & Rasmussen, 1978; Myrberg, 1981). They can be very intense (> 130 dB re 1Pa), which may be important in announcing the size or the physical strength of an animal, which in turn may attract females and or scare away competitors (Ladich et al., 1992; Myrberg et al, 1993; Crawford et al., 1997). Sound production during non-spawning periods commonly occurs during intra- and interspecific aggression or when fish are disturbed or frightened (Brawn, 1961; Myrberg, 1981, Ladich, 1997). If sounds are used during agonistic encounters they are usually accompanied by visual agonistic displays (Ladich, 1990; Hawkins, 1993). Those sounds vary from low frequency grunts and drumming sounds (40- 1700 Hz) to higher frequency creaking sounds, clicks and stridulation sounds (1-6 kHz) in various species (Ladich 1997).
Cod, has well-developed drumming muscles and is known to produce grunts under a variety of circumstances (Brawn, 1961; Hawkins & Rasmussen, 1978; Soldal & Totland, 2002). Grunts had their main energy below 1 kHz, with peak frequencies varying between 50 and 500 Hz, were 60 to 200 ms long with source levels between 120 to 133 dB re 1 µPa (Brawn, 1961; Hawkins & Rasmussen, 1978; Midling et al., 2002; Nordeide & Kjellsby, 1999). Outside the spawning season, both females and males produced grunts in aggressive context or when frightened, whereas during the spawning season only male cod produced grunts in connection to aggressive behaviour, to scare away other less vigorous males or immature females (Brawn, 1961). Free ranging cod might also produce a series of knocks, with frequencies below 0.6 kHz in addition to grunts (Midling et al., 2002).
I have carried out bio-acoustic studies at the Department of Arctic Biology at the University of Tromsø from 1999-2001. This was done in connection with my master studies, where I was investigating the potential use of echolocation by harp and hooded seals. In that context, I have recorded clicks in the presence of both seals and cod that in subsequent follow up investigations with a human diver were found to originate from the cod (Vester et al., 2004).
When cod is charged by a large predator, for example a seal, it tries to get away with a sudden swim movement and produces loud click sounds (please listen to the sound). These clicks are higher, 4-8 kHz and louder 150 dB, than grunts, and we can easily hear them. The purpose and function of these clicks is still unknown. But it has been discussed that cod might use these clicks as an acoustic defence against predators. The clicks may be produced while cod contracts all its muscles to get away from the predator. During my studies, Harp seals hesitated or did not eat cod at all after it produced clicks and one possible explanation may be that the clicks startled the seals.
See article: Vester, H.I., Folkow, L.P. and A.S. Blix, February 2004. “Click sounds produced by cod (Gadus morhua)” J.Acoust.Soc.Am. Vol. 115, No. 2, pp. 914-919.
Reproduction behaviour of cod
”Skrei” grows up in the Barents Sea and starts its southward migration for spawning when > 7-15 yrs old. The migration happens in 200-400 m depth and they can swim at least 20 km/day. They arrive at their spawning ground (Lofoten/Vestfjord and the coats of Møre) at the end of January- February, and spawning follows in March-April. The older females arrive first at the spawning grounds and males follow. At the end there are 50 % females and 50 % males. The males produce grunting sounds (please listen to the sound) and it has been proposed that females can hear the strength and fitness of a male by their sounds (e.g. a larger cod will produce louder and deeper sounds). If a female accepts a male, she puts her snout on his head and they swim together in great speed belly to belly, while they release eggs and milk, which they self mix with their tail fins. Eggs will float to the open Sea and drift towards northwards. They hatch after 2-3 weeks and migrate passively with the stream north over to the Barents Sea, which they reach in fall as young fish of 8-9 cm length. They spend 3-4 years in the Barents Sea before they start migrating for food (for references see Per Pethon, 1998 and www.fishbase.org).
Grunting sounds are also produced during aggressive behaviour between males, but females are able to produce grunts, too.
Cod (Gadus morhua)
Cod-family: 80 species, 24 in Norway
Distribution: North Eastern Atlantic cod lives mostly in the Barentsea and Svalbard. It migrates to the Norwegian coast for feeding and spawning. Important spawning places are outside Vesterålen and Lofoten and in the Vestfjord and, along the coast of Møre and Romsdal.
Colour: Light brown or greyish, with dark spots. White lateral line, overbite and prominent barbel.
Size: max. 180 cm
Weight: max. 55.6 kg
Age: max. 40 years
Habitat: The Norwegian Arctic migrating cod lives mostly pelagic (freely in the water) and conducts migration in 200-400 m depth.
Biology: Sexual mature cod (> 7yrs), which is called “skrei”, migrates in January at least 20km/day to their spawning places along the Norwegian coast. Spawning happens in January-April in 50-200m depth. A big female can spawn up to 5 million eggs. Males produce grunting sounds and when a female accepts him, she will show it by putting her snout gently on his head. Then spawning starts: they swim belly to belly fast through the water while releasing egg and sperm, which they blend by moving their tail fins. The fertilised eggs float to the surface and move northwards with the water stream. They hatch after 2-3 weeks and young fish arrive in the Barentsea in autumn, where they search the bottom area and grow up. When 4 yrs old, sexual immature cod starts to migrate in winter for food from the Barentsea to the coast of Finnmark to feed on capelin and is then called “loddetorsk”.
Sexual maturity: > 7-15 years
Prey: Fish predator, mostly herring, capelin, gobbies, crustaceans and other small animals.
Fishing: Important for fishing since the Viking age. Cod is preserved as dried fish and salted dried fish. Norway fished 36.045 tons of skrei in 2005.
Distribution: In the North-Atlantic from Biscay to the Barents Sea, local fish, lives close to the coast. Also in Iceland, Greenland and East coast of North America.
Colour: The colours vary according to its environment. Algae cod can become orange with dark spots. But it is mostly light brown with dark spots. The sideline organ is white.
Size: max. 80 cm
Weight: max. 20 kg
Age: min. 7 years
Habitat: From coastal areas to 600m deep. Mostly in shallow waters close to the bottom. Stationary fish that remains closely to the coast.
Biology: Spawns in January-April in 50-200m depth, eggs sink to the bottom and stay stationary.
Sexual maturity: 2-3 years
Prey: Fish predator, mostly herring, capelin, gobies, crustaceans and other small animals.
Fishing: Norway fished 184.916 tons of coastal cod north of 62° in 2005.
Vulnerable on the Red List (IUCN): At least 20% of the population declined over the last 10 years due to exploitation.
Websites: www.fishbase.org / www.imr.no
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