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Noise pollution and its effect on schooling fish

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Snapper 1

A school of bigeye snappers (Lutjanus lutjanus) create a unique formation at Chumphon Pinnacle, Koh Tao with the help of a delicate sensory system.

The Lutjanidae, more commonly, ‘snapper’ family are often found living in marine ecosystems throughout the tropics and subtropics. Across the family, the majority of species occupy an important ecological niche as mesopredators (mid food web). The predatory diet of most snappers comprises small fish and invertebrates. Throughout the variety of habitats they inhabit, many species form large schools – a common attraction for divers and photographers, who often share close encounters with, or become entirely surrounded by the school.

Snappers are an important member of coral reefs, promoting diversity within the trophic level they feed on. They are also an important food source for several large predatory fish. The numerous interactions they share with other species, both as predators and prey, highlight their importance within the ecosystems they belong to. 

In recent years, there has been extensive research into the collagen produced by species of snappers, including that of bigeye snappers. Collagen is widely used for medical and biomedical purposes. Certain properties of the collagen found in the skin and bones of snappers differ to those commonly found in mammalian collagen. This has caught the attention of the scientific community, and may have useful application across a variety of industries, further highlighting their importance.

 

Snapper 2

An orange spotted trevally (Caragoides bajad) darts in and out of a school of snappers. A long exposure photo catches the movement of predator and prey.

 

Members of the Lutjanidae family are heavily targeted across the world. Throughout their distribution, snappers are highly popular fish to appear in restaurants, local markets, and supermarkets. Additionally, their use in other areas of the food industry further increase their demand. Several species of snapper, including bigeye snappers, are used in the production of surimi. They are selected for their significant ‘gel-forming abilities’ – a biomolecular characteristic that is of particular importance in achieving the texture of surimi (similar to ‘crab sticks’). Thailand is one of the biggest producers of surimi, favouring bigeye snappers for its production. They have therefore become a heavily targeted species.

As mid-trophic members, snappers risk predation from several higher-trophic fish species (e.g. trevallies, barracudas, mackerels etc.). Schooling behaviours often reduce one’s likelihood of being the subject of a predatory pursuit, and snappers are commonly found swimming in tight formations during the day. Like other schooling fish, snappers benefit from using a lateral line system – a sensory system running ‘head-to-tail’ down either side of their body – highly sensitive to changes in pressure and water movement. By detecting vibrations created by their neighbours’ movements, these hydrodynamic cues are used by all members of the school in order to maintain a cohesive, tight shape and formation, as well as coordinate same-directional swimming. The lateral line also helps detect movement of nearby predators, and is of particular use when direct vision is compromised.

 

Above: A diagram of a snapper, showing its lateral line system, used to detect nearby signals and cues.

Below: A tight ball formation of bigeye snappers (L lutjanus) is maintained as school members use their lateral line to detect hydrodynamic cues from their neighbours’ movements.

Snapper 3

 

Increased anthropogenic activity in the same area where schools are formed has often brought with it significant noise pollution, often caused by boat engines and diving compressors. Construction of marine renewables such as wind farms and turbines also produces a considerable amount of noise, which has previously demonstrated its devastating and long lasting effects on marine life within a given radius. In both applications of the lateral line as described above, misidentifying, or entirely missing a cue could be a matter of life or death. Numerous studies have shown that noise pollution significantly affects schooling patterns, swimming speeds and depths, predator detection, turning frequencies, turning synchrony etc. – all of which are important behaviours in avoiding predation. 

As both the frequency and intensity of noise pollution continues to increase, this may significantly disrupt key predator-prey interactions of snappers and other schooling species. In addition to members of the snapper family, a number of other species also form large schools. Studies have revealed that schools formed by lower trophic, pelagic grazers may offer several advantages in locating favourable, and more profitable areas for feeding. Schooling behaviours negatively affected by noise pollution may therefore result in large populations feeding in suboptimal areas, further demonstrating the ecosystem-wide effects of noise pollution on a variety of schooling species.

 

Chumphon Bait Ball copy 2 

A large school of pelagic grazers - smooth-tailed trevallies (Selaroides leptolepsis) form offshore. The large school may offer advantages to all of its members when individuals are searching for favourable areas to graze.

 

Both long-term population monitoring, and behavioural research of schooling species are areas of particular importance for the successful conservation of these species. Population monitoring of schooling species often provides a useful insight into the community structure of a particular marine ecosystem. Ongoing monitoring of schools can also act as an early indicator of large-scale, rapid change to a particular area, often simultaneously highlighting local threats to that environment (e.g. the sudden mass reduction in size of large schools could indicate extensive fishing had recently occurred in the area). Gathering long term data on fish behaviours, alongside changes to the existing community, may also provide a valuable insight into the community effects of noise pollution, particularly in ‘noisy’ areas such as dive sites where compressors are run directly over the site.

Findings obtained from long term research and monitoring could be of significant importance for future marine park management, particularly in areas of high boat traffic or use of onboard dive compressors. Controlling noise pollution around dive sites may be an important management step in sustaining an environment where divers and large schools are able to continue sharing close encounters. 

 

 

-- Click here to see more of Piers' underwater photos of marine organisms. -- 

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