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Fish Behaviour Could Affect Stock Measurement Accuracy

29 May 2015

EU - European research performed off the coast of Mallorca has thrown doubt on the accuracy of stock level measurement techniques, as fish were less likely to take the bait when there were more fishermen present.

Almost 70 per cent of marine fish stock are fully or over-exploited according to the UN’s Food Agriculture Organisation (FAO). Or could it be just a trick played by clever fish?

Josep Alós and Robert Arlinghaus have been investigating fish populations in the region since January 2014, using innovative mathematical models and monitoring methods.

They found that the more fishers there are at a specific location, the less fish are inclined to take the bait.

To get to this result, the researchers studied the behavioural responses of two types of fish in 54 different locations with the same habitat characteristics, but different angling pressures.

The two fish studied were the carnivorous painted comber (Serranus scriba), and the annular seabream (Diplodus annularis), an algae eater.

As fishers tried to catch their prey, an autonomous underwater video recording was used to measure the behaviour of fish.

The painted comber, which cannot afford to think twice before attacking its mobile prey, was expected to be generally more aggressive towards baits than the seabream.

However, the scientists found otherwise. Although they were quite aggressive in environments with low fishing pressure, the fish actually tended to become increasingly shy as the number of baits increase.

According to the team, this changing behaviour could be explained both by genetic change towards increased shyness and learning from experience, resulting in increased capture avoidance.

"These results suggest that recreational angling may contribute to patterns of hyper depletion in catch rates without a corresponding change in the fish population where catch rates decline stronger than the abundance of fish," said Mr Alós, researcher at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries and co-author of the study.

But does it mean that reported fish stock depletion across the world could have been influenced by clever fish behaviour?

"Reports on the dramatic decline of fish populations in the ocean which were only based on fishery-dependent data, for example data from the long-line fishery of tuna, cod or swordfish, could also have their cause in enhanced gear-avoidance behaviour of those fishes.

"We have to rethink our monitoring of fish stocks and take the behavioural changes into account.

"Maybe some areas with high fishing intensity host more fish than we believe," concluded Mr Arlinghaus, study leader and researcher at the Humboldt-Universität zu Berlin.

The study was funded under the FISH&FISHERS project, which aims to improve fishing mortality estimations while studying the spatial interactions between fish and fishers.

Ultimately, the team hopes that their findings will contribute to better protecting marine ecosystems, preserving biodiversity and making the development of fisheries more sustainable.

Further Reading

Find out more about the project by clicking here.

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