Giant steps help hungry fish to find food

Hungry sharks, swordfish and other ocean predators have adopted different hunting behaviours depending on how much food is readily available. They use at least two particular methods to detect their next meal, according to research published in Nature on 24 June 2010

Professor Graeme Hays from the Department of Pure and Applied Ecology at Swansea University, said: “How animals search their environment for prey is a long standing question that has perplexed biologists. However, this question has recently started to intrigue scientists from other disciplines and has developed into a major cross-cutting theme in the scientific world.

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“Emanating from theoretical physics and mathematics has been the consideration of what movement patterns - for example, random, clumped, sparsely distributed etc - are best to find items distributed in space.”

To gain a better understanding, an international research team - funded through the Natural Environment Research Council’s Oceans 2025 programme - attached electronic tags to 55 individual fish from 14 different species of shark, tuna, billfish and ocean sunfish. They monitored the movements of these predatory ocean wanderers to test whether or not they show predictable behaviours in the face of changes in food availability.

Lead investigator, Professor David Sims of the Marine Biological Association Laboratory in Plymouth explained: "Theory predicts that predators should stay local where food is abundant, but when it becomes diminished, they should adopt a movement pattern known as a Lévy flight to optimise searches for targets at unknown locations."

A Lévy flight is a mathematical 'walk' pattern characterised by giant steps (long, straight movements) interspersed by smaller steps. By using this search pattern the fish are able to increase their chances of finding new food patches, however widely scattered they may be.

The researchers collected detailed movement data by tracking the fish as they encountered different habitats in the open ocean, from productive zones - such as those around the Galapagos Islands - to more food-limited areas.

Together the team analysed over 12 million vertical movement steps spanning 5,700 days of data and 14 different species – by far the largest data-set of its kind ever assembled.

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Professor Sims said, "In much the same way as treasure-hunters use metal detectors, the fish have developed different ways of scanning the oceans in search of their next meal. Where food is plentiful they remain in that area, searching at a leisurely pace as they almost stumble across their 'finds'. When the source becomes sparse they take giant steps that conform to a Lévy pattern to find a new location where they might have more success."

He added, "The results of our study enable predictability of movement patterns of fish under decreasing prey conditions, as might be expected with environmental changes such as fishing depletions or climate change. This knowledge should help us to manage fish stocks more efficiently in the future."

The importance of these findings will reach far beyond marine biology. If distantly related fish have naturally evolved search patterns approximated by a Lévy walk, it seems highly likely that other animals, and possibly even people, can adopt similar behaviour in response to environmental changes.

Archaeological and genetic evidence points to the rapid colonisation of new lands by human hunter-gatherers. The study by Plymouth University, in collaboration with Swansea University and other higher education institutions, could indicate that Lévy-type behaviour might have evolved naturally in these early humans as they adapted to finding better conditions and sources of food.

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"Giant steps could become more common amongst both sea and land animals as conditions change during future climate warming," added Sims.
As part of the ongoing research, Professor Hays’ team is tracking predators and prey species, such as plankton, to examine the interactions between predator and prey. For further information about this and related research at Swansea University, visit: www.swan.ac.uk/bs/turtle/ and www.swansea.ac.uk/biosci/researchandimpact/

Working as part of this team, Professor Graeme Hays deployed electronic tags onto the World’s largest species of bony fish, the ocean sunfish.

Photo credit: Gower Coast Adventures:
www.gowercoastadventures.co.uk/
Photo caption: A jellyfish being equipped with a miniature dive computer as part of ongoing work to look at the foraging ecology of marine species.