Artefacts offer Pacific shark species absence clues

Weapon constructed from shark teeth (Image: PLoS One) Historical artefacts, such as this shark teeth weapon, are an "under-utilised source of data"

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Nineteenth Century tools made from sharks' teeth suggest that two species of shark used to populate the Central Pacific but are no longer present.

Using artefacts from museums, a team of US researchers found that spot-tail and dusky sharks used to inhabit the reefs surrounding the Gilbert Islands.

The unusual historical data would help evaluate the success of ecological conservation measures, they added.

The findings have been published in the scientific journal PLoS One.

In their paper, the team from the Field Museum of Natural History, Chicago, and Columbia University, New York, said indigenous artefacts often represented an "under-utilised source of data".

"By examining the materials... we can gain access to the flora and fauna present during the time of their construction," they wrote.

"When these materials are assigned to a particular species, they can indicate which species were present in the past."

They said historical artefacts could be used to provide important insights in the absence of historical ecological data and provide an important first step in the assessment of the effectiveness of current conservation methods.

They observed that as shark teeth were "diagnostic to species", the artefacts allowed the team to identify some of the species that where present in the waters around the islands when the weapons were manufactured, between 1840 and 1898.

Ecologically important

"When combined with historical records, these identifications allow us to reconstruct the shark community," they explained.

"In doing so, we are able to identify how the baseline of an apex predatory community has shifted over time."

Spot-tail sharks (Carcharhinus sorrah), listed in the IUCN Red List as Near Threatened, are now primarily found in coastal regions around the Indian Ocean and the south-eastern Asian shores of the Pacific Ocean.

Meanwhile, dusky sharks (Carcharhinus obscurus) are listed as Vulnerable because they are slow-growing and reach ages in the region of 40 years (compared with eight years for spot-tail sharks).

Sharks are considered to be vital organisms within Central Pacific ecosystems, influencing the food chains within reef habitats and the composition of species found in and around reef community.

Map showing location of Gilbert Islands

The researchers highlighted that these two species, like other species around the globe, were facing severe conservation threats, such as direct pressure from fisheries.

"A major source of mortality comes from [the demand] for shark fins, often for sale in Asian markets for shark fin soup," they said.

The researchers added that, globally, shark populations were declining and had "dropped by as much as 99% in area where there was active fishing pressure".

In the case of the Gilbert Islands (now know as Kiribati), the scientists suggested that the loss of the two shark species from the reefs' ecosystem may have had an impact.

They observed: "When baseline perception are shifted to a more degraded state, they hamper conservation actions by providing goals that may be less ambitious or less capable of true conservation and restoration.

"In order for conservation measures to recapture the vivid splendour of past coral reefs, it is critical to describe what a healthy reef community looked like in the past."

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