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Overfishing threatens shortfin mako shark populations

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The shortfin mako shark is facing a dangerous amount of overfishing, the data from a new study suggests.

Conducted by researchers from the University of Rhode Island and Nova Southeastern University’s Guy Harvey Research Institute (GHRI), and published in Journal Proceedings of the Royal Society B, the results have experts fearing for mako shark populations.

Many shark species have suffered a decline, primarily as a consequence of fishing activities. While sharks can be deliberately targeted in commercial fishing, a large proportion of them are caught accidentally. An accurate value of the number of sharks killed by fishing is vital for conservation and preservation efforts – however, these numbers are typically only obtained through the self-reported values given by the fisheries themselves, and may therefore be unreliable.

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“Traditionally, the data obtained to determine the rate of fishing mortality, a key parameter used to help gauge the health of shark stocks, has depended largely on fishermen self-reporting any mako sharks they may have caught,” explained Dr Mahmood Shivji, director of the GHRI and senior author of the study.

“The challenge is that not all fishermen report the same way, or some may underreport or even not report the mako shark captures at all, so these catch data are known to be of questionable reliability.”

The shortfin mako shark (Isurus oxyrinchus) are considered at particular risk to exploitation owing to their late age of maturity and low reproductive rate, meaning that their population recovery rate is extremely slow.

To get a better idea of the true impact of fishing on the mako shark population, the researchers employed satellite linked radio tags (SLRTs), using a technique called satellite telemetry. These tags were fixed onto the dorsal fin on sharks off the Yutican Peninsula and Northeast US coast. Each time the sharks surfaced, the tags were able to communicate with satellites and triangulate their position.

At the end of the research period, it was found that 30% of the sharks had been captured in fisheries, with fishing-related mortality rate 10 times higher than previously estimated. Longline fisheries were the cause of the majority of these incidents.

“Using satellite tags for makos and possibly other fished species can be a time-efficient way and a fisheries-independent tool for gathering useful fisheries-interaction data, including answering fundamental questions about the levels of fishing survival and mortality,” said Dr Michael Byrne, the study’s lead author.

“The tracking data also showed these mako sharks entered the management zones of 19 countries, underscoring how critical it is for countries to work together closely to manage and conserve these long distance oceanic travellers.”

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An important point to make is that, if shark populations dwindle, the consequences could be severe – and not just for the sharks. As upper trophic level predators, they have a significant influence on the populations of their prey. With less sharks, these communities could themselves face a significant threat to their stability.

Dr Bradley Wetherbee from the University of Rhode Island hopes that these results may encourage the use of more reliable data gathering techniques to manage populations. “It’s vital that we habe the most accurate data possible to aid decision-makers in managing marine life populations sustainably,” he commented.

Luc Bourne

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