Shark attacks have been grabbing headlines in the United States and Australia over the last few weeks, raising much concern among summer beach goers and posing a challenge to scientists who are trying to figure out what may have caused the sudden spike.
In the US, there has been a string of 10 shark attacks in the past month along the 800km coast of North and South Carolina, facing the Atlantic Ocean - an area which normally has an average of about six attacks a year.
Over in Australia, there have been eight attacks off the coast of the eastern state of New South Wales this year, compared with just three over the whole of last year.
While it seems like sharks have suddenly taken an interest in humans, scientists say that there are a number of possible reasons, including climate and human activity in the water, that may have led to the increase in attacks.
The latest incident in the US occurred last weekend, in Surf City, North Carolina. The attack was not fatal and the 32-year-old victim was treated for bites to the forearm and right hand.
An earlier attack on an eight-year-old in the same area happened in just knee-deep water.
Experts say a number of environmental and ecological factors have brought some sharks closer to shore. Turtles, for example, have been identified as a possible cause.
Mr George Burgess, director of the Florida Programme for Shark Research at the University of Florida, said tiger sharks and bull sharks are fond of eating turtles and this is the season when turtles are nesting on the beaches of North Carolina.
The sudden spike in attacks could be because "this is a particularly good year for nesting", said Mr Burgess, "with two times as many nests on the beach as usual".
The Menhaden - a type of herring - which is "favoured by sharks", is also particularly abundant this year, said Mr Burgess.
"All these things make it good to be a shark," he added.
Then, there are possible climate-related explanations.
Professor Roger Rulifson, who studies fisheries and fish ecology at East Carolina University, says one hypothesis is "the sudden increased water temperatures early in the summer, which could have caused a mass migration of sharks rather than migrations northwards in waves. If true, then this would cause the sharks to spread out, even going into shallow waters, in search of food."
Low rainfall, said Mr Burgess, has also caused the salt content in the waters nearer the shore to increase, thus attracting more fish closer to shore and, in turn, more sharks which feed on those fish.
In Australia, however, the converse seems to be true. High rainfall in New South Wales may be the reason sharks are venturing closer to the beach.
This seeming contradiction boils down to the different sizes of the estuaries in the two areas.
Dr Daniel Bucher, a senior lecturer of Marine Biology and Fisheries at Southern Cross University in Australia, points out that estuaries are smaller in New South Wales than in the Carolinas.
"Where there are big estuaries, sharks like tigers and great whites will use them, but leave when it rains," he said. But in the case of New South Wales, where the estuaries are smaller, these sharks "do not use them but are attracted to the entrances and nearby beaches after it rains", he said.
However, experts do not think there is a need for the public to be too alarmed by recent attacks.
Mr Burgess says the shark population is probably "at an all-time low" due to over-fishing, and the increase in attacks is largely a result of increased human activity in the sea.
Mr John West, coordinator of the Australian Shark Attack File at Taronga Conservation Society Australia, agrees with this assessment: "More people in the water for longer and throughout the year will increase the risk of encountering a shark."
If one does encounter a shark, Prof Rulifson said, the person needs to "whack them in the nose as hard as possible. If that doesn't work, then go for the eyes. The last resort would be the gills".
Mr Burgess also advises people to go into the sea in groups and avoid doing so at dusk and dawn when sharks are most active.
He adds: "We are all eco tourists and have to accept some risks... but I think it's safe to say we are not under siege by sharks."
This article was first published on July 11, 2015.
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