The Pacific republic will ban trade in shark products and commercial shark fishing throughout its waters.
Tourism, including diving, is a staple of the Marshall Islands archipelago, which is home to just 68,000 people. Sharks and their near relatives such as rays are seriously threatened by issues such as habitat loss and fishing.
About a third of ocean-going sharks are on the internationally-recognised Red List of Threatened Species.
“In passing this [shark protection] bill, there is no greater statement we can make about the importance of sharks to our culture, environment and economy,” said Senator Tony deBrum, who co-sponsored the bill through the Marshallese parliament.
“Ours may be a small island nation, but our waters are now the biggest place sharks are protected.”
To put the sanctuary in context, it covers roughly the same area as Indonesia, Mexico or Saudi Arabia, and is about eight times bigger than the UK.
The move will extend the area of ocean in which sharks are protected from about 2.7 million sq km to 4.6 million sq km (1.0 to 1.8 million sq miles).
Under the bill, commercial shark fishing and any trade in shark products will be banned, and any of the fish accidentally caught must be released alive.
Certain designs of fishing gear will be banned from Marshallese waters; and violators of all these measures face fines of up to £200,000.
Last month, a group of eight countries including Mexico, Honduras, the Maldives and Northern Mariana Islands signed a declaration announcing they would push for more shark protection across the world.
Because they grow and reproduce relatively slowly, sharks are especially vulnerable to factors such as accidental or targeted fishing.
Shark protection measures are also likely to help marine biodiversity overall, as they restrict the rights of fishing vessels and require greater scrutiny of landings.
However, with the Marshall Islands as with Palau and some other countries, there are questions over the capacity of authorities to monitor fully such huge expanses of ocean.
For the full article: BBC News