Image copyright Boston Globe via Getty Images Image caption A ceremony is held by the carcass of Wolverine, a North Atlantic right whale, prior to its necropsy.

Canadian officials are looking into what killed six highly endangered North Atlantic right whales in June.

Necropsies are being performed on four of the six dead whales recently found in the Gulf of St Lawrence.

Measures have been put in place to prevent more deaths by reducing the potential for ship strikes and fishing gear entanglement.

They include speed limits for larger vessels in designated areas and shipping lanes.

Some areas in Atlantic Canada and Quebec where the whales have been spotted have also been closed to snow crab and lobster fisheries.

The investigation was announced the day before a sixth dead whale was spotted of the coast of the Gaspé Peninsula on Thursday. Officials are currently assessing recovery and necropsy options.

Canadian officials said the dead whales have been found in various locations, and there is currently no apparent pattern as to the cause.

So far, preliminary results show the cause of death of one whale - a 40-year-old female named Punctuation - as due to sharp trauma consistent with being struck by a vessel.

The results of a necropsy on a nine-year-old male, Wolverine, performed earlier this month have so far been inconclusive.

A necropsy will be performed on a 34-year-old male, Comet, in the coming days, and there are plans for one on another unidentified carcass.

North Atlantic right whales were hunted virtually to extinction by the early 1890s. They have been listed as endangered since 1970 and remain one of the world's most endangered large whale species.

The current population estimate for the North Atlantic right whale is just over 400.

Image copyright Boston Globe/Getty Images Image caption An aerial view of a right whale spouting from its blow hole while feeding in US waters

The latest deaths are being met with concern by conservation officials in both the US and Canada, who fear a repeat of 2017, when 12 deaths were reported in Canadian waters and five in US waters.

The cause of those deaths - when they could be determined - was either blunt force trauma from a suspected vessel strike or acute entanglement from fishing gear.

There has been some good news for the species - seven calves have been spotted by researchers this year. Scientists reportedly did not spot any right whale newborns in 2018.

The whales have been increasingly present in the Gulf of St Lawrence in recent years, likely due to a shift in the source of plankton for food.

Efforts to protect the whales have been complicated in part by the fact the whales are edging to the north and the east of where they were spotted in 2017.