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Mild winter puts squeeze on Saimaa ringed seal

If seals can’t find suitable nesting sites they will give birth on the open ice, reducing their pups’ chances of survival.

An endangered Saimaa ringed seal pup in mid-April 2019. Image: Ismo Marttinen

The ongoing mild and relatively snowless winter in southern Finland is posing a threat to Saimaa ringed seal populations. During the birthing season, the endangered animal needs plenty of snow and ice, both of which are in short supply this year.

"From the seals’ perspective the situation is dreadful. We’re in January and we are experiencing an autumn storm," said Saimaa ringed seal researcher Tero Sipilä.

Sipilä said that during his 40 years working for state-owned parks and wildlife management agency Metsähallitus, he has never seen similar conditions at the beginning of January.

Seals generally give birth around 26 February, so there is still time for the weather to change enough to allow them to make suitable dens for newborn pups.

The seals favour secluded waters sheltered by islets, but if they cannot find ice in such locations, pregnant animals will seek out ice-bound bays.

If they cannot find suitable cover, the seals will be forced to give birth on open ice, exposing their newborns to many perils.

Pups likely to die without nests

During winters with little ice in the Archipelago Sea and marine areas near Estonia, Baltic Sea pups have been detected in islets.

"Their chances of survival have not been precisely mapped, but they seem low," Sipilä noted.

The winter of 2006 – 2007 was mild in Lake Saimaa, making it impossible for the Saimaa seals to build nesting sites. This means that a majority of cubs were born on open ice. As a result mortality among newborns increased by a magnitude of four, with one-in-three young cubs perishing.

It was during that winter that volunteers began assisting the endangered species by building nesting sites. The practice has helped halve number of nesting deaths among seals.

It seems likely that volunteers will once more have to come to the seals’ rescue this year. A decision on building artificial nests will be made in two weeks, leaving conservationists only a couple of weekends to create the much-needed sanctuaries for the females to find.

"If the weather doesn’t get colder, we will have to choose the sites based on where there is ice and snow to plough," Sipilä explained.

Man-made nests to the rescue

The Saimaa ringed seal population currently numbers about 400 individuals, with just a few dozen cubes born every year.

"In good natural conditions, nest mortality is less than 10 percent; with the artificial nests it is 14 percent and on the open ice more than 30 percent," the researcher added.

Last year volunteers counted 88 new cubs and the spring census revealed that just one of them died. Sipilä said that without the man-made nests the number of deaths would have been 12 – and up to 30 on open ice.

"The combined effect of a bad winter and deaths due to fishing activity and interference with nests is so detrimental that the growth of the seal population would have stopped without the artificial nests," Sipilä observed.

He added that while the seal population can withstand mild winters, the same is not true of deaths caused by fishing and other disruptions.

"We measure the proportion of pups that reach sexual maturity and it would be one in four. That would be enough to ensure the population remains viable."