That can be seen in the DNA of todays bowheads. The bowheads habitat is the edge of the ice pack. If the passage had been open then there is a fairly good chance that would of been some amorous play moves going on up there between the stocks.

Before man had a significant impact, most bowheads died because they became intrapped in the ice in the autumn. Their corpses would often become incorporated in the beaches, and these former beaches are now well above present sea level. If no whale bones from lets say 3-4k years ago are found in an area that has a lot of bones for the periods on either  side of that then it begins to look like that area was choked full of ice between 3-4k.

That evidence can be collaborated with evidence of more whale remains  being found down-ice of the area above. Also wood that has been incorporated in the beaches in some areas are only believed to be able to get there during icy periods.

Anyhow, yah there is pretty solid reconstruction that has been done using this evidence to say that last year was the most ice free year in several thousand years. So this is all excellent news for the Baffin Bay stock (much fewer members of the two stocks) and they are in for some lumbering frolicking times.