"Males develop enlarged anal fins, the fin at the bottom of the body, and our best guess is the reason they do that is so they can dig a trench into the gravel," Purchase told The Broadcast. The scales of the male fish also change during the spawning process.

What is capelin rolling?

The arrival of the capelin heralds the beginning of summer weather, along with a chance to spy humpback whales coming to feed on these slender, speedy fish. ... And they're also a traditional delicacy, fresh from the ocean for a short season.

Do capelin roll on high or low tide?

Spawning occurs mainly when the water temperature is between 6° and 10oC and is most commonly observed at night. In the estuary and Gulf of St. Lawrence, the capelin “roll” on the shores between mid-April and July. They rely on the tides to carry them to the beach.