Males wait until daytime boat traffic dies off to make mating 'roars'
By Larry Pynn, Vancouver Sun, Dec 9, 2015
Marine scientists call it a roar, though it sounds more like someone breathing heavily on the other end of the phone. It’s the underwater mating call of a male harbor seal and it could be at risk of being drowned out by the din of motor vessels. The first study of its kind has found male seals in the Straight of Georgia are choosing evening times to vocalise when underwater noise from vessel engines, especially pleasure craft and commercial fishing boats, is lowest.
Katrina Nikolich, a master’s student in biology at Western Washington University in Bellingham, described the Straight of Georgia as an “incredibly noisy” region due to vessel traffic and it’s logical for male seals to hold off on their calls until vessel noise is “significantly” lower. “You can be in a crowded bar ... but if the music is blaring really loud they still won’t hear you.”
Nikolich said in an interview that male seals are thought to make their roars to deter other males and potentially attract females and establish territories close to haulouts. No one has witnessed harbor seals mating because it is done in the water, she noted. An estimated 105,000 harbor seals live along the B.C. coast, about 40,000 of them in the Straight of Georgia. There are so many of them and we think we know so much about them, but this is something we know very little about,” Nikolich said.
She conducted her study at Heron Rocks off southern Hornby Island - an area with a high density of seals - using an underwater hydrophone to pick up sounds during the breeding season from June through September. Seals are born on rocks from May to August along the B.C. coast. Nikolich teased out 1,800 recorded calls over 20 random days, compared with decibel levels from shipping noise, and found a “striking” correlation. The number of roars increased to 49.4 per hour at night, from 15.4 during the day, while noise levels dropped to 86.3 decibels from 98.5 during the same periods.
Studies elsewhere have found male seals make their calls more frequently when tides are high and more seals are forced off their haulouts and into the water, but that’s not what Nikolich found in the Straight of Georgia. “The number of calls per hour, regardless of the time of day, increased as the noise level decreased,” she said. Nikolich said more studies are required to definitely draw a connection, including studies into preferred feeding times of males and females and a look at other regions to see if male seals behave similarly. Males elsewhere have been observed to feed less often during the breeding season.
Nikolich noted male harbor seals are also noted for their displays, such as slapping their flippers on the water surface, as a way of posturing to rivals - sometimes at distances of a few hundred meters. The males don’t maintain harems but rather just “jump out in front of her and catch them on their way, wherever they are going. It’s a really funny image.”