Rorqual bleu auKrill Buffet
Six blue whales in one day, in an area of less than 50 square kilometers, that’s the kind of observation that has become less common in the St. Lawrence Estuary in the last ten years. It is a good indication that the krill buffet must be rewarding, since these whales are feeding on krill swarms at the same spot for many hours.
On a beautiful late September day, flat calm water in the estuary, a few nautical miles off the coast in the center of our research area, four blue whales are feeding at or just below the surface. The sound of their powerful blows carries a long way over the water. There are two single animals and one pair, lunging through the krill concentration every few minutes.
One of the single animals is a female named « Jawbreaker », known to the researchers of the St. Lawrence since 1991 and a regular visitor in the St. Lawrence Estuary. For a certain period of time, she joins the pair and they are feeding in a trio, but then she quits the area and heads offshore towards deeper water. There’s another pair of blue whales out there, feeding close to the St. Lawrence Seaway. In this pair, there’s another female first sighted in 1991, named « Chameau » (camel in French), known to be suffering from a lordosis-type deformation of the spine. She had a calf in 2002 that was seen again in 2011, the first blue whale calf ever that was resighted in the St. Lawrence after the first year.
Blue whale calves are born after a gestation period of 10-11 months, they are 6-7 meters in length at birth and weigh about 2.5 metric tons. A calf gets 380-570 liters of milk per day, with a fat content of 35-50 percent, until it is weaned after 6-9 months, by which time it has doubled in length. During that period, the calf grows by approximately 90 kg per day, or 3.75 kg per hour! Both genders reach sexual maturity at the age of 5-15 years. It is estimated that blue whales can live to at least 80-90 years of age. On the Northern Hemisphere, adult males average 24 m in length and females 25 m, while their conspecifics on the Southern Hemisphere average 25 m and 26.5 m, respectively.
The blue whales in the Gulf of St. Lawrence are part of the Northwest Atlantic population, designated « endangered » by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) since 2002. This population is estimated to count less than 250 mature individuals and there are strong indications of a low calving rate and low rate of recruitment. Today, the biggest threats for this species come from ship strikes, entanglement in fishing gear, disturbance from whale watching activity, noise pollution, chemical contamination and habitat degradation. A study published in 2005 revealed that the krill biomass in the St. Lawrence had decreased by 67% compared to 1995. Blue whales are also very vulnerable to long-term changes in ocean climate, which can affect the abundance of zooplankton.
The oceanographic features of the Gulf of St. Lawrence make it an exceptional feeding ground for blue whales of the Northwest Atlantic, because the bathymetry and tidal currents create ecological hot spots, where prey accumulations are predictable. Several dozen blue whales feed in the gulf and the estuary every year, sometimes as early as in March or April. Even though blue whales can deal with significant ice cover, they do get trapped in sea ice at times. In March 2014, nine blue whales died trapped in thick ice that had accumulated west of southern Newfoundland. That’s a tremendous loss for an endangered population.
It is a great privilege to be able to observe these animals in the St. Lawrence Estuary, but the survival of the Northwest Atlantic blue whales is very unsure if the population trends don’t change. On that September day, we have got some important insights into the feeding behaviour of these blue whales and ID photos from five of the six animals in our research area.
Dany Zbinden, Gessica Gambaro