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Avoiding the Hunter

Schooling fishs’ response to any unusual situation which might put them in danger, is to create a dense ball of moving bodies to escape the predator who is going after one prey item at a time. This strategy, however, is quite the wrong response when dealing with a hunting rorqual whale whose intention is to bunch the fish for a bigger mouthful. Similar to the bubble net feeding of humpback whales for instance, the St. Lawrence minke whales have developed their own tricks to coral schooling capelin fish.

Hunting in the Saguenay Fjord

In 2000 certain minke whales started to hunt in the waters of the Saguenay and, over  time, developed novel behaviours never seen before in the St. Lawrence. In the Fjord environmental conditions and prey distribution are strikingly different to the adjacent Estuary as tidal forces are light, the water mass is well mixed and visibility is almost zero.

During a feeding cycle these minkes apply a combination of rather slow respiration and entrapment surfacings in various numbers (mean 2.67), directions and intervals prior to a feeding strike. These behaviours seem to have acoustic and/or visual properties intended to startle prey causing them to aggregate in a central place, or to temporarily confuse prey while the whale continues corralling. About 99%  of  the fast and powerful feeding strikes at the end of the corralling are either performed as lateral or ventral arcs.

Entrapment manoeuvres

Entrapment manoeuvres are characterized by the minke whales unusually slow speed. The grooves, although visible, are not expanded and no water is purged out.

Chin-up Blow

In a chin-up blow the whale surfaces slowly dorso-ventrally pushing its head well into the air, taking a breath and re-submerging, sometimes even rolling laterally. The white throat, not expanded, is well visible.

Lateral Chin-up Blow

A lateral chin-up blow is performed by surfacing slowly laterally (usually on its right side) the whale twists its head to take a breath, rolling back laterally when re-submerging. Chin-up blows might be rather related to underwater entrapment movements than entrapping the prey at the surface.

Head Slap

Similar to an oblique lunge, the whale lunges at an angle of 30 to 45°, yet the throat is not expanded and no water is purging out. At the highest point, the whale bends its head back and slaps it onto the surface which creates a huge splash and sound. When first observed in 2000 we named it a Frog Lunge as it certainly brings to mind  a frog leaping out of the water.

Lateral Head Slap

For a lateral head slap the whale surfacies slowly laterally (usually on its right side) the whale either remains on her side or turns back into dorso-ventral plane in order to slap. Or it breaks the surface in dorso-ventral plane and turns onto its side prior to the slap.

Exhale on the Dive

After a regular surfacing the whale does a strong exhale right below the surface throwing a fountain of frothing water into the air  which produces  a roaring sound.

Underwater bubble blast

The whale releases air while fully submerged. To the observer only a huge air bubble appearing at the surface is visible and indicates the whale’s position.
 

Stay tuned for any new manoeuvres the minke whales of the St. Lawrence might invent in coming years.