Sixty pilot whales strand in the mud flats at Port Levy, near Christchurch, New Zealand.  Sixteen die but rescuers push forty-four back into the water and they swim back to the open sea. (link)

by Captain David Williams

Deafwhale Society, Inc.

Here's another prime example of a seaquake-injured pod of whales running aground. 

On 6 January 2010, a magnitude 5.7 shallow-focused (reported 10km by USGS) earthquake erupted in the seabed about 89 miles west-northwest of Auckland Island.  The quake occurred at night (9:45 pm local time) and was epicenter along the Macquarie Ridge, a prime feeding grounds for pilot whales. 

The entire Macquarie Ridge is dotted with possibly a hundred seamounts.  In fact, this particular earthquake occurred near the base of an undersea mountain, the top of which rises to within 60 meters of the surface.  This is indeed prime feeding country for pilot whales.  If you are a pilot whale, here's where you feast and get fat.

The earthquake had predominate horizontal motion and likely shifted the nearby seamount from side to side converting its slope into the faceplate of a gigantic transducer and causing the surrounding water pressure to suddenly increase and decrease several hundred pounds per square inch every second or so for ~30 seconds.  In fact, it is even conceivable that the flat top of the mountain exhibited up and down piston motion as the s-waves spread out from the epicenter.

Regardless of exactly how the rapid pressure changes came about, the net results was that the air contained in the head sinuses of each diving whale expanded and contracted too excessively, resulting in a barotraumatic injury in the pertygoid sinuses that are so important for proper function of bioacoustics.  The whales survived the earthquake but were no longer able able to dive and feed themselves due to severe pain and loss of echo-location abilities.  Nor were they able to echo-navigate.   Bottom line, the pod quickly became lost and were no longer able to care for themselves.  Not having a sense direction, the pod huddled together for protection against sharks, and just starting swimming in a random direction.

More likely than not, the pod had previously been injured by several undersea earthquakes in the past and had recovered; however, the seaquake waves from this particular quake were just a bit too powerful or the pod was caught is shallow water (under 100 meters) where the percentage of air volume change in the sinuses would be much greater than at deeper depths due to reduced ambient pressure.

After the quake, the pod swam off in a downstream direction.   But how do we know?  Common sense.  When any animal, including man, swims in a current without a sense of direction, the resistance presented by the flowing water controls the compass direction toward which they swim.  It's a matter of physics.  Anything and everything moving through a current without a sense of direction will be turned downstream in the path of least resistance.  Its ten times easier to swim with the flow than against it. 

Surface currents in the area flow in a general northwest direction toward West Cape and Resolution Island near the western tip of South Island, New Zealand. 

However, rather than swim north along the western shore of South Island, the pod got caught up in the Southland Current that flows east toward Stewart Island and then swings around the southeast tip of South Island and turns northeast toward the Banks Peninsular as shown in the map on the left.

The trip from the epicenter to the beach was ~830 miles as the crow flies, but more likely closer to 1,200 miles considering that the pod likely alter course several times to escape shark attack and ship traffic.

The trip from the epicenter to the beach took 17 days meaning the pod likely travel downstream about 75 miles per day but they probably swam ~125 miles per day or ~5 miles per hour.

They likely entered the bay leading to Port Levy on the incoming tide on 22 January, followed in by up to 20 large sharks.  The sharks kept them penned into the bay until the next rising tide which washed them onto the mud flats at high tide.  When the tide went out, they were left high and dry. 

The rescuers pushed ~45 whales back to deep water and they departed the area and were last seen in open ocean. 

But this don't mean the whales were saved.  They are still injured and have no sense of direction.  Sharks are still dogging the pod and taking any stragglers. 

The surface current heads from Bank Peninsula straight toward Chatham Islands, ~550 miles from where they stranded the first time.  The odds favor a second stranding on Chatham Island by the end of the January.

Thus, the Deafwhale Society is going on record.  Rescuers in the Chatham Islands should be on guard for a pilot whale beaching any day now.

There's a little more to the story:

The day after the mag 5.7 event occurred, on 7 Jan 2010, another dangerous earthquake erupted 50 miles west of the Macquarie Ridge axis, but not near the ridge itself.   The strange thing about this quake is that it was listed as only 2km below the surface. If this is true, the seafloor was certain split open during this event.  

The next question is whether or not there were any pilot whales feeding near the epicenter? 

If the epicenter was properly located, the water is over 5,000 meters deep and not likely to be a feeding ground for whales.  Thus, there is no warning issued for this event and it is eliminated from suspicion.

The point here is to show the process of selecting which earthquake might have caused a particular stranding.

Capt. David Williams

(Any telephone inquiry should be directed to our Florida landline: 954-306-1549)