Science Heresy - January 2012

Strategy to Avoid Mass Strandings

Every year we experience the tragedy and occasional triumph of whale strandings and rescue attempts, with the hard work of many volunteers and official organisations involved.

Chris Arthur, of Parks and Wildlife, says "it is not known "why so many whales strand themselves on particular beaches". I submit that it should be known by "experts" after so many incidents and facts that point to one persuasive hypothesis.

The whales navigate using a system of echo-location, which relies on them receiving a continuous flow of information from the terrain ahead, produced by echoes of their transmitted sound signals, much as radar uses radio signals.

Virtually all strandings have occurred on gently shelving beaches in the presence of heavy surf.

At the best of times, a shelving beach will produce a very poor echo as such slopes absorb most of the energy arriving at them: a heavy surf produces a barrage of "white noise", swamping any weak echoes and leading the animals to believe that either open water lies ahead or confusing them by "jamming" their signals.

If a chain of reflecting objects, suitably designed corner reflectors, buoys, or possibly even a line of big rocks, were to be placed out to sea, well before the beach, the echoes from this chain would warn the whales that there was some sort of hazard ahead. I suggest that a trial at one of our "hot-spots", like Ocean Beach, could be conducted at a fraction of the cost of these rescue attempts.

Perhaps the continuing occurrence of these events may inspire a new generation of researchers to be the one that finally ends these tragedies.

Ian Milne
Seven Mile Beach

The Mercury 20/11/11

A corner reflector

(designed to reflect radar rather than sound)

The most obvious apparatus for reflecting whale navigation signals is a corner reflector. A corner reflector consists of three  intersecting mutually perpendicular plane surfaces and has the property of reflecting radiation back along the path by which it arrived so giving a large return signal. Such devices are commonly used for reflecting light and radar waves. The "cats eyes" along the centre of the road are corner reflectors. Light weight corner reflectors made from aluminium foil are hung below meteorological balloons to allow them to be tracked by "wind-follow" radar. Wave theory tells us that a reflector needs to be at least one half wavelength in size in order to be effective.

Most baleen whales make sounds at about 15–20 hertz (Wikipedia). The velocity of sound in sea water is 1560 m/s. Hence the wavelength of a whale sound is around 100 m. A corner reflector needs to be larger than half a wavelength so its minimum dimension is 50 m. Perhaps such a gadget could be constructed from stainless steel mesh and either allowed to lie on the bottom outside the breaker zone or suspended in mid-water by a suitable flotation device.