Strike Zone: Sri Lanka is a leading place for whales getting hit by ships
Sri Lanka is one of the best places in the world to glimpse large whales, especially charismatic sperm and blues. Unfortunately, it may also be one of the leading places for large whales getting maimed or killed by big, fast-moving ships.
In spring 2012, for example, a container ship steered into Colombo harbour bearing a blue whale carcass across its bow. Around the same time, another blue carcass was spotted floating just off the south coast.
A WDC (Whale and Dolphin Conservation) web site reports five deadly ship strikes on blues and two on sperm whales in Lankan waters during 2011. The high danger lies in the fact that both ships and whales for their different reasons cruise at the continental shelf edge a few miles offshore.
In the name of reducing ship strike on whales, blues in particular, marine biologists have been studying factors that may lead to high blue concentrations in particular spots.
They are concentrated over ocean floor canyons—remnants of ancient river valleys when sea levels were lower—extending out from Lankan shores.
Down-drifting nutrients from decomposed plants, animals and animal waste continuously settle in these depths but sometimes come gushing to the surface through cold water ‘upwellings’ from down there.
These nutrient upwellings—just add sunlight—fuel phytoplankton blooms, which in turn feed outbreaks of krill and other pinkie-size crustaceans that blue whales specialize in gulping down.
Mapping is valuable as pure science of course, but one hopes also that identifying blue high-density zones could reduce ship strike by showing where shipping lanes could be re-routed or ships required to slow down.
Ship strike risk stems both from a collision/blunt trauma as with the 2012 Colombo harbour whale and from propeller laceration as with the one found floating in the south a few days later: whales too shallow beneath a passing ship may actually get sucked into the gouging blades.
Marine biologists speculate that for every blue whale ship strike recorded another ten go unrecorded because the carcasses sink or drift into the untraveled ocean.
Ship strikes are the most serious threat facing endangered blue whale numbers. Other experts downplay the ship strike threat, especially in light of other dangers blues confront. Even sceptics might concede, however, that high strike levels could undermine the viability of a blue population resident in Lankan waters.
It is more than a little puzzling why whales get hit rather than moving out of harm’s way. Large ships emit deafening engine and propeller noise. Sound carries better through water than through air.
Whale hearing is exceptionally acute, though their ears are hydrodynamically buried beneath a layer of skin. Whales may exhibit startle-flight responses even to the click of an underwater camera. Why don’t they hear ships coming and evade? It would seem that even the slowest swimmers should have plenty of warning and time.
We know very little at the moment but there are a number of possible explanations for this apparent heedlessness. All of them may be true at various times and places and some may also interact with one another.
Large adult whales have for millions of years had little to fear from loud noises or anything else. If it doesn’t sound like orcas (killer whales), great whites or colossal squid, any of which might rip into you or snag your juveniles, you probably don’t need to worry about it.
Evolution proceeds not only by developing body structures and behaviours with survival value but also by forgoing those with none. It disfavours wasting energy on absent dangers, energy that can be used instead for developing useful structures and behaviours.
For aeons until very recently, large whales have had no reason to maintain habits of avoiding noisy things, which might just as plausibly draw their curiosity. Unlike sudden noises that startle, approaching ships begin with a scarcely noticeable sound and build to a roar gradually.
The era of large fast-moving ships is little more than a hundred years old: a small handful of generations at most for whales. This is scant time for avoidance to emerge even as a learned behaviour, much less as an instinct.
Some suggest that whales could often be sleeping when struck, though this explanation stumbles over the fact that one whale brain hemisphere stays awake while the other sleeps: to maintain breathing, which in whales is not automatic as it is with us.
Whales may often simply be too preoccupied with what they are otherwise doing–feeding, playing, communicating, courting, mating, nursing–to notice ships bearing down on them.
In some places, moreover, there may be so many ships and so much ambient noise—some of it echoing off the seafloor and other objects both natural and human-made—that whales fail to isolate the source, proximity and direction of impending danger, accurately.
Especially chilling is the possibility that whales don’t hear approaching ships at all, despite the glaring racket. There may be at least three reasons for this. First is a possible ‘bow-null’ effect: the ship’s hull itself blocking engine and propeller noise, thereby creating an acoustic shadow directly ahead of the proceeding vessel.
Second is a possible refraction effect. Sound waves starting horizontally near the surface get bent and pulled downward by colder, denser water just below, where they propagate more efficiently. Noise, therefore, passes beneath whales at the surface until just before the ship is upon them.
Third, is a possible reflection effect sometimes called ‘Lloyd’s mirror.’ Sound waves generated slightly underwater may reach whales in a ship’s course by two different routes: directly and by reflection off the water/air boundary at the surface.
Until the ship is quite close, direct and reflected sound waves will reach the whale’s ear almost simultaneously because the distance travelled by the reflected wave up to the surface and back down to the whale will be small compared with the horizontal distance travelled.
Arriving simultaneously, direct and reflected sound waves are, however, 180 degrees out of phase with each other as they reach the whale’s ear. This is due to the fact that the reflected wave has bounced off the surface en route. In this opposite phasing, the trough of the reflected wave cancels the crest of the direct wave and vice versa.
The result: silence. As the ship gets closer, the up/down distance travelled by the reflected sound wave will grow in proportion to the horizontal distance travelled. Travelling proportionately further, the reflected wave will reach the whale more and more belatedly compared with the direct wave.
The crest/wave cancellation due to simultaneous arrival will dissipate and the whale will begin to hear the oncoming ship. Too late.
The terrible irony if whales sometimes surface precisely to escape infernal ship racket below and find best relief smack in the path of onrushing behemoths. Peace and quiet, so lovely….
(From the archives of Echelon Magazine; first published in July 2015)