Sunday, 6 April 2014

Deep Dives to Dark Depths

There are something like ninety species of cetacean, which are usually grouped into fourteen families. Perhaps the most obscure and little known of these families - and among the most obscure of all mammal families - is that of the beaked whales. It's actually quite a large family, with over twenty species, representing almost a quarter of all known cetacean species, and around a half of all those that are noticeably larger than dolphin-sized. And, since they typically weigh a ton or more, they're hardly small themselves.

The main reason we know so little is where they live. These are deep-water animals, that rarely come in close to shore, and so just aren't seen very often. Combine this with the fact that they're mostly of little interest to whalers, and it becomes apparent that studying them is neither easy, nor likely to be commercially motivated.

A lot of what we do know comes from strandings, where the whales leave their usual deep water habitats with fatal consequences. (Whales that die at sea usually decompose and sink long before they can be washed ashore, although it does happen). Strandings are most likely to occur in seas with high human activity and consequent noise pollution - in the case of beaked whales, for instance, there are many strandings along the coasts of the Mediterranean. If we'd like to prevent, or at least reduce, this sort of thing, then it would certainly help if we knew more about the whales and their behaviour.

The species most commonly found in these strandings is Cuvier's beaked whale (Ziphius cavirostris), not least because it's the most widespread even when it's healthy. At about two-and-a-half tons in weight and twenty feet in length, it's about average for a beaked whale, and it lives in every major body of salt water outside the Arctic and Antarctic, bar a few shallow places such as the Yellow and Red Seas and the Persian Gulf. Indeed, they generally prefer to swim in waters at least 1 km (3,300 feet) deep.

Research has shown a strong correlation between mass strandings of this species and the use of mid-frequency active sonar. That's not to say that it's the only cause of mass strandings of beaked whales, since these have been recorded since at least the 1870s, but such strandings have been far more common since this type of sonar was introduced in 1950, and there are some that have been clearly linked to specific naval exercises.

On the other hand, this is the main tool used by naval forces to detect hostile submarines, and so isn't something that the military is generally keen to give up on.  Tests in which fake sonar was directed at beaked whales of the coast of southern California showed that some of them were obviously spooked, and tried to escape from the sound. At the same time, however, a real sonar signal was coincidentally emitted into the area by a ship on exercises, and the whales that would have heard that didn't seem to bothered.

The reason for this apparent contradiction may well be that the whales weren't observed for very long. We only have their immediate reactions, and no longer-term context in which to put them. In other words, what do they do when everything's normal? We know so little about them, that that's really not clear.

To fill in that gap, researchers have looked at the behaviour of Cuvier's beaked whales swimming in the Southern California Anti-Submarine Warfare Range - exactly the sort of place where you'd be worried about this sort of thing. The range lies to the west of San Clemente, an island which is itself fifty miles or so off the Los Angeles coast.

They attached radio tags to eight whales swimming in the area, and followed them for three months to see where they went and how often - and how far they dived. From previous studies, we knew that Cuvier's beaked whales typically dive to about 275 metres (900 feet), into dimly lit waters where there is too little light for photosynthesis even in the clearest of oceans, but where there are still plenty of fish to be found. Such dives typically last about 20 minutes, and the whales may perform several in a row.

However, they occasionally dive much deeper, and it's thought that it's on these longer dives that they get most of their food. The headline news from this study was that the recordings shattered the previous records for both depth and duration of known dives - not just by this species, but for any air-breathing animal.

Those records had previously been held by elephant seals, which can descend to 2,300 metres (7,500 feet). When you consider that the deepest anyone had previously recorded a Cuvier's beaked whale going was 1,100 metres (3,600 feet), you can see that that must have taken some beating. In fact, the new record now stands at 2,992 metres (9,800 feet, or 1.8 miles), and during the longest dive, the whale held its breath for an astonishing 135 minutes, breaking the seals' record by a quarter of an hour.

At these remarkable depths, the temperature holds steady at just a few degrees above zero. There is not a hint of light, and the pressure reaches almost 300 atmospheres. Here, we must assume, the beaked whales find and eat the deep-sea squid that are their primary food source.

But, digging a little deeper than this headline, we find some other interesting facts, too. On average, the whales only spent two minutes at the surface between their dives, suggesting that they must be incredibly good at taking in oxygen. Even before the deep dives, when presumably they were bracing themselves for the exertion ahead, three minutes was typically quite enough. Only on those rare occasions where they performed two deep dives in a row did they spend any time recovering in between (up to 35 minutes), and not always even then, so clearly it wasn't essential.

The whales dived at all times of day and night, suggesting that, like dolphins, they don't need sleep in the way that we do, and that the squid they hunt basically stay put around the clock. They were more likely to stay at, or just below, the surface during the night. Taking all this together, it's no wonder they're so rarely seen!

The thing is, why had we not noticed this before? These record-breaking dives weren't so rare that we should have failed to spot them even in the limited, short-term studies we'd conducted before. So the conclusion must be that there's something special about the whales in this area. It could be that that there just happen to be particularly good feeding grounds at that depth in this part of the world - a possibility that isn't exactly easy to test.

However, it's also worth noticing that the whales tended to stay within the same area of the sea. Which, in this case, happens to be where the US Navy tests its sonar. Is this somehow affecting the whales' behaviour? The researchers concluded that the sonar is unlikely to be directly responsible for deep diving behaviour, because some of the longest dives were actually recorded well outside the testing range, on rare trips  as far north as seas off Santa Barbara and as far south as Guadelupe Island in Mexico.

On the other hand, the whales performed deep dives less frequently than those in previous studies elsewhere, typically about once every three hours or so. If that's a response to the sonar, it might be worrying, since it would likely restrict their time spend hunting. Yet they keep coming back to the same sound-polluted area, despite the fact that their occasional forays elsewhere prove there has to be food further afield.

To be honest, its hard to say how much, or how little, the sonar use is affecting the whales. Before we could determine this, the US Navy would have to provide really detailed records of what it was doing, and when, which isn't very likely. While it's hard to imagine that it has no effect at all, given the previous studies with 'fake' sonar, it's also entirely plausible that the whales are, by now, at least pretty used to the noise.

Is the long-term behaviour of beaked whales being affected by noise pollution off California? Maybe. The study is inconclusive on that point. But it does at least tell us something about these mysterious animals that we never knew before.

[Photo © Heinrich Schatz, obtained from the Smithsonian Institution website. Use permitted for educational, non-commercial purposes only.]

No comments:

Post a Comment