Orcas-Silent running

by Matthew Norton

When it comes to feeding yourself in the sea, different prey use different antipredator adaptations which in turn requires the predator to use different methods to catch their prey. This makes things interesting for opportunistic predators, who target a variety of different prey species and so must be flexible in their hunting strategy, such as orcas (also known as killer whales). The diet of these large dolphins includes fish, sharks, squid and a range of marine mammals, but each orca population, of which there are many, target only some of these prey species and seem to ignore others.

Orca article image 1 — Orcas hunt various animals that include (clockwise from top left) fish such as herring, salmon and even large sharks as well as seals, porpoises, dolphins, humpback whales and even the occasional octopus or squid.

The most well known difference in the prey that orcas target is between resident populations, who stay in one area and eat fish, and transient populations, who move from place to place and attack other marine mammals. The fish-eating orcas also rely more on echolocation (they produce high frequency ‘clicking’ sounds that bounce off solid objects in the water) to find their food whereas mammal eating orcas are more likely to listen out for their prey without making any sound themselves.

Echolocation would be just as effective in detecting marine mammals as it would for fish, but marine mammals are more likely to hear those high frequency ‘clicking’ sounds, especially dolphins who use echolocation themselves, and be warned of approaching orcas. Even for these powerful predators, the element of surprise can be vital to the success of a hunt with even large whales sometimes running away if they hear orcas coming.

Orca article image 2 — Echolocation is used by many toothed whales (a group that includes all dolphins ) to find food by sending out high frequency ‘click’ sounds and listening for the echoes that are bounced back when the clicks hit something solid in the water. Using these clicks they can make a mental map of their environment, including the location of a potential fish.

Even between fish-eating orca populations (and probably marine mammal eating populations) there are differences in the prey species they target and the hunting strategies they use. For example, orcas living in Norway and Iceland catch herring by herding them into tight groups and then slapping their tails to stun the fish.

Meanwhile, the shark and ray eating orcas found around New Zealand and Australia (although orcas have been seen eating sharks elsewhere in the world) can use various hunting methods to handle these tricky prey. These include grabbing bottom dwelling rays by the tail with their teeth and dragging them out of hiding places, sometimes with the orca’s tail hanging out of the water, using tail slaps to confuse sharks and even flipping white sharks on to their backs to immobilize them. It is worth mentioning that reported cases of orcas eating sharks and rays are rare and these hunting behaviours could be one-off, or could be one of many unique shark hunting techniques used by orcas.

The different hunting techniques that orcas have developed are all part of the continuing battle of wits that evolution creates between every predator and prey combination. This in return has lead some of the orca’s prey species to evolve clever antipredator responses, with some fish being able to hear the high frequency sounds used in echolocation and smaller dolphins using sounds that are outside an orca’s hearing range. Nonetheless, orcas are still one of the seas most impressive and effective predators.

From a human perspective

It was difficult to know where to start for this section, orcas have affected our lives for so long by being a source of wonder in the seas and a source of inspiration in television and film with the Free Willy films being particularly famous. Sadly, many orcas who came into contact with humanity were abducted from the wild and forced into a life in captivity for our entertainment. Fortunately, the harm caused to orcas by a life in captivity has received worldwide attention since the documentary “Blackfish” came out and sparked major protests against aquariums that still keep whales and dolphins in captivity.

Even in their natural environment, orcas may not be completely safe from humans meddling in their lives, they do share the seas with us after all. For example, our boats can sometimes disturb wild orcas if they live around a busy shipping lane, or are approached by a whale-watching boat that gets too close. As isolated incidents, these disturbances may not cause the orcas too much harm, but if they are repeated over and over again then it may cause them to waste a lot of their valuable time and energy while avoiding our boats.

Orca article image 3.jpg — We make noise in the sea through all kinds of activities including pile driving to set up wind turbines, engine noise from boats and the sonar from submarines.

Worse still, the noise pollution from some of our activities at sea, which include general boat traffic, sonar from submarines and pile driving (i.e. slamming a large piece of metal into the seabed), can disturb orcas and other marine wildlife that are nowhere near us. All that noise makes it harder to be heard in the ocean, which is especially bad for social animals like orcas that use various calls to talk to each other. Noise pollution can also drive orcas away from the surrounding area for long periods of time, which in some cases is the intended effect with humans using noise to keep marine mammals away from fish farms and to round up whales and dolphins to take into captivity.

These are the impacts of noise pollution on orcas that we know about, but it is worth pointing out that noise pollution has a whole range of effects on sea life. In the most extreme cases it can actually damage the ears, or whatever other structure an animal may use to hear, and cause hearing loss. In other cases, noise pollution can lead to dangerous behavioural changes such as unusual aggression, a lack of response to certain sounds and for whales and dolphins, panic and disorientation during dives which could lead to decompression sickness. Even though these effects of noise pollution have not been specifically reported in orcas (as far as I know) they could still be happening.

Noise pollution is one of many threats that marine life is facing in the modern world and while the world under the waves is a naturally noisy place, the loud sounds that we make are certainly an unwelcome addition.