Feeding Strategy

Techniques used by individual groups 

• Some bottlenose dolphins in Australia have become known as ‘spongers’. They grab a sea-sponge and dive down to the seabed with it. Holding the sponges tightly in their mouths, they then poke them into the sandy seabed, disturbing fish in hiding. The fish emerge, the sponge is dropped, the meal is eaten, and the tool picked up for further foraging. The sponges protect the dolphins’ noses from scuffs, scrapes and stings, in the same way as we would protect our hands with gloves when gardening or clearing rubbish from a beach.

• Other bottlenose dolphins living in Shark Bay, Australia, carry large conch shells in their mouths when fishing. The dolphin fills the shell and then visits the surface to shake it so that the seawater drains out, leaving small fish trapped in the bottom.

• In the shallow waters of Florida Bay in the US, dolphins use their speed, which can exceed 20 miles an hour, to swim quick circles around schools of mullet fish, stirring up curtains of mud that force the fish to leap out of the water into the dolphins’ waiting mouths.

• Dusky dolphins off the coast of Patagonia herd schools of anchovies into neat spheres and then take turns gulping them down.

• In several locations including Brazil, India and Myanmar, dolphins have teamed up with fishermen and fished cooperatively together for generations.  Shore-based fishermen wait for the dolphins to signal that they have rounded up fish before they cast their nets, the dolphins then easily catch disorientated fish that spill out around the nets.

• The “bubble net” is a neat trick exhibited by humpback whales. A group of humpbacks locates a school of fish. Attacking them is likely to disperse them, so instead they swim beneath the fish in ever-shrinking circles, blowing air bubbles as they go. The densely packed bubbles rise in similarly shrinking circles, effectively trapping the fish as the 'net' gets tighter, and then at the right moment, the whales swim up the net, mouths agape, and swallow the fish in great clusters. The whales all have different roles: some are bubble-blowers, others dive deep to ensure the fish are driven upwards, and some even make calls to help herd the prey.

• Orcas also work cooperatively when hunting. In Norway they work together to herd schools of herring into tight balls. Swimming round and under their prey, flashing their white patches to scare the fish and create a fish bait ball, the orcas then spin around and slap the balled fish with their tails, stunning them for easy catching.

Echolocation 

The ability to echolocate appears to be present in all toothed whales. The sounds used by cetaceans to navigate or to find prey differ markedly from the whistles, grunts and moaning songs they use to communicate with each other. It is rather like the system bats use to hunt in the dark and involves creating 'sound pictures' by making clicking noises and listening to the echoes that come back, but unlike bats that echolocate using extremely high-pitched calls beyond the range of most people's hearing, the 'clicks' used by cetaceans are at least partly audible. 

The amount of information obtained through echolocation is astonishing. The time lapse between each click and its echo enables the whale or dolphin to estimate how far away an object is; the form of the echo helps to identify its size and shape; and the varying strength of the echo provides information on the direction in which the object is moving. Even prey hidden under the sand isn't safe because echolocation is so sophisticated that a dolphin can 'see' through the sand and tell the difference between hidden rocks and hidden fish just by the type of echo produced.

The study of dolphins at sea suggests that echolocation is used in a variety of different ways. For much of the time relatively simple, low frequency sounds are emitted to provide a dolphin with a picture of its surroundings, such as variations in the seabed and water depth. The volume of the sound determines the dolphin's range of view, from the immediate vicinity to over a kilometre. If the dolphin detects something of interest, it will emit rapidly repeating clicks of a broader frequency. As the dolphin moves closer to its target it will focus its beam and use higher frequencies. These higher frequency clicks will provide maximum information about a subject.

An echolocating bottlenose dolphin can make up to a thousand clicking noises per second.

A typical echolocation sequence is as follows:

1. During normal swimming with no specific target, a general low-frequency echolocation signal of fairly pure tone is used this provides an animal with information on topography of the area including water depth, changes in sea floor profile and position of coastal features. The scan range will be determined by the time between the signals (clicks) and how much energy there is in each signal. For efficient echolocation one individual click must be sent out and the echoes received before the next click is emitted.

2. Once a new echo is received, the first requirement is to determine distance and direction, and to collect more detailed information on the target itself. The dolphin emits a series of clicks with a broad frequency band, the echoes of which can give many different pieces of information. The high frequencies give the most detailed information, but they are absorbed quite quickly by the water and so are useful only at close ranges.

3. Once the bearing of the target is established, the dolphin focuses the signal on the target. This concentrates the power of the higher frequency components and gives a more detailed picture of the target. By moving its head from side to side it can also gain information about target size and movement.

4. As the range closes between dolphin and target, the dolphin can use much higher frequencies in its echolocation clicks and hence get even more detailed information. At this stage the clicks will start to come more closely together and produce what sounds like continuous 'creaking'.

5. Finally, at very close range, it may be necessary to determine texture or other fine structural information. In this case use of short-range sonar with very high frequencies would be necessary. It has been suggested that taking objects into the mouth may be associated with a close range acoustic sensory system rather than with the sense of touch.

Hydroplaning

Some bottlenose dolphins use a technique called 'hydroplaning', which is a bit like body surfing. The dolphins chase fish into the shallows and then propel their bodies forward in water that is only a few centimetres deep in an effort to seize a fish.

Scholarly links

For more information on this topic please explore these scholarly articles noted below:

• www.sciencedirect.com/topics/agricultural-and-biological-sciences/echolocation 

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