02.08.2018 – Strength in numbers

Today our tours included several dolphin sightings with Bottlenose dolphins (Tursiops truncatus), Short beaked common dolphins (Delphinus delphis), Striped dolphins (Stenella coeruleoalba)and Atlantic spotted dolphins (Stenella frontalis)! People love dolphins because they are so interactive and often curiously approach our boats. Every so often, however, even the most inquisitive dolphin species won’t bother to approach us if they are preoccupied with more necessary daily activities such as hunting. Today we were able to observe several sub-groups hunting schools of fish, who were simultaneously attacked from above by Cory’s shearwaters (Calonectris borealis).

Life isn’t easy for schooling fish in Madeira. Apart from the frequent airborne strikes through marine birds and attacks in the water from various marine predators including several cetaceans, many species are also very sought after as cuisine fish or live bait by humans. Several schooling fish may even be small juveniles of predatory fish; young Garfish (Belone belone), for example, are seemingly an exceptional favorite amongst dolphins here in Madeira.

Although both terms are used rather loosley, it is important to differentiate between schooling and shoaling behavior amongst groups of fish. Schoaling is when a group of fish congregate to socialize and these individuals usually disperse when predators approach. Schooling is when fish swim in the same direction in a coordinated manner with several leading animals managing the the group. The entire group thus swims as a unit and behaves like one whole organism and has evolved amongst several fish species as a tool to escape predators. This impressive group coordination is possible because of the lateral line organ, a sensory line extending along the flanks of all bony fish interrupting their scales. The lateral line allows the animal to sense slight changes in water movements and this triggers an immediate directional response in the animal, which will correspond to the individuals swimming close to it.

This behavior amongst fish follows the general consensus „strength in numbers“, where the individual finds protection within the group. While large solitary-hunting predatory fish face problems pursuing such schools, dolphins have understood that hunting coordinated groups requires highly organized teamwork. When approaching such schools, dolphins use their biosonar to disrupt the lateral line organ and simultaneously surround and intimidate the group of fish, gradually driving them to the surface. The aim here is to confuse the group until it separates into smaller units which are easier to pursue. Short-beaked common dolphins around Madeira, for instance, often use air bubbles emitted from their blowholes to drive the fish to the surface. Hunting strategies amongst dolphins vary according to species, prey and region and are taught to younger animals by their peers by means of social learning.

While one can’t help but feel sorry for the frantic schools of fish escaping these formidable predators, it’s impossible to not admire the clever, extraordinary group strategies used by dolphins to hunt them.

By Paula Thake

Sightings of the day

Ribeira Brava

09:00 Bottlenose dolphins, Short-beaked common dolphins

13:30 Bottlenose dolphins, Short-beaked common dolphins

Stenella

09:00 Atlantic spotted dolphins (snorkeling)

12:00 Short-beaked common dolphins

18:00 Short-beaked common dolphins, Striped dolphins