From the German Bight to the deep sea: dog sharks from the North Sea migrate far out into the North Atlantic, where they carry out vertical migrations in a daily rhythm: they climb to the surface at night and sink hundreds of meters during the day. Researchers have now discovered the reason for this using transmitters and sonar measurements: the sharks follow their favorite food, the squid.
There are also sharks in our waters: the dogfish (Galeorhinus galeus) is the largest shark in the German North Sea, measuring up to two meters in length. It occurs worldwide in the shallower waters of the continental shelves and shallow coastal seas of the temperate climate zones, but has also been detected in high seas and in greater water depths. However, the fish belonging to the smooth sharks are considered to be threatened with extinction, because fishing in particular has severely decimated their stocks. Because they grow slowly and mature late, top dog shark populations struggle to recover from major losses.
Sharks with satellite transmitter
It is all the more important to know the habitats and regions in the oceans that are important for dog sharks – this is the only way they can be protected effectively. Scientists from the Thünen Institute for Sea Fishing in Bremerhaven have therefore been researching dog sharks in the German North Sea for a long time. Since 2017, they have been regularly marking dog sharks with satellite transmitters during their summer gatherings around Heligoland. By following the movements of the sharks, they can understand in which areas of the sea, for example, the sharks give birth to their offspring or spend more time seasonally and thus identify particularly important habitats.
When evaluating this data, Matthias Schaber from the Thünen Institute and his colleagues made a surprising discovery. They found that some dog sharks native to Helgoland not only unexpectedly migrated far into the open Atlantic – one animal even migrated to the waters around the Portuguese island of Madeira. In addition, the dog sharks also changed their usual swimming behavior and dived to depths of less than 700 meters. This vertical migration in the North Atlantic also showed recurring patterns: At night, the animals stayed mostly in near-surface layers with a water depth of up to 150 meters, while during the day they stayed at a depth of 400 to 600 meters.
Following the squids
In order to find out why the dog sharks carry out this vertical migration, Schaber and his team compared the depth profiles of the sharks with sonar measurement data that came from research vessels in the North Atlantic. From such data it is already known that there are so-called echo scattering layers in all oceans of the world, which are characterized by an enormous density and biomass of organisms – from the small luminous shrimp to jellyfish and squid to small and larger fish. These organisms usually live in average water depths of a few hundred meters. “By superimposing the measurement data from the sharks and the echo sounders, we were able to clearly show that the dog sharks in the open Atlantic stayed almost continuously within these echo scatter layers,” explains Schaber.
This suggested that the dog sharks in these strata also hunt their prey – and therefore follow it. “However, many of these animals migrate to the sea surface every day under cover of darkness and return to the dark depths of the oceans during the day,” explains Schaber. “In terms of biomass, this migration is considered the largest migration in the animal kingdom – and the dog sharks are apparently migrating with them.” As the research team determined using the sonar data, the sharks primarily follow the movements of their favorite prey, the squid. “The dog sharks change their swimming behavior in high-sea areas that are characterized by a relative lack of food near the surface in order to be able to skim off their favorite prey, the squid, from the rich source of the Mesopelagic migrants,” says Schaber. “This has never been demonstrated in such detail before.”
Source: Johann Heinrich von Thünen Institute, Federal Research Institute for Rural Areas, Forests and Fisheries; Specialist articles: Scientific Reports, doi: 10.1038/s41598-022-05989-z