Movement patterns (SP 3)

Data on variations in movement patterns in the North Sea

Positioning of jacket foundation in 2013 at DanTysk offshore wind farm in the German North Sea. Photo: Vattenfall

The dynamics of a population is likely to depend on the individuals’ ability to disperse among areas with high prey densities as well as their ability to forage efficiently in these areas. In the study by Nabe-Nielsen the dispersal patterns of the simulated animals were calibrated to resemble those of satellite-tracked animals by adjusting the modelled animals’ ranges and dispersal speeds. The animals’ foraging efficiency was linked to their fine-scale movement patterns in the model. These were calibrated to resemble those of an animal tagged with dead-reckoning sensors, which resulted in realistic autocorrelation structures for turning angles and movement speeds. The model used spatial memory behaviour to produce realistic home ranges.

As the porpoises’ dispersal patterns in the North Sea are likely to differ from the ones observed in areas where their movements are more bounded by land the model is likely to produce more realistic predictions if explicitly incorporating movement patters for animals observed in North Sea. Dispersal pattern may also vary among different parts of the North Sea, and it is therefore preferable obtaining data from animals tagged on different locations.

Satellite tracking

Although some of the animals tagged by Skagen (northern Denmark) moved into the North Sea, it would strengthen the model considerably if more animals are satellite-tracked. These should be caught on the southern part of the Danish west coast, and preferably also in Scotland (in collaboration with University of St Andrews). The animals that were tagged in the IDW and by Skagen were caught in pound nets, but this fishing method cannot be used in the harsh conditions of the North Sea. Instead animals will be caught actively using floating drift nets. To further increase the sample size additional animals will be caught in Skagen using pound nets. In total we aim to equip a total of 15 new animals with Argos satellite tags.

As porpoises are likely to forage on different prey species in the IDW and the North Sea, their fine-scale movements may not be entirely similar in the two areas. These are also likely to depend on the water depth and to vary among individuals. We therefore propose obtaining fine-scale movements data from 5–10 of the animals that are also equipped with Argos tags. These movements are recorded with Fastloc GPS tags that detach themselves from the animals after some days, while the Argos tags remain on the animal for up to 1.5 years. In contrast to the animals tagged for Sub-project 1 these animals should not be exposed to noise from air guns or similar, as this would prevent us from assessing their natural movement patterns.

Field campaigns

In order to obtain data on both fine-scale movements and dispersal patterns of porpoises from different parts of the North Sea we plan on tagging new animals in the years 2013-2014. Each year shall include a 2-week field campaign in order to catch porpoises actively in the Danish part of the Wadden Sea, a 2-week field campaign in Scotland together with researchers from University of St Andrews as well as a continuous effort to catch and tag porpoises at Skagen. A large fraction of the porpoises caught in Skagen disperse into Skagerrak and the North Sea. If it proves impossible to obtain the necessary permits for working in Scotland we shall allocate our effort to tagging additional porpoises in other parts of the North Sea.