Killer whale (Orcinus orca) predation on harbour porpoise (Phocoena phocoena) in Hardangerfjord, Western Norway

The Hardangerfjord has a high density of harbour porpoises, making it stand out among Norwegian fjords as a particularly important habitat. A new predator-prey relationship emerged in 2017, with killer whales entering the fjord in an intermittent pattern and preying on the residing harbour porpoise population. Killer whales, as top predators, have the potential to significantly influence marine communities through their impact on the abundance, behavior, and distribution of their prey. It is important to understand the predator-prey dynamics in an area to maintain the ecological integrity of aquatic ecosystems. This thesis aimed to investigate the population-level impact of killer whale predation on the harbour porpoise population in Hardangerfjord. The killer whale’s predation rates were successfully estimated by integrating bioenergetics and predators’ presence estimates, followed by an implementation of the predation rate into an individual-based model. Multiple scenarios with predation rates ranging from 0.12-0.28 had to be run in the model, to account for uncertainties in the parameters used. When the highest predation rate (p = 0.28) occurred over a 20-year period, the killer whales reduced the harbour porpoise population by 90 %. In this scenario, the porpoise population would be classified as ‘endangered’ on the IUCN red list, and they would be at risk of going extinct. This research has provided a deeper understanding of the predator-prey dynamics between killer whales and harbour porpoises, giving valuable insight into the conservation and management efforts in this region. The harbour porpoise is a species of conservation concern globally due to high bycatch levels. Thus, this investigation will establish baseline data for future research, which can help develop strategies to further protect the harbour porpoise species. Further understanding of the killer whales’ and porpoises’ distribution, diet, and movement patterns within Hardangerfjord, will be an important step towards ensuring their conservation.