The whale I'm talking about is the very small harbour porpoise. Unfortunately the population in the Baltic sea has declined rapidly and is now estimated to only about 600 individuals, which is very little. Many porpoises get entangled in fishing nets and drown every year, because they do not discover the fishing nets in time. To help porpoises avoid entanglement, acoustic alarms, also called pingers, have been developed. Pingers emit high frequency displacement sounds that make porpoises avoid the area around nets. They have reduced by-catch in several fishery trials and are now mandatory in several countries. There are however some potential negative side effects to these conventional pingers, since they emit sounds continuously. In the NIPPER project, which my thesis is a part of, we tested an alternative - the "interactive" pinger. It only emits displacement sounds when a porpoise has triggered it with its sonar, hence only when a porpoise is in the area! On this homepage you can read more about the results of the study, and also find out more about the harbour porpoise!