The objective of this thesis was to examine diet of pikeperch (Sander lucioperca) in the Turku archipelago. The most recent similar diet analysis was conducted in 2007. However, the list of species in Baltic sea is constantly evolving. There are approximately 20 different non-indigenous species found from the Turku archipelago (Helcom 2012). One of the most notable and recent of these non-native species in Turku archipelago is estuarine mud crab (Rhithropanopeus harrisii), which has spreaded rapidly since the first sighting in 2009 (Fowler, etc. 2013), and is detected as a regular part of diet to some different fish species (Ovaskainen 2015). One object was to find out whether estuarine mud crab would also be part of natural diet of pikeperch.

Pikeperch samples for this thesis were caught between 9.9.–6.11.2013, except for 41 individuals which were caught from Sauvo in November 2014. Fish were caught by regular gillnets and Nordic Coastal -gillnets, which were used in examination fishing in area of Kaitvesi.

12 Different prey species or groups were found from 220 examined stomachs. Pikeperches in different size classes were concentraded in slightly different prey species. The most important separate prey group in size classes under 35 cm were mysid shrimps (Mysidae). However, different fish species consisted the largest part of diet in all size classes. In size classes under 35 cm, perciformes like pikeperch, gobies (Pomatoscistus sp.) and ruffe (Gymnocephalus cernuus) were common prey species. In size classes over 35 cm diet consisted only of fish. These larger size classes were concentraded more in cyprinid species and pelagic species rather than perciformes. Estuarine mud crabs were not found from stomachs.