Resampling Huitfeldt-Kaas: Freshwater fish distribution in south-eastern Norway in 2019

629 of the same lakes from the original source data collected by Hartvig Huitfeldt-Kaas were resurveyed between July 2019 and December 2019. All lakes revisited were located within the counties formerly recognised as Akershus, Buskerud, Hedmark, Oppland, Oslo, Telemark, Vestfold and Østfold (now Oslo, Viken, Innlandet and Vestfold og Telemark). The number of lakes in the original source data was unevenly distributed between counties, and also between municipalities within each county. The municipality with the most lakes recorded was Aurskog-Høland in Akershus county (now Viken) with 36 lakes, while multiple municipalities only contained one or few lakes. Information on which species different lakes contain at present was gathered through interviews with local stakeholders and informants, and online grey literature. The informants consisted mainly of people involved with volunteer work in local hunting- and fishing associations in different municipalities within the study area. Contact information was obtained from association websites. They were first contacted by email, in which they were given a brief background on the purpose of the project and asked if they knew which freshwater fish species were present in the different lakes of interest in their local area. The text was standardized and the only thing that was changed was the municipality and lakes so it was “personalized” for each informant that was contacted. Those who responded but had no information regarding the lakes in question were asked if they could provide contacts for more relevant informants. Those who failed to respond were contacted again by email after a couple of weeks. If this second attempt was unsuccessful, they were contacted by phone for information regarding the lakes in their area. Out of the more than 190 informants that were contacted, roughly half responded initially. In many cases, the informants only had information for a subset of the lakes in question. These respondents were also asked if they could provide contacts for more relevant informants. Information on fish inventory data through grey literature was obtained for 77 of the lakes within the study area. This information was obtained by simple Google search where the name of the lake was combined with search words such as “fiske”, “fiskearter”, “prøvefiske”, “kartlegging”, “biologisk kartlegging”, “undersøkelse” and “biologisk undersøkelse”. For many of the lakes there was no sufficient information available online. Only reports, surveys and research from government bodies, independent research institutes and organizations which managed and sold fishing licenses were included. Absences were assumed if a fish species was not mentioned as being present in a given lake by any informat. In addition to occurrence status, means of establishment, probable year of introduction, occurrence remark, lake name, latitude and longitudes were compiled for each species. UUIDs were also produced to be used as UUID values. Additionally, lakes were matched to an event ID and location ID. All present species composition data from informants and grey literature was recorded consecutively in a custom-made fish inventory data app. This was constructed in RStudio using the shiny package. Further information regarding the app is available upon reasonable request from Anders Finstad. Data was downloaded from the app into a .csv file for further analysis. Lakes were categorized by county and municipality. Other information like lake ID, vernacular lake name (if existing), and information source was also included in the spreadsheet.