Betydelse av fiskens ålder vid bedömning av ekologisk status

The aim was to show how fish age complements other fish metrics used for assessment of ecological status. As fish age is seldom reported from current environmental monitoring programs, available knowledge was summarised, for age determination, age structure, recruitment and growth of fish species frequently occurring in Swedish lakes. Examples included perch (Perca fluviatilis), bream (Abramis brama), ruffe (Gymnocephalus cernua), roach (Rutilus rutilus), smelt (Osmerus eperlanus), Arctic charr (Salvelinus alpinus), whitefish (Coregonus lavaretus), vendace (Coregonus albula) and brown trout (Salmo trutta). Age structure was indicated as proportion of young fish and maximum age. The proportion of young fish was lower in acidic than in neutral lakes, and it decreased at higher latitude. Within species, the oldest fish was usually younger than the largest fish, and maximum age increased with latitude. For annual samples, a recruitment index (RI) was based on abundance within year classes, estimated using data from three subsequent years. RI revealed better relationships between recruitment and water quality, compared to presence or absence of small roach. Fish length at age was not clearly related to fishing pressure, acidity or climate, probably because fish growth is often limited by intra- and inter-specific competition. Back-calculated first year growth of perch was a more promising indicator for monitoring of climate change. In three case studies, age data increased the understanding of why simpler fish community metrics fluctuated or changed over time. The case studies further indicated some weakness in the current assessment criteria. Some reference values from multiple regression models could be questioned, supported by historical knowledge and data in time series. With more data on natural variation in the proportion of young fish, this kind of metric could be included in improved assessment criteria for ecological status.