Har det skett en förändring i abborrens (Perca fluviatilis) tillväxt i sjön Tåkern mellan år 1978 jämfört med år 2019?

Sammanfattning: The growth in perch (Perca fluviatilis) is generally slow but varies depending on living conditions. For instance, growth is more rapid in warmer water temperature and when there is reduced intra- and interspecific competition for food. The cormorant (Phalacrocorax carbo) was established in Lake Tåkern in 1994 and could possibly have an effect on the growth of perch due to predation of small perch and competition for food with larger perch. The aim of this study was to examine whether the growth of perch had changed from year 1978, when a previous study on perch was preformed, to a recent fish survey in 2019 in lake Tåkern. The aim of the study was to assess if any changes in growth could be explained by predation and competition from cormorants. The aim of the study was also to examine whether there was a difference in growth between females and males. The age was determined by using the opercular bone and an age analysis was done to compare the growth between year 1978 and 2019. The results showed that the growth of 1-3-year-old perch was higher in year 1978 than in 2019. This was contrary to the expected outcome, that the predation from cormorants should had decreased competition for young perch in 2019. Previous studies on perch have shown a difference between males and females with an age over 2 years old. However, by the results there is no support that differences between males and females have affected the difference in growth at ages 1-3 years old between 1978 and 2019. As there was no difference between the sexes, any difference in growth between 1978 and 2019 could not be explained by an altered sex ratio. Since there were only a few older individuals 2019 comparison of growth for individuals over 4 years old were not possible to do. The conclusion is that there was no support that cormorants reduced the competition for smaller perch. Cormorants could possibly have contributed to low number of older individuals in 2019.