Impact of the Warm Summer of 2018 on Growth of Roach (Rutilus rutilus) in Lake Tåkern, Sweden

Sammanfattning: Climate change will lead to higher temperatures and longer summers in the future, which will likely influence the growing season of fish living in temperate lakes. The warm summer of 2018 in Sweden matches prognoses for normal summers at the end of the century and can thus be used to investigate the effect of temperature related factors on fish growth. In this study I used back-calculation of the growth of roach (Rutilus rutilus) caught in Lake Tåkern, Sweden, to find differences in growth during 2018’s hot summer versus the period 2012-2017. I compared growth during these years with results from a similar study from Lake Tåkern in 1978. For this comparison I used 1977 as a representative year for the 1970’s. I applied sclerochronology to the scales to determine age and growth. The results show that growth in terms of length increment was faster in 2018 than in 2012 – 2016. The results indicate that 2018 had an effect on the whole roach population, since significant differences were found across age groups. Additional comparisons between 1977 and 2018 showed no significant difference. Differences in roach growth rate between 2012-2018 could be caused by the differences in mean temperature during the roaches growing season, since 2018 was abnormally warm, and the difference between 1997 and 2018 could be attributed to 1977’s fish death. This means that if the pattern of climate change continues, roach growth rates will increase in the future regardless of age group.