Are perch (Perca fluviatilis L.) getting larger or smaller in Swedish lakes?

Space- for-time approaches are often used to indicate current or future changes over time. A European gillnet standard facilitates spatial-scale comparisons of fish communities in European lakes. Fish size was generally lower and densities higher in warmer lakes, but less is known about trends over decadal time scales. We analysed the size of European perch ( Perca fluviatilis L.) in 2121 Swedish lakes sampled during 1996– 2021. The aim was to test whether size changed over time, and whether trends were similar in southern and northern regions. We analysed mean length and occurrence or relative abundance of size classes from the smallest ( < 100 mm) to the largest individuals (≥350 mm), and length at 1 and 5 years. The large data set was used to find general trends over time within regions, and within-lake trends were tested for 40 time-series lakes. The mean length of perch decreased in the southern and increased in the northern region. The proportion of the smallest perch increased