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

Abstract

AbstractSpace‐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 in the southern, but not in the northern region. The most general trend was an increasing occurrence and proportion of the largest‐sized perch in both regions in the large dataset. The average trends in the southern region of the large dataset also appeared as predominant trends within time‐series lakes, although opposite trends occurred within some lakes in each region. This study also revealed that perch size at age 1 often increased within lakes over time, while more variable trends were found for size at age 5. The duration of the growth season has generally increased during the time frame of the present study, while other changes may differ between lakes in the same region. We recommend more studies based on time series of fish monitoring, including studies on possible reasons for the increased occurrence of very large perch in Swedish lakes.