Colony location, colony stmcture, and population status of the Long-tailed Mayfly on certain sections of the upper Tisza river

Abstract

The geographic range of the Long-tailed Mayfly (Ephemeroptera: Palingenia longicauda), once widespread in Europe, is currently confined to the Tisza river system. Although the species has long been known for its spectacular swarm, exact data on its density or population size have been lacking to date. We mapped mayfly colonies on river Tisza in 11 sections threatened by riverbank erosion control projects and estimated population size by quantitative sampling of the larvae. The number of larvae was highest in sections where the riverbed was not altered by riverbank protection, was made up of soft or hard, eroding clay and had stable, relatively fast water flow. We found riverbanks inhabited by mayfly larvae on 44% of the total length surveyed (14,534 m). The proportion of colonies with high densities of larvae (1450-1500 individuals per m2) was 11%, that of average colonies (800-820 individuals per m2) was 28%, whereas low-density colonies (420-150 individuals per m2) made up 61 % o f the sections inhabited by mayflies. The total population of the studied sections was estimated at 40-50 million individuals, which corresponds to a nature conservation value of minimum 77 billion and maximum 102 billion Hungarian Forints. The most important effect threatening the mayfly population is erosion control works, because these projects eliminate the densest colonies and the most important habitat for the species.