Widespread long-term declines of littoral areas in protected and unprotected Czech fishponds

Abstract

Fishponds play a key role in current pondscapes in many developed countries. They provide multiple ecosystem services affected by trade-offs mediated by different types of fishpond management. Many fishponds were impacted by the shift towards more intensive aquaculture and widespread eutrophication in the 20th century. These changes can undermine various ecosystem functions including the maintenance of aquatic and riparian biodiversity in the littoral areas, but their extent is not well documented. Here we use aerial photographs to investigate the changes of littoral areas of 46 protected and 20 unprotected fishponds in the Czech Republic between the years 1950 and 2019. Protected fishponds had initially larger littoral areas than the unprotected ones, and the difference persisted over time. However, littoral areas decreased markedly in 38 protected (mean area decrease by 22.5%) and all unprotected (mean area decrease by 16%) fishpond categories, especially during the second half of the 20th century. Within protected fishponds, the trend was unaffected by the reserve establishment year, fishpond area and conservation target. Our results suggests that legal protection did not prevent the initial catastrophic loss of littoral areas and facilitated at most minor recovery in the last two decades, with negative implications for long-term maintenance of aquatic diversity. We attribute the losses of fishpond littoral areas to high fish stocks and eutrophication resulting from additional feeding, pond manuring, and ongoing nutrient inputs from the pond catchments. We conclude that littoral areas of fishponds urgently need effective protection. This would require a paradigm shift towards less intensive fish stock management, more frequent summer drainage, and effective reduction of all nutrient inputs to increase the water quality. Such measures could help recover the littoral areas and the associated biota.