Structural equation models suggest that bottom‐up processes override top‐down processes in boreal pikeperch (Sander lucioperca) lakes

Abstract

Abstract Inter‐relations between water quality and piscivore populations have remained poorly studied but their quantification is crucial for the ecosystem‐based management of inland waters. Here, we aimed to estimate the relative strength of bottom‐up and top‐down trophic interactions between water quality and fish community composition using a comparative approach. Our main hypothesis was that lakes with an abundant piscivore (mainly pikeperch, Sander lucioperca, Percidae) stock would have better water quality than similar lakes with a poor piscivore stock. We used structural equation models on biomass catch per unit of effort data from standard gillnet test fishing to examine the effects of predators (pikeperch, pike [Esox lucius, Esocidae] and piscivorous perch [Perca fluviatilis, Percidae]) on cyprinids (Cyprinidae), and the effects of cyprinids, nutrients, and lake depth on water quality measured as chlorophyll‐a concentration and turbidity. We derived the dataset covering a single summer snapshot of 138 Finnish pikeperch lakes from national databases. Nutrients and lake depth index, the main principal component describing positive effects of nutrient concentrations and negative effects of lake depth, had a positive effect on chlorophyll‐a concentration and turbidity. Pikeperch had a detectable negative effect on both planktivorous and benthivorous cyprinids. While pike also had a negative effect on benthivorous cyprinids, both groups of cyprinids had a positive effect on pikeperch and pike. Supporting the strong, fish‐independent bottom‐up regulation, no significant relationships were detected between the cyprinids and water quality. This study suggests that the bottom‐up effects override the top‐down effects, and despite pikeperch is an efficient piscivore, the observed stock levels did not cascade down to improved water quality. Thus, in lake restoration, reductions of nutrient loading appear more efficient than biomanipulation aimed to strengthen the top‐down links.