Optimizing seine net methodology for lake restoration: insights from seasonal and diurnal aggregations in roach (Rutilus rutilus)

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Abstract
Biomanipulation via removal of benthivorous and zooplanktivorous fish is a common method for restoring eutrophic lakes. By affecting zooplankton and nutrient resuspension, these fish increase algal biomass and reduce water clarity. Seine netting is widely used for fish removal, but its efficiency requires further assessment, particularly in relation to the target species’ behaviour. This study examines roach ( Rutilus rutilus ), a prevalent zooplanktivorous fish in European lakes, aiming to (1) estimate seine net efficiency by assessing overall removal efficiency and specific haul efficiency based on the spatial–temporal overlap between individual fish and net trajectories in small shallow lakes and (2) assess the behaviour of acoustically tagged adult roach at varying population densities to optimize removal timing and locations. Results showed a high overall removal efficiency (93.3%) across 23 hauls, a high specific haul efficiency of 88.8% ± 28.8%, and an overall low escape rate of 6.4%. Roach exhibited seasonal and diurnal aggregation patterns, dispersing in littoral zones during spawning, but forming dense pelagic aggregations in autumn mornings, especially at low densities. This period therefore appears optimal for cost-effective and repeated removal efforts. Further studies across different lake systems are necessary to generalize findings on roach aggregation and seine net efficiency.
Funding Information
  • Poul Due Jensens Fond (Grundfos Foundation)
  • Danmarks Tekniske Universitet

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