Suitability of motorized under-ice seining in selective mass removal of coarse fish

The efficiency of modern, fully motorized under-ice winter seining technique was studied in the selective mass removal of coarse fish in the eutrophicated Lake Vesijarvi, southern Finland in 1992–1994. The total catch in 57 seine hauls was 69000 kg and the average catch per haul 1210 kg (250–6500 kg). The average biomass removed from the fishing area per haul was 120 kg ha−1 (SD 116) in the deepest basins (depth > 15 m), where smelt (Osmerus eperlanus L.) dominated the catches. The corresponding catch was 192 kg ha−1 (SD 187) in a shallower basin (< 15 m) where the catch comprised mainly cyprinids, all age classes of bream (Abramis brama L.) and bleak (Alburnus alburnus L.) and juveniles (1 + and 2+) of roach (Rutilus rutilus L.) and bleak. The catches were composed of aggregated fish since there was no correlation between the length of the haul and weight of the catch and the biomass removed from the fishing area was high. The under-ice pelagic schooling pattern of fish distribution was confirmed by scanning sonar monitoring which was effectively utilized in fishing. Winter seining is a suitable method for selective fish removal both in fisheries management and in biomanipulation of temperate lakes which are ice-covered for several months during a year.