The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early stages of perch (Perca fluviatilis) and pikeperch (Sander lucioperca) in the bathypelagic layer of a canyon-shaped reservoir

The long-term effort of the Fish Ecology Unit is obtaining of the "true picture" of the fish stock in lakes and reservoirs. Absolutely essential for its finding is the knowledge of the efficiency of our sampling techniques. In continuity on the former attempts to observe the efficiency of fish sampling tools we focused on the avoidance behavior of the earlier stages of percid fry, which often utilize deep (bathypelagic) layers of the water column during the day. The efficiency of different sized (0,5m x 2m, 1m x 2 m, 2m x 2m, 4m x 2m) and coloured (black, white) ichthyoplankton trawl nets was investigated during daytime in the bathypelagic layer of the Římov Reservoir (Czech Republic) in late May 2008. Eight to 16 mm long perch (Perca fluviatilis) and pikeperch (Sander lucioperca) fry were the dominant species in fry catches in this layer. We did not find any significant differences between the densities of either species when black and white nets were compared or when different sized nets were compared. Neither were there any significant differences in body lengths of the fish caught in the different trawls. Our results indicate that both perch and pikeperch fry in the bathypelagic layer are quite passive, without observable avoidance behaviour to the ichthyoplankton trawl nets. This is in contrast to active avoidance of the same species and sizes previously observed in the epipelagic layer.

Jůza, T., Čech, M., Kubečka, J., Vašek, M., Peterka, J., Matěna, J. 2010. The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early fish stages. Fisheries Research 105: 125-133.

Jůza, T., Čech, M., Kubečka, J., Vašek, M., Peterka, J., Kratochvíl, M., Frouzová, J., Matěna, J. 2012. The influence of the trawl mouth opening size and net colour on catch efficiency during sampling of early stages of perch (Perca fluviatilis) and pikeperch (Sander lucioperca) in the bathypelagic layer of a canyon-shaped reservoir. Fisheries Research 123-124: 21-25.