Monitoring of European lake trout spawning in small tributaries

An on-going monitoring since 2005 of European lake trout spawners migrating into small tributaries (Q(347) < 100 l/s) of Lake Zurich and Lake Pfaffikon, Switzerland, revealed that migration only takes place during higher water flows due to the small size of the creeks. The catchment areas of the tributaries range from 4.9 to 15.1 km(2). The mean length of 402 spawners from five tributaries ranged from 45 to 55 cm with an overall mean of 52 cm for all individuals. It correlated positively with the low water flow of the respective tributaries. Compared to spawners from large tributaries of Swiss lakes, the mean length of the small creek spawners is far lower. All individuals were marked with fototagging for later individual recognition. Less than 10 % of the spawners could be observed in more than one spawning season. This low rate of multiple reproductions is in accordance with data from other Swiss lake trout populations and indicates that most of the lake trout spawners in Switzerland seem to be semelparous.