Behavior of European Grayling in a Recently Colonized Scandinavian Lake

Lesjaskogsvatnet is a Norwegian lake that experienced a founder event in the late 19^th century that temporarily allowed an influx of European grayling (Thymallus thymallus), a typically fluvial palearctic salmonid. While adult grayling reside in the lake for most of the year, they require stream habitat for spawning. Over twenty streams with potential spawning habitat feed into Lesjaskogsvatnet and can be categorized as either “warm” or “cold” based on water temperatures during the spawning season. Previous experiments  revealed that juvenile grayling from the cold streams grow faster than those of the warm streams to compensate for later spawning. The mechanisms that contributed to and maintain this case of adaptive divergence in life history traits remain unknown. A previous genetic study moreover revealed strong gene flow among the local populations; but there is to date no knowledge about the spatial admixture of individuals during most of the year in the lake. Therefore we conducted an observational telemetric study of adult grayling with the following objectives: (1) quantify individual dispersal and “straying” from spawning streams, (2) compare habitat preferences and movement behavior between individuals from the cold and warm streams, and (3) provide novel information on the lacustrine habitat use and behavior of an adfluvial population of grayling.  In the spring of 2009, 38 mature grayling were tagged with acoustic transponders during their spawning migration in 3 streams, two “warm” type and one “cold”. Transponder data (including presence, depth, and temperature) was recorded by five acoustic receivers anchored throughout the lake. Preliminary results indicate that the nine fish with usable data showed a great diversity of behaviors, occupying different depths in the water column and sometimes showing clear patterns of daily vertical migration. All of the three fish tracked across seasons showed a dramatic shift in water depth after the lake froze, residing close to the lake surface throughout the winter. While individuals were usually detected at the transceiver closest to the stream where they were tagged, nearly half were detected at further transceivers and two individuals were detected on the opposite side of the lake from where they were tagged (>5 km).  Based on the diversity of occupied depths and vertical migratory patterns we observed, our preliminary results indicate that grayling may occupy different behavioral niches in Lesjaskogsvatnet.