2009, Probst, Wolfgang Nikolaus, Eckmann, Reiner

The diel vertical distribution of young-of-the-year (YOY) burbot Lota lota in the pelagic zone of Lake Constance was compared to light intensity at the surface and to the light intensity at their mean depth. Lota lota larvae inhabited the pelagic zone of Lake Constance from the beginning of May until the end of August. From early June, after the stratification of the water column, fish performed diel vertical migrations (DVM) between the hypolimnion and epilimnion. The amplitude of DVM increased constantly during the summer and reached 70 m by the end of August. Lota lota started their ascent to the surface after sunset and descended into the hypolimnion after sunrise. As the YOY fish grew from May to August, they experienced decreasing diel maximum light intensities: in May and early June L. lota spent the day at light intensities >40 W m-², but they never experienced light intensities >01 W m-² after the end of June. From this time, L. lota experienced the brightest light intensities during dusk and dawn, suggesting feeding opportunities at crepuscular hours. The present study implies, that YOY L. lota in the pelagic zone of Lake Contance increased their DVMamplitude during the summer to counteract a perceived predation risk related to body size and pigmentation.

2009, Probst, Wolfgang Nikolaus, Thomas, Gregor, Eckmann, Reiner

The near-surface distribution of young-of-the-year perch was observed with a towed upward beaming echosounder system (SIMRAD EK60 with a circular 7◦ transducer). Perch aggregated densely in the epilimnion during daytime and dispersed evenly below the surface at night. Shoaling commenced in late June when perch metamorphosed from the larval to juvenile stage. Average shoal width was 6.6 m and average shoal height was 2.35 m in July, when perch were observed for the last time in the pelagic zone of Lake Constance.
Upward echosounding revealed the presence and near-surface distribution of pelagic juvenile perch and therefore this method can be used as a complementary survey tool to get more precise information about the distribution, behaviour and abundance of near-surface fish.

2009, Probst, Wolfgang Nikolaus, Stoll, Stefan, Hofmann, Hilmar, Fischer, Philipp, Eckmann, Reiner

The selection of spawning depth by Eurasian perch Perca fluviatilis was investigated in an experiment using artificial substrata in Lake Constance during the spawning season of 2007. The experiment compared spawning behaviour at substrata between 0.5 and 15 m depth at two sites exposed to different regimes of ship-generated wave action. The total abundance of egg ribbons did not differ significantly between the two sites, but the preferred spawning depth was deeper at the wave exposed site (5 m) compared to the sheltered site (2 m). While water temperatures could not account for the observations, differences in wave exposure may explain the different spawning depth preferences. At both sites, large egg ribbons were generally found in deeper water, and large egg ribbons occurred more frequently at the sheltered site. Because the egg ribbons of perch are likely to have a size-dependent susceptibility to hydrodynamic stress, large females may be expected to select deeper spawning locations where the effects of surface waves are considerably attenuated.