Spawning migration of grayling Thymallus thymallus (L., 1758) in a Northern German lowland river
Archiv für Hydrobiologie Volume 152 Number 1 (2001), p. 99 - 117
published: Jul 19, 2001
ArtNo. ESP141015201003, Price: 29.00 €
The spawning migrations of seven grayling Thymallus thymallus (L., 1758) externally tagged with radio transmitters were studied in the River Ilmenau (Lüneburg Heath, Lower Saxony) from 11 March to 16 April 1991. Upstream runs extended over a maximum distance of 11.3 km (mean ± S.D.: 8.2 ± 2.8 km), where the migrations of two grayling were even obstructed by weirs. Periods of active migration alternated with resting periods with home range behaviour. During periods of mobility the fish covered upstream distances of 1.15 km/d on average. Migratory activity was essentially diurnal, peaking around noon, whereas migratory activity during nighttime was generally low. No correlation was found between the observed movements and the river discharge, variations in water level or water temperature. Whilst resting periods during the ascent were characterized by activity ranges up to c. 100 m length (mean ± S.D.: 66 ± 51 m), the temporary activity ranges of resident grayling distinctly decreased during the following weeks (mean ± S.D.: 26 ± 17 m). Finally, most fish descended again towards the end of the study period and four grayling even demonstrated post-reproductive homing. The observed long distances of spawning migrations in the River Ilmenau seem to depend on the limited range of suitable spawning grounds for this lithophilous fish species. Therefore, impassable weirs in the river basin must be regarded as a main threat to grayling in this lowland river.