Differences In Spawning Redds of Brown Trout In Geologically Different Streams

The redds of brown trout (Salmo trutta), the most common fish species of Austria, were monitored in two geologically different reaches in 2010 and 2011. The Große Mühl River (5^th order) originates in the Bohemian mountains (1.160 m.s.l.). River morphology is classified as ‘plane bed’, discrete bars are missing, the substrate is dominated by cobbles and sand, and gravel fractions are rare. The key bedrock features within the spawning sites are augen-gneiss, biotite and fine- to medium-grained granites. The second investigated stream, Weissenbach, is a 2^nd order headwater brook originating in a pre-Alpine (800 m.s.l.) limestone region of Lower Austria, with gravel dominated substrate.

In the Große Mühl River spawning peaks mid-October – mid-November whereas in the Weissenbach, it lasts from mid-November to January. Spawning trout are ca. 25-30 cm long in both streams but the excavated redds differ significantly in size and form. In the silicate stream, the average redd size varies around 2.1 m² while it is 0,7 m² in the limestone brook. Volumetric sediment samples were taken from the spawning sites, dried, sieved and characteristic grain sizes (d₉₀, d₈₄, d₅₀, d₁₆, d₁₀ and d_m) were determined for both. Additionally, different redd parameters (length/with of pot and tail, flow velocities, distance from the bank, neighbouring structures etc.) were measured. Temperature was recorded by data loggers.

The pronounced differences in the timing of spawning and size/form of spawning redds are thought to be due in part to local adaptations to different temperature- and flow regimes (e.g. stationary ice cover for 2 months at the Gr. Mühl River) however, the predominate determining factor is the different substrate quality present in these reaches.