Characterization of Brown Trout Spawning Activities in Alpine River Systems

SEDRIVER is an interdisciplinary project that aims at detailing the possible consequences and effects of climate change on sediment transport and brown trout habitats in alpine and pre-alpine river systems. Climate change will have distinct effects on streams and brown trout especially in Switzerland, where alpine regions are prevalent. Primarily in winter, the ongoing climate change is expected to lead to an increase or an alteration in flooding and sediment transport. Due to this, accurate knowledge of spawning time, spawning behavior and egg burial depths of brown trout at different altitudes are crucial to pinpoint environmental changes in alpine and pre-alpine systems. However, such data is generally lacking in Switzerland and other countries within the Alpine region.

In the following study, five rivers of different drainage systems (glacial, nival, pluvial) and stream size at altitudes from 300 - 1600m were investigated. The main goal was to get broad information about  spawning activities of brown trout in Switzerland. The spawning time was determined by redd counting from September 2010 until January 2011. Discharge, water temperature and day length were recorded as parameters that might trigger the onset of spawning and determine the duration of spawning activity. Most of the published work concerning burial depths is dealing with river systems of lower altitudes. In this work, the focus was on rivers of a mean altitude of between 800m and 1500m. In these rivers, special conditions such as high slope, rapidly changing discharge and highly dynamic sediment transport are influencing fish populations. Over 170 redds were surveyed by measuring micro-habitat features (depth, velocity, substrate) and determining the egg burial depths.

Analysis of the data indicate an earlier onset of spawning for rivers at high altitudes. Changes in discharge and temperature show also significant correlations with the occurrence of redds. Surprisingly, egg burial depths were found to be distinctly smaller (mean burial depths: 37mm - 51mm) than reported by any study so far. Although the study rivers differed in substrate composition, spawning trout tend to use the same substrate sizes at redds in all rivers. Due to the different characteristics of the study rivers (e.g. river size, altitude, mesohabitat characteristic, discharge regime), habitat use with regard to velocity and water depth seems to cover the whole range of data reported so far.