Modelling dispersal patterns of fish larvae in a large river

Fish are widely used as indicators in riverine systems for ecological integrity, as they i.a. integrate large spatial scales during their life cycle by their different use of habitats. The linkage between spawning and nursery habitats is crucial for the sustainability of fish populations, as fish larvae present critical stages in the life cycle. Dispersal patterns of larval fluvial fish are investigated on the temporal as well as spatial scale by introducing individually marked fish of different ontogenetic stage at different hydro-morphological habitats in the main river channel of the Danube. Using an integrated habitat modelling approach the observed patterns are superimposed by particle traces derived from the application of a three-dimensional hydrodynamic model in order to define the triggers and mechanisms of larval dispersal patterns. Implications about the connectivity of spawning and nursery habitats are made based on this approach; this knowledge can be used in applied restoration ecology for large-scale restoration concepts.

Fish are widely used as indicators in riverine systems for ecological integrity, as they i.a. integrate large spatial scales during their life cycle by their different use of habitats. The linkage between spawning and nursery habitats is crucial for the sustainability of fish populations, as fish larvae present critical stages in the life cycle. A reduction in connectivity is assumed to be a main factor for population losses. Larval stages are most important for the size of fish stocks, their dispersal is crucial for the preservation of the population.Within this interdisciplinary project, the dispersal patterns of larval fluvial fish were analyzed by means of laboratory studies and field experiments. Mark recapture experiments showed that inshore geo- morphology as well as hydraulic factors affect larval drift and the settlement in nursery habitats. A high inshore retention, and an enhanced connectivity between adjacent habitats (spawning-nursery, nursery-nursery) was observed in natural shores when compared to stabilized and modified shores of the river. These findings should be considered in upcoming river management.In contrast to passive diffusion of artificial particles, even early developmental stages of fish show clear active components and a distinct rheoreaction. Based on the findings of the flume experiment, concerning life stage and velocity dependent movement patterns and durations, a larval model based on rheoreaction was developed. It inherits those findings which are used to simulate the type of movement pattern and its duration (frequency of change of movement patterns) of individual larvae at any simulated time step. Furthermore, the concept of the biased and correlated random walk model was modified to be in accordance with the actual movement pattern to represent rheoreaction of larvae. Therefore a new approach of reorientation and horizontal swimming speed was considered, whereby the intensity of the rheoreaction is essentially adjusted by the parameter sensing ability and thus effects the simulated larval trajectory. The new rheoreaction model has the potential to estimate larval dispersal patterns of freshwater fish larvae, where flow velocity is regarded as a main factor for orientation and navigation. Hence, it facilitates the understanding of dispersal processes in general. Furthermore, this method offers the chance to improve also larval models for marine environments.