Abstract
The dispersal of fish larvae in rivers might result from water movement but also from larval behaviour. Although potentially crucial for dispersion, knowledge of the role of behaviour is still fragmentary. This study intends to contribute to the question of how riverine fish larvae drift or move. All dispersal-relevant movement patterns of larvae of a characteristic rheophilic species were analyzed based on the parameters (i) swimming activity, (ii) direction of movement, and (iii) the orientation towards the current vector. Experiments were conducted in a novel flume mesocosm at three different flow scenarios covering the current velocity range of natural habitats. Mean current velocities in these scenarios were under, near, and over the “critical current velocity”, above which fish larvae are not able to constantly hold their position in the water column. Three consecutive larval stages were tested to account for possible ontogenetic shifts in movement behaviour, both during the day and at night. Our results strongly suggest that the assumption of mainly passively drifting larvae has to be refused; in total, 92.6% of all observed movement events were characterized by swimming activity and directed orientation, whereas only 7.4% could be assigned to passive drift. During downstream movement, a significant portion of movement events (57.1%) was attributed to larvae that orientated in an upstream direction and performed active swimming movements.
Highlights
Dispersal of fish larvae in lotic environments was often assumed to be a primary consequence of water movement (Wolter and Sukhodolov 2008)
Basic behavioural features of fish larvae have already been successfully incorporated into elaborate 3D models of physical–biological interactions, which have increasingly become an integral tool for understanding larval fish dynamics in the sea (Gallego et al 2007), while in rivers
The flow scenario was a significant factor for the proportions of most movement patterns (mP), significance was detected for active upstream (Au), active–passive downstream (Ap), passive downstream (P), traversing (T), and the cumulative proportion of all downstream movements
Summary
Dispersal of fish larvae in lotic environments (streams, rivers, estuaries, and marine habitats) was often assumed to be a primary consequence of water movement (Wolter and Sukhodolov 2008). A growing number of studies demonstrate that other factors beside the movement of water may be responsible for dispersal outcomes in fish larvae (Leis 2007). The importance of larval behaviour for dispersal has been recognized during the past decade (Fiksen et al 2007; Gallego et al 2007; Leis 2007), and the need to break “the behavioural black box” has been realized (Pineda et al 2007). Basic behavioural features of fish larvae have already been successfully incorporated into elaborate 3D models of physical–biological interactions, which have increasingly become an integral tool for understanding larval fish dynamics in the sea (Gallego et al 2007), while in rivers
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