Discharge and temperature alterations trigger behavioral responses in early-life stages of trout
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Date
2025
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Doctoral Thesis
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Abstract
Over the past hundred years, the intensified construction and operation of hydropower facilities has substantially altered and fragmented river ecosystems globally. Combined with morphological modifications such as river channelization, these anthropogenic activities contributed to substantial degradation of freshwater ecosystems and their biodiversity−effects that are evident today. Nevertheless, striking a balance between societal needs and the conservation of freshwater ecosystems requires a nuanced understanding of how aquatic organisms respond to anthropogenic disturbances. Pumped storage hydropower plants have gained traction for their capability to produce electricity on demand and to store excess energy from intermittent renewable energy sources like wind and solar. This capacity to buffer rapid fluctuations in electricity production and demand plays a pivotal role in maintaining the stability of the electricity grid and its transition away from fossil fuels. However, the operation of storage hydropower plants can induce unnaturally rapid alterations in river discharge, known as hydropeaking. Hydropeaking can additionally alter the thermal regime of rivers, a phe- nomenon known as thermopeaking, should the temperature of the released water differ from the recipient river. In the worst case, these operationally induced disturbances can lead to stranding or uncontrolled drift of juvenile fish. The ecological severity of hydro- and thermopeaking is expected to be linked to the speed at which discharge, water levels, and water temperature change, as well as the ability of aquatic organisms to cope with these rapid spatio-temporal alterations.
In this thesis, I used controlled laboratory experiments to reproduce distinct discharge and temperature disturbances mimicking hydro- and thermopeaking, and quantified the movement behavior of the early-life stages of trout (Salmo trutta) to understand their capacity to respond to such alterations.
I set out to achieve that by developing a novel laboratory method to quantify the fine-scale movement behavior of fish in environments resembling natural habitats (Chapter 2). By combining a translucent gravel base layer with infrared illumination, the novel setup allowed for automated image-based fish tracking through the air-water interface. Then, I incorporated local depressions, cobbles and self-filling bluff bodies to mimic hydrodynamic heterogeneity present in the riverbed. Using this innovative approach, I quantified fish posture parameters (e.g. head orientation and tail offset) and trajectories that informed fine-scale movement responses elicited by fish to respond to the flow dynamics imposed during the experiments.
Given the success obtained with the setup I devised, in Chapter 2 I describe it in detail and discuss the challenges and recommendations to obtain high-quality fish movement data.
In Chapter 3, brown trout parr were exposed to hydropeaking conditions and their fine-scale movement responses were simultaneously quantified. A laterally inclined flume section was used to generate heterogeneous hydraulic conditions and enabled the continuous capture of individual positions and body postures of both wild- and hatchery-reared trout. The experiments revealed that juvenile fish negotiate the rapid variation in discharge by swiftly (within minutes) adjusting their spatial occupancy and exploratory behavior. During increase in discharge, fish moved perpendicular to the main flow direction to shallow areas as these became submerged. During peakflow, fish maintained position at low flow velocity zones around cobbles and exhibited swimming behaviors, including bow-riding and entraining, likely to min- imize energy expenditure. During the decrease in discharge following peakflow, fish abandoned areas falling dry by moving laterally. In the treatment with the higher down-ramping rate, the time to initiate relocation was lower while the relocation speed was higher. These findings establish a causal and quantitative relationship between down-ramping rate and relocation speed and underline the importance of particular flow regions (e.g. near cobbles) in which hydraulic conditions enable fish to use energy saving strategies during hydropeaking conditions.
In Chapter 4, brown trout parr, acclimated to 12 °C, were exposed to rapidly forming cold- and warm-water interfaces with temperatures ranging from 4 to 20 °C. Here, state-of-the-art tracking of individual fish was combined with the simultaneous visualization of the interface between warm and cold water in a lock-exchange flow tank. This allowed to precisely relate the fine-scale fish movement response to the position of the thermal interface. The experiments revealed that juvenile brown trout possess an exquisite capacity to rapidly respond to fine-scale temperature disturbances, avoiding exposure to cold water (≤8 °C) and exploiting access to warmer water (12−20 °C). Contrary to the current paradigm, based on long-term exposure experiments, that fish become physiologically impaired and swim more slowly when exposed to cold water, our results revealed a surprising combination of behaviors: when in cold-water regions, fish increased their swimming velocity and increased their rate of turning upwards (i.e. towards warmer water), thus combining thermokinesis and thermotaxis to avoid cold water exposure. In contrast, in warm-water treatments fish repeatedly crossed thermal interfaces towards water up to 8 °C warmer, without exhibiting any avoidance response, suggesting a behavioral strategy to exploit warm water to gradually adjust body temperature. Collectively, these findings provide critical insights into the fundamental aspects of movement behavior during hydropeaking and thermopeaking in natural streams, offering valuable information for habitat modeling and the development of mitigation strategies for hydropower operation. At the same time, this work introduces new methods for performing fish tracking experiments in the laboratory to study the behavioral response of fish to environmental stimuli, and these could be more broadly applied to a wider range of research and management questions. These findings will be of interest to a broad and interdisciplinary audience both for their scientific relevance to the fundamental understanding of fish movement responses and thermoregulation and for their potential to enhance the management of brown trout populations in hydropower affected rivers.
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Examiner : Stocker, Roman
Examiner: Boes, Robert
Examiner : Schmutz, Stefan
Examiner : Pinheiro, António
Examiner : Holzner, Markus
Examiner: M. Silva, Luiz G.
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ETH Zurich
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Subject
Hydropeaking; Ecohydraulics; Thermopeaking
Organisational unit
09467 - Stocker, Roman / Stocker, Roman