Non-invasive method of measuring resonance properties of fish swim bladders

Abstract

Fish face elevated mortality risks at hydraulic structures such as dams and hydropower turbines. In response, this thesis proposes a novel, non-invasive technique for measuring swim bladder resonance in live, free-swimming fish and evaluates whether resonance-specific acoustic stimuli can alter their behaviour. The research begins by establishing experimental protocols using surrogate bubbles, whereby inflated latex balloons mimic fish swim bladders. Laboratory tests confirm the ability to detect resonance peaks in these surrogates. Subsequently, recently euthanised fish serve to extend these measurements in a more biologically representative context, allowing factors such as swim bladder shape, volume, wall thickness, and damping factors to be included. Finite element modelling then helps validate measured resonance properties, matching theoretical expectations when realistic morphological parameters are applied. Building on these foundations, a custom cylindrical test tank was created for non-invasive in vivo measurements and behavioural studies. Acoustic tests in this facility confirmed that fish could swim freely and remain unanaesthetised throughout the measurement process, preserving natural buoyancy and movement. Resonance peaks were successfully measured and observed in individual fish, revealing notable inter-individual variation. In several cases, morphological factors such as body length, height, and weight were correlated with resonance frequency shifts, indicating that fish with larger body size or swim bladder volume often showed lower resonance frequencies. With the support of computed tomography, comprehensive finite element simulations of swim bladder resonance behaviours coupled with the frequency response of the test tank were performed to validate the results of the experimental measurements and achieved satisfying alignment. The final experiments explored whether fish, upon exposure to tonal signals tuned near their measured swim bladder resonance, would exhibit stronger startle reactions than under off-resonance conditions. Behavioural observations, supplemented by statistical modelling, demonstrated that fish responses were more pronounced when the stimulus frequency tuned closely to the measured swim bladder resonance. While not all individuals displayed equally robust reactions, a clear trend emerged, suggesting that resonance-driven oscillations of the swim bladder can heighten the perceptual or physiological response of the test fish. Collectively, these findings lay the groundwork for a more focused and potentially species-specific deterrent strategy. By enabling precise non-invasive in vivo resonance measurements, the method permits greater insight into the interplay of acoustic excitation, swim bladder oscillations, and fish behaviour. This approach also shows the potential for practical applications in endangered species conservation and fisheries management, where minimising fish mortalities and improving passage efficiency remain pressing concerns.

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Identifiers

ORCID for Paul White: orcid.org/0000-0002-4787-8713

ORCID for Paul Kemp: orcid.org/0000-0003-4470-0589

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