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Acoustic telemetry represents the state-of-the-art technology for monitoring behaviour of aquatic organisms in the wild. Yet, the performance of different systems is rarely evaluated across species and environments. In this study, we evaluate two different acoustic telemetry systems, a commonly used analogue pulse-position-modulation-based system (VEMCO PPM) and a newly developed high-residency digital binary phase shift key-based system (VEMCO HR2), in ability to track downstream migrating Atlantic salmon smolt (Salmo salar) and European eel (Anguilla anguilla) around hydropower facilities. High-precision GPS were used to evaluate precision and accuracy of hyperbolically positioned data derived from each system. The PPM-based system had higher detection range than HR2 and generated more positions per transmission for eels migrating close to bottom than for surface-oriented salmon smolts. HR2 generated tenfold more positions per time unit than PPM, were less sensitive to noise, achieved submetre positional precision, and were considerably more accurate than PPM-derived positions after filtering. HR2 was deemed more capable than PPM in fine-scale positioning at moderate distances at hydropower facilities.

La télémétrie acoustique représente la fine pointe de la technologie pour la surveillance des comportements d’organismes aquatiques dans la nature. La performance des différents systèmes de télémétrie est toutefois rarement évaluée pour différentes espèces et différents milieux. Nous évaluons deux systèmes de télémétrie acoustique, soit un système analogue basé sur la modulation de la localisation des impulsions (VEMCO PPM) couramment utilisé et un système numérique mis au point récemment à long temps de résidence basé sur la modulation binaire à déplacement de phase (VEMCO HR2), afin d’établir leur capacité de suivre des saumoneaux de saumon atlantique (Salmo salar) et des anguilles européennes (Anguilla anguilla)en dévalaison près d’installations hydroélectriques. Le GPS de haute précision est utilisé pour évaluer la précision et l’exactitude de données de localisation hyperbolique obtenues de chacun des systèmes. Le système PPM a une meilleure portée de détection que le système HR2 et produit plus de localisations par transmission pour les anguilles migrant près du fond que pour les saumoneaux qui demeurent plus près de la surface. Le système HR2 produit dix fois plus d’emplacements par unité de temps que le système PPM, est moins sensible au bruit, atteint une précision submétrique de la localisation et donne des emplacements considérablement plus exacts que le système PPM après filtrage. Les capacités du système HR2 sont jugées plus grandes que celles du système PPM pour la localisation à échelle fine à des distances modérées d’installations hydroélectriques.

Structures for guiding fish around migration barriers are frequently used for maintaining connectivity in regulated riverine systems. However, for non-physical barriers, experimental studies providing direct and detailed observations of fish–barrier interactions in rivers are largely lacking. In this study, we quantify the efficiency of bubble barriers (alone or in combination with light stimuli, and in both daylight and darkness) for diverting downstream migrating Atlantic salmon (Salmo salar). Both a laboratory-based migration experiment and a large-scale field experiment in a regulated river were used to evaluate efficiency of bubble barriers. In the latter, we used acoustic telemetry to provide in situ measurements of how downstream migrating Atlantic salmon smolts interact with bubble barriers. We show that bubbles divert smolts with high efficiency in both a laboratory flume (95%) and in natural settings (90%). This latter efficiency is higher compared to an already present physical barrier (46%) covering the upper two meters of the water column in the large river. The bubble barrier did not affect flume migration in darkness, suggesting that visual cues are crucial for the observed repelling effect of bubbles. We conclude that bubble barriers can be effective, largely maintenance free and low-cost alternatives to physical structures currently used to divert salmon away from high-mortality passages.

Salmonid repeat spawners are precious individuals for wild populations due to their high fecundity and previous spawning experience, making them important in environmental policy. However, repeat spawners rarely exist above hydropower dams in regulated rivers as the mortality of post-spawners (kelts) when passing through turbines during downstream migration is very high. To mitigate this problem, there are different technical solutions that potentially guide fish toward available fishways. Bubble barriers represent one alternative to costly physical guiding structures, but the efficiency of bubbles for guiding downstream migrating kelts has not been tested. In this study, we evaluate a 100 m long bubble barrier in guiding salmonids—both smolts and kelts—away from the main current and toward an alternative fishway in Ume River, a large regulated river in northern Sweden. We used both acoustic telemetry and sonar to measure the guiding effect of the bubble barrier for downstream migrating fish. We found that more than twice as many salmonids chose the alternative fishway when the bubble barrier was turned on. This was true both for smolts and kelts, suggesting that bubble barriers can be used to guide salmonids of different life stages in rivers with flow rates over 500 m3 s−1. Indeed, our study indicates that bubble barriers are low-cost structures that could be rapidly applied in many regulated rivers to support salmonid migration.