Habitat and movement selection processes of American lobster/jakej within a restricted bay in the Bras d'Or Lake/Pitu'paq, Nova Scotia, Canada.

American lobster inhabit the unique, brackish Bras d'Or Lake system, although densities are low compared to areas with similar habitats in the Atlantic Ocean. Nevertheless, lobsters are an important part of local First Nation (Mi'kmaq) food and culture. We used acoustic telemetry and habitat mapping, combined with local Mi'kmaw knowledge, to document the movements and habitat use of adult lobsters within a section of the Lake. Movement patterns of acoustically tagged individual lobsters were analyzed with both resource selection functions and integrated step selection functions using data obtained from a high-resolution VEMCO Positioning System within a restricted bay in the Bras d'Or Lake. The resource selection function suggested stronger selections of substrates that contained a combination of soft and hard sediments. While the integrated step selection functions found substantial individual variability in habitat selections, there was a trend for lobsters to exhibit more resident behaviour on the combined soft/hard substrates despite the fact these sediments provided little in the way of obvious shelters for the animals. Adult lobsters at this site have very little risk of predation, which presumably allows them to freely exhibit exploratory behaviours and reduce their association with substrates that provide shelters.

Performance and detection range of acoustic receivers in mangrove habitats.

Acoustic telemetry has been used to monitor the movement of aquatic animals in a broad range of aquatic environments. Despite their importance, mangrove habitats are understudied for the spatial ecology of elasmobranchs, with acoustic telemetry rarely used inside mangrove habitats. One reason for this may be a general assumption that acoustic signals would not be able to be detected by receivers in such shallow, structurally complex, environments. This study tested whether acoustic receivers can be used inside mangrove habitats to track the movement of sharks and rays. Thirty-eight receivers were deployed in a mangrove system in Pioneer Bay, Orpheus Island, Great Barrier Reef, including inside mangroves, mangrove edges, and adjacent reef flat areas. The detection range and receiver performance metrics, such as code detection efficiency, rejection coefficient, and noise quotient, were examined and tested among habitats. The results highlighted that the signal from transmitters was successfully detected inside mangrove habitats as well as on the adjacent reef flat. The range to detect at least 50% of transmissions was up to 20 m inside mangroves and up to 120 m outside mangroves. The performance metrics of acoustic receivers inside the mangrove habitat were characterized by low background noise, low rejection rates, and reasonably high code detection efficiency. Furthermore, this study tested the application of this method on juvenile blacktip reef shark Carcharhinus melanopterus and mangrove whipray Urogymnus granulatus, and demonstrated that it can be used to successfully track animals inside mangrove habitat. This novel method could reveal further information on how sharks and rays use mangrove habitats.

Temperature and body size affect movement of juvenile Atlantic cod (Gadus morhua) and saithe (Pollachius virens) at nearshore nurseries.

Seasonal migrations of marine fish between shallow summer feeding habitats and deep overwintering grounds are driven by fluctuations in the biotic and abiotic environment as well as by changes in the internal state. Ontogenetic shifts in physiology and metabolism affect the response to environmental drivers and may lead to changes in migration timing and propensity. In this study, we investigated the effect of temperature and body size on migration timing and depth distribution in acoustically tagged Atlantic cod, Gadus morhua, and saithe, Pollachius virens, during the period of seasonal migration from shallow summer habitats. The results from our study revealed a wide range of horizontal and vertical distribution of age 1 and 2 G. morhua within the fjord. Larger G. morhua inhabited deeper, cooler waters than smaller juveniles, likely reflecting size-dependent thermal preferences and predation pressure. Conversely, juvenile P. virens occupied primarily shallow waters close to land. The variation in depth distribution of G. morhua was mainly explained by body size and not, against our predictions, by water temperature. Conversely, the dispersal from the in-fjord habitats occurred when water temperatures were high, suggesting that seasonal temperature fluctuations can trigger the migration timing of P. virens and larger G. morhua from summer habitats. Partial migration of small juvenile G. morhua from in-fjord foraging grounds, likely influenced by individual body condition, suggested seasonal migration as a flexible strategy that individuals may use to reduce predation and energetic expenditure. Predation mortality rates of tagged juveniles were higher than previously suggested and are the first robust predation mortality rates for juvenile G. morhua and P. virens estimated based on acoustic transmitters with acidity sensors. The results have relevance for climate-informed marine spatial planning as under the scenario of increasing ocean temperatures, increasing summer temperatures may reduce the juveniles' resource utilization in the shallow summer nurseries, resulting in lower growth rates, increased predation pressure, and lower chances of juvenile winter survival.

Validation of a new acoustic telemetry transmitter for the study of predation events in small fishes.

Acoustic telemetry has emerged as an important tool for studying the movement and behavior of aquatic animals. Predation-sensing acoustic transmitters combine the functions of typical acoustic transmitters with the added ability to identify the predation of tagged animals. The objective of this paper was to assess the performance of a newly miniaturized acid-based predation-sensing acoustic transmitter (Innovasea V3D; 0.33 g in air). We conducted staged predation events in the laboratory where acoustically tagged rainbow trout (Oncorhynchus mykiss) were fed to largemouth bass (Micropterus nigricans) at 3.3-7.0, 9.0-10.8, 16.0-20.0, and 22.0-25.8°C. We also conducted false-positive tests where tagged rainbow trout were held at 10.0 and 16.8°C without the risk of predation. Predation events were successfully identified in 92% of the staged predation trials. Signal lag (i.e., the time required for a predation tag to indicate that predation occurred) ranged from 0.11 to 6.29 days and decreased strongly with increasing water temperature and increased with increasing body mass of the tagged prey. Tag retention in the gut of the predator was much more variable than signal lag and was influenced by water temperature and individual predators but not by prey mass. No false positives were detected after 60 days at either temperature (n = 27 individuals). Although the relationships between water temperature, signal lag, and retention time are likely species-specific, the data reported here provide useful information for the use of these transmitters to study predation in wild fishes, especially for temperate, freshwater fish.

Marine predators segregate interspecifically by space and time in a sheltered coastal bay.

Marine predators are vital to the healthy functioning of coastal ecosystems, but to understand their roles, it is necessary to elucidate their movement ecology, particularly in relation to one another. A decade's worth of acoustic telemetry data (2011-2020) from Algoa Bay, South Africa, was investigated to determine how two mesopredatory species (teleosts: dusky kob Argyrosomus japonicus, n = 11, and leervis Lichia amia, n = 16) and two top predatory species (sharks: ragged-tooth sharks Carcharias taurus, n = 45, and white sharks Carcharodon carcharias, n = 31) used and shared this bay ecosystem. Multi-annual seasonal fidelity to the bay was exhibited by all species, but differences in residency were observed among species. Similarly, species used space in the bay differently-the teleosts moved less and had movements restricted to the central and western inshore regions of the bay. Conversely, the sharks roamed more, but detections were concentrated in the western part of the bay for C. taurus and in the eastern part of the bay for C. carcharias. Social network analysis showed that species segregated in space and time on a fine scale. However, there was some interaction observed between C. taurus, L. amia, and A. japonicus, but to varying degrees. This is likely because of strong habitat preferences exhibited by each species and predator-prey relationships between these predatory guilds. Results highlight that the sheltered marine Algoa Bay is a resource-rich environment, supporting multiple predators with different hunting strategies albeit similar prey preferences. Finally, these species are likely afforded some protection by the current Greater Addo Elephant National Park Marine Protected Area in the bay but are vulnerable to fishing pressure when they leave this ecosystem.

Larger yellow-phase Japanese eels show short-range homing in a freshwater river.

The homing behavior and site fidelity to habitats in various fishes, including anguillid eels (genus Anguilla), are fascinating. However, little is known about how yellow-phase eels exhibit homing behavior and the sensory mechanisms involved. Using acoustic telemetry, we investigated the homing behavior of 18 Japanese eels, A. japonica, with total lengths ranging from 204 to 570 mm, in a narrow freshwater river in inland central Japan, where salinity gradient, tidal current, and magnetic sense cannot be used for their homing, but where olfaction could play a role. The tagged eels captured upstream and downstream were released downstream and upstream, respectively. The results showed that large eels, over approximately 400 mm in total length, exhibited homing behavior to their original sampling locations (likely to shelters and foraging sites, where they probably spent a longer time than in other locations and grew successfully) from outside their home ranges, predominantly during the dark period. Homing success was not affected by the two capture locations, indicating that eels did not use olfactory cues for short-range homing in freshwater rivers.

Seasonal occurrence and individual variability of bull sharks, Carcharhinus leucas, in a marine reserve of the southwestern Gulf of California.

Background: Studying how the bull sharks aggregate and how they can be driven by life history traits such as reproduction, prey availability, predator avoidance and social interaction in a National Park such as Cabo Pulmo, is key to understand and protect the species. Methods: The occurrence variability of 32 bull sharks tracked with passive acoustic telemetry were investigated via a hierarchical logistic regression model, with inference conducted in a Bayesian framework, comparing sex, and their response to temperature and chlorophyll. Results: Based on the fitted model, occurrence probability varied by sex and length. Juvenile females had the highest values, whereas adult males the lowest. A strong seasonality or day of the year was recorded, where sharks were generally absent during September-November. However, some sharks did not show the common pattern, being detected just for a short period. This is one of the first studies where the Bayesian framework is used to study passive acoustic telemetry proving the potential to be used in further studies.

Long-distance migrations and seasonal movements of meagre (Argyrosomus regius), a large coastal predator, along the Iberian Peninsula coast.

BACKGROUND: The meagre, Argyrosomus regius, is a large coastal predatory fish inhabiting waters from the north-eastern Atlantic and Mediterranean Sea, where it is targeted by commercial and recreational fisheries. Previous genetic studies have found an unexpectedly high population differentiation not only between the Atlantic and the Mediterranean, but also along the Atlantic coast. However, the reasons underpinning this genetic barrier remained unclear. Likewise, even though the species is amongst the world's largest marine teleosts, knowledge about its movement ecology and migratory behaviour remains notably scarce, and primarily reliant on fisheries-dependent data. METHODS: In this study, we used a combination of acoustic telemetry and pop-up satellite archival tags to investigate the movements of 22 adult meagre (70-143 cm total length) along the Southwestern coast of the Iberian Peninsula. RESULTS: Our results strongly suggest that the previously reported genetic differentiation is not maintained by limited adult dispersal/movement, as hypothesized. On the contrary, we documented some of the longest individual annual migrations ever recorded for a coastal teleost, up to > 2000 km, with frequent back-and-forth movements between the West and Southern Iberian coasts. Moreover, their detected regional movement patterns support the existence of a marked seasonal behavioural shift, with individuals being less active and moving to deeper waters during winter, and are consistent with spawning philopatry associated to their summer reproductive movements. Finally, we identified putative aggregation areas that may harbour important feeding/overwintering grounds. CONCLUSIONS: These findings shed new light on the movement and behaviour patterns of meagre that may be of particular importance for the conservation and spatial management of this species throughout its range, and open the door to further research on functional connectivity.

Abiotic drivers of the space use and activity of gray reef sharks Carcharhinus amblyrhynchos in a dynamic tidal environment.

Predators display rhythms in behavior and habitat use, often with the goal of maximizing foraging success. The underlying mechanisms behind these rhythms are generally linked to abiotic conditions related to diel, lunar, or seasonal cycles. To understand their effects on the space use, activity, and swimming depth of gray reef sharks (Carcharhinus amblyrhynchos), we tagged 38 individuals with depth and accelerometer sensors in a French Polynesian atoll channel exposed to strong tidal flow, and monitored them over a year. C. amblyrhynchos used a larger space during nighttime and were more active at night and during outgoing currents. Shark activity also peaked during the full and new moons. The swimming depth of sharks was mostly influenced by diel cycles, with sharks swimming deeper during the day compared to nighttime. The dynamic energyscape may promote the emergence of discrete behavioral strategies in reef sharks that use the south channel of Fakarava for resting and foraging purposes. Turbulence imposed by outgoing tides induces additional foraging cost on sharks, shifting their hunting areas to the southern part of the channel, where turbulence is less pronounced. Understanding when and where sharks are active and foraging is important for our understanding of predator-prey dynamics and ecosystem dynamics. This study highlights how abiotic rhythms in a highly dynamic environment likely generate spatiotemporal heterogeneity in the distribution of predation pressure.

Tagging mortality in acoustic telemetry studies: Insights from a multispecies analysis.

The widespread adoption of acoustic telemetry has transformed our understanding of marine species' behavior and movement ecology. However, accurately interpreting telemetry data, especially concerning tagging mortality, is essential for drawing valid conclusions. In this study, we scrutinized tagging mortality in 223 individuals across 14 species and evaluated the impact of tagging methodologies, including capture method and size effects. Results reveal high tagging survival rates overall, attributable to the resiliency of the studied species and the rigorous animal welfare protocols followed during tagging procedures. Our results highlight the importance of tailoring array designs to the specific mobility patterns of the studied species for accurate survival assessments. This research contributes to generalizing mortality assessments and clearing the path for more precise and reliable telemetry studies in the future.

Spatial connectivity of reef manta rays across the Raja Ampat archipelago, Indonesia.

The reef manta ray Mobula alfredi is present throughout most island groups that form the Raja Ampat archipelago, Indonesia. The species is protected regionally and nationally and is currently managed as a single homogeneous population within the 6.7 million ha archipelago. However, scientific evidence is currently lacking regarding the spatial connectivity and population structure of M. alfredi within this archipelago. Using network analysis and an array of 34 acoustic receivers deployed throughout Raja Ampat between February 2016 and September 2021, we examined the movements of 72 subadult and adult M. alfredi tagged in seven regions of Raja Ampat. A total of 1094 M. alfredi movements were recorded and were primarily concentrated between nearby receiver stations, highlighting frequent local movements within, and limited long-distance movements between regional acoustic receiver arrays. Network analysis revealed highly connected nodes acting as hubs important for M. alfredi movements. A community detection algorithm further indicated clusters within the network. Our results suggest the existence of a metapopulation comprising three demographically and geographically distinct subpopulations within the archipelago. They also reveal the importance of Eagle Rock as a critical node in the M. alfredi movement network, justifying the urgent inclusion of this site within the Raja Ampat marine protected area network.

Coexistence, resource partitioning, and fisheries management: A tale of two mesopredators in equatorial waters.

Rock hind (Epinephelus adscensionis) and spotted moray (Gymnothorax moringa) are ubiquitous mesopredators that co-occur in the nearshore waters of Ascension Island in the South Atlantic Ocean, where they have significant cultural and subsistence value, but management of their non-commercial take is limited. This isolated volcanic system is home to high biomass and low species diversity, which poses two key questions: How can two mesopredators that perform similar ecological roles coexist? And if these two species are so ecologically similar, can they be managed using the same approach? Here, we combined acoustic telemetry, stomach content analysis, and stable isotope analysis to (i) explore space use and diet choices within and between these two species and (ii) to assess appropriate species-specific management options. Although rock hind had high residency and small calculated home ranges (0.0001-0.3114 km2), spotted moray exhibited shorter periods of residency (<3 months) before exiting the array. Vertical space use differed significantly across the 20-month tracking period, with individual differences in vertical space observed for both species. A hierarchical generalized additive model using 12-h averaged depth data identified that rock hind occurred lower in the water column than spotted moray, with both species occupying moderately deeper depths at night versus day (+1.6% relative depth). Spotted moray depth was also significantly predicted by lunar illumination. Aggregating samples by species and tissue type, Bayesian ecological niche modeling identified a 53.14%-54.15% and 78.02%-97.08% probability of niche overlap from fin clip and white muscle, respectively, whereas limited stomach content data indicated a preference for piscivorous prey. Variability in niche breadth between years suggests these species may exploit a range of prey items over time. These findings indicate that although these two species perform a similar ecological role by feeding on prey occupying the same trophic levels, subtle differences in movement behaviors between them suggest a one-rule-fits-all management approach is not likely the most effective option.

Emigration of post-spawned twaite shad Alosa fallax, an anadromous and iteroparous fish, in a highly fragmented river.

Anthropogenic barriers are widely known to negatively impact the spawning migrations of anadromous fishes, by delaying or preventing passage upstream. Although the impacts of barriers on emigrating post-spawned adults are less well studied, they could potentially impact the fitness and subsequent return rates of iteroparous species. In this study, passive acoustic telemetry was used to track the emigrations of 53 twaite shad Alosa fallax in the River Severn basin in their first spawning migration a year after being tagged, giving insights into their emigration movements and the impacts of anthropogenic weirs on these movements. A. fallax began their emigrations after spending varying amounts of time and migrating various distances within the river, with late-emigrating individuals moving fastest and most directly. Emigrations became faster and more direct the further downstream individuals were from their furthest upstream extent. Downstream passage delays at weirs increased emigration times by a median of 61%, with environmental conditions (i.e., temperature, flow, and tidal influence on river level) having little influence on downstream passage at weirs with no modifications to facilitate fish passage. As weir-induced emigration delays are suggested to deplete energy reserves (when energy levels are already depleted post-spawning), limit spawning opportunities (by preventing access to downstream spawning habitat), and expose individuals to increased predation risk and suboptimal conditions (e.g., high temperatures), these delays can potentially diminish the benefits of iteroparity. The evidence presented here suggests that more consideration should be given to the potential impacts of anthropogenic barriers on the emigrations of iteroparous species when assessing river connectivity or undertaking barrier mitigation.

Diamond Gymnura natalensis and duckbill Aetomylaeus bovinus rays undertake nationwide coastal migrations.

Migration is a critical aspect of ocean ecosystems, and understanding this phenomenon answers ecological and management questions. Given the difficulty in tracking ocean animals across large distances, the extent to which different ray species perform long-distance movements, such as migrations, remains unknown. This study used passive acoustic telemetry to track the movements of endemic diamond Gymnura natalensis and critically endangered duckbill Aetomylaeus bovinus rays along the South African coastline using a collaborative nationwide network of coastal acoustic receivers for up to 7 years. Duckbill rays were detected significantly more frequently than diamond rays, but both species moved between the south and east coasts of South Africa (traveling up to 1167 km). Tagged individuals were detected significantly more often in their tagging locations during summer months but traveled significantly further distances during winter months. Furthermore, movement models fitted to individual duckbill rays' annual net-squared displacement identified most individual annual movements as migratory. This evidence suggests that both diamond and duckbill rays make eastward winter migrations and return to specific areas along the coastline during the summer months. The exceptions to this were diamond rays tagged on the east coast that were not found to migrate seasonally, which supports previous research that there is intraspecific variability in migrations for ray species. These findings have implications for understanding ray migration not only on a global scale but also locally for spatial management interventions and population delineation.

Early signals of Posidonia oceanica meadows recovery in a context of wastewater treatment improvements.

Natural ecological restoration is a cornerstone of modern conservation science and managers need more documented "success stories" to lead the way. In French mediterranean sea, we monitored Posidonia oceanica lower limit using acoustic telemetry and photogrammetry and investigated the descriptors driving its variations, at a national scale and over more than a decade. We showed significant effects of environmental descriptors (region, sea surface temperature and bottom temperature) but also of wastewater treatment plant (WWTP) effluents proxies (size of WWTP, time since conformity, and distance to the closest effluent) on the meadows lower limit progression. This work indicates a possible positive response of P. oceanica meadows to improvements in wastewater treatment and a negative effect of high temperatures. While more data is needed, the example of French wastewater policy should inspire stakeholders and coastal managers in their efforts to limit anthropogenic pressures on vulnerable ecosystems.

Migration timing and marine space use of an anadromous Arctic fish (Arctic Char, Salvelinus alpinus) revealed by local spatial statistics and network analysis.

BACKGROUND: The ice-free season (typically late-June to early-October) is crucial for anadromous species of fish in the Arctic, including Arctic Char (Salvelinus alpinus), which must acquire adequate resources for growth, reproduction, and survival during a brief period of feeding in the marine environment. Arctic Char is an important food fish for Inuit communities across the Arctic. Understanding drivers and patterns of migration in the marine environment is thus essential for conservation and management of the species. METHODS: We used passive acoustic telemetry to characterize migration patterns of 51 individual anadromous Arctic Char during the ice-free season in the marine environment of Coronation Gulf (Nunavut, Canada; 2019-2022). Based on recent genetic evidence, some tagged individuals were likely Dolly Varden (Salvelinus malma malma), a closely related species to Arctic Char. Using local Getis G* and network analysis, we described movement patterns and identified high-use locations in the marine environment. We also related freshwater overwintering location to migration timing and movement pattern. RESULTS: Comparing groups of fish that overwintered in distinct locations, we found: (i) limited evidence that marine movements were associated with overwintering location; (ii) minor differences in use of marine space; and, (iii) timing of freshwater return differed significantly between overwintering groups, and was related to length and difficulty of the migratory pathway in freshwater. Results from both network analysis and local Getis G* revealed that, regardless of overwintering location, coastal locations were highly used by fish. CONCLUSIONS: Overwintering locations, and the migratory routes to access overwintering locations, affect the timing of freshwater return. Preference of fish for coastal marine locations is likely due to abundance of forage and patterns in break-up of sea ice. Similarities in marine space use and movement patterns present challenges for managing this and other mixed stock fisheries of anadromous Salvelinus spp. Absences or periods of time when fish were not detected prevented comprehensive assessment of movement patterns. Local Getis G*, a local indicator of spatial association, is a helpful tool in identifying locations associated with absences in acoustic telemetry arrays, and is a complementary method to network analysis.

Balancing risks and rewards of alternate strategies in the seaward extent, duration and timing of fjord use in contemporary anadromy of brown trout (Salmo trutta).

BACKGROUND: Anadromy comprises a successful life-cycle adaptation for salmonids, with marine migration providing improved feeding opportunities and thus improved growth. These rewards are balanced against costs from increased energy expenditure and mortality risk. Anthropogenic-induced environmental changes that reduce benefits and/or increase costs of migration e.g., aquaculture and hydropower, may therefore result in adaptations disfavouring anadromy. We tagged brown trout (Salmo trutta) smolts (N = 175) and veteran migrants (N = 342), from five adjacent riverine populations located in Sognefjorden, the longest Norwegian fjord-system supporting anadromous brown trout populations (209 km). Over four years, 138 acoustic telemetry receivers were deployed to track migrations of tagged individuals from freshwater and throughout Sognefjorden. Detected movements were used to fit migration models and multi-state mark-recapture models of survival and movement for each life-stage. Seaward migration distance was modelled to examine the fitness consequences from alternate migration strategies, with these models used to simulate the extent of fjord-use by individuals and accompanying growth, fecundity and survival consequences. We compared these findings with mark-recapture data collected prior to aquaculture and hydropower development. RESULTS: The telemetry data revealed that the outermost-fjord region was utilised by all populations albeit by few individuals. However, historical recaptures were located at a greater distance from the river mouth (87.7 ± 70.3 km), when compared to maximum migration distances of present-day counterparts (58.6 ± 54.9 km). River of origin influenced observed migratory behaviour and differential survival was estimated for each population and life-stage. The simulations based on telemetry-data models revealed a 30% and 23% difference in survival among populations for smolts and veteran migrants, respectively. At the individual-level, a long-distance migration strategy was rewarded with enhanced fecundity. However, the main contribution to population-level fecundity was overwhelmingly derived from middle-distance migrants, due to higher mortality rates and limited numbers of long-distant migrants. CONCLUSIONS: We conclude that present-day anadromy is precarious, but potential risk varies considerably between life-stages and populations, even within a single fjord system. Our findings suggest that selection for extended migration is under pressure, we therefore stress the importance of monitoring and management actions to secure genetic variation pertinent to preserve fitness gains of anadromy.

The use of predator tags to explain reversal movement patterns in Atlantic salmon smolts (Salmo salar L.).

Acoustic telemetry has seen a rapid increase in utility and sophistication in recent years and is now used extensively to assess the behavior and survival rates of many aquatic animals, including the Atlantic salmon. As part of the salmon's complex life cycle, salmon smolts are thought to make a unidirectional migration from fresh water to the sea, which is initiated by changes in their physiology. However, some tag movement patterns do not conform with this and can be difficult to explain, particularly if the tagged fish has been eaten by a predator. This study combines the use of predator tags with machine learning techniques to understand the fate of migrating salmon smolts and thereby improve estimates for migration success. Over 3 years between 2020 and 2022, 217 salmon smolts (including wild and hatchery-reared ranched fish) were acoustically tagged and released into an embayment on the west coast of Ireland. Some tagged smolts were observed to return from the estuary back into a saline lagoon through which they had already migrated. To distinguish between the movement of a salmon smolt and that of a predator, predator tags were deployed in migrating smolts in 2021 and 2022. The addition of a temperature sensor in 2022 enabled the determination of predator type causing the returning movement. A significant number of predator tags were triggered, and the patterns of movement associated with these triggered tags were then used with two types of machine learning algorithms (hierarchical cluster analysis and random forest) to identify and validate the behavior of smolts tagged without extra sensors. Both models produced the same outputs, grouping smolts tagged with predator tags with smolts tagged without the additional sensors but showing similar movements. A mammalian predator was identified as the cause of most reversal movement, and hatchery-reared ranched smolts were found to be more likely predated upon by this predator than wild smolts within the lake and the estuary. However, overall migration success estimates were similar for both wild and hatchery-reared ranched fish. This study highlights the value of predator tags as an essential tool in the overall validation of detection data.

A shift in habitat use patterns of brown trout (Salmo trutta): A behavioural response to macrophyte removal.

Mass development of macrophytes is an increasing problem worldwide and they are frequently removed where they are in conflict with local waterway users. Yet, macrophytes can provide important refuge and nursery habitats for fish. Little is known about the consequences of macrophyte removal for fish behavioural space use and habitat selection. We hypothesised that macrophyte removal would affect brown trout (Salmo trutta) movement during the partial removal of the aquatic plant Juncus bulbosus (L.) in an oligotrophic impounded Norwegian river.We tagged 94 brown trout and tracked them using passive acoustic telemetry for 10 months and mapped the cover of J. bulbosus. Trout behavioural patterns were quantified as space use (utilisation areas 50% and 95%) which was linked to habitat use and selection for J. bulbosus. Removal of J. bulbosus influenced space use of brown trout by reducing the core utilisation area by 22%. Habitat use and selection were likewise influenced by removal with increased use and selection of areas with low J. bulbosus cover (<25%) with corresponding reduction in high J. bulbosus cover (>25-75%). Finally, diurnal differences in space use and habitat use were found, with 19% larger utilisation areas at night and higher use of areas with low J. bulbosus during daytime. Yet, all effect sizes were relatively small compared to the size of the study area. This research provides a detailed case study on the effects of macrophyte removal on fish behavioural patterns in a section of a large Norwegian river with macrophyte mass development. We found no large effects of removal on trout behaviour but noted an increased use of areas with low macrophyte cover. This research is relevant for water managers and policy makers of freshwater conservation and provides a template for using acoustic telemetry to study the effects of macrophyte removal on fish.

Continuous sound from a marine vibrator causes behavioural responses of free-ranging, spawning Atlantic cod (Gadus morhua).

Marine vibrators are a new technology being developed for seismic surveys. These devices can transmit continuous instead of impulsive sound and operate over a narrower frequency band and at lower peak pressure than airguns, which is assumed to reduce their environmental impacts. We exposed spawning Atlantic cod (Gadus morhua) to sound produced by a prototype, but full-scale, marine vibrator, and monitored behavioural responses of tagged cod using acoustic telemetry. Fish were exposed to 10 × 3 h continuous sound treatments over a 4-day period using a randomised-block design. Sound exposure levels were comparable to airgun exposure experiments conducted previously with the same set-up ranging from ∼115 to 145 dB re 1 µPa2s during exposure. Telemetry data were used to assess 1) whether marine vibrator exposure displaced cod from the spawning ground, through estimation of residence and survival probabilities, and 2) fine-scale behavioural responses within the test site, namely swimming depth, activity levels, displacement, and home ranges. Forty-two spawning cod were tagged prior to the exposure, with 22 present during the exposure. All 22 tags were equipped with pressure sensors and ten of these additionally with accelerometers. While no premature departure from the spawning site was observed, cod reacted to the exposure by decreasing their activity levels (by up to 50%, SE = 7%) and increasing their swimming depth (by up to 2.5 m, SE = 1.0 m) within the test site during the exposure period. These behavioural responses varied by sex and time of day. Cod reactions to a marine vibrator may be more pronounced than reactions to airguns, possibly because continuous sound is more disturbing to fish than intermittent sound at the same exposure levels. However, given sample size limitations of the present study, further studies with continuous sound are necessary to fully understand its impact and biological significance.