Acoustic Telemetry Research Articles

Ecological connectivity of invasive and native fishes in a historic navigation waterway

Regulated waterways, interconnected by navigation barriers (locks and dams), are uniquely difficult to manage, given interest in enabling native species connectivity while minimizing invasions. Canada's historic Rideau Canal Waterway, a 202 km navigable route located in eastern Ontario and connected by 24 lockstations, embodies this challenge. The lock(s) and water-control dam that compose each lockstation may respectively offer a connectivity pathway, though to what extent is unclear. We used acoustic telemetry (native largemouth bass ( Micropterus nigricans) and northern pike ( Esox lucius), and invasive common carp ( Cyprinus carpio); n = 224) to evaluate fish connectivity relative to lock operations and environmental data over three years (2019–2021). Thirty-five passages by 23 native fishes were recorded, with 49% of passages through locks. No common carp passages were detected; movements indicate that they favour higher flow areas downstream of dams, regions with no pathway upstream. Most passages were downstream and, of concern to obligate upstream migrators, we found that multi-flight and higher-lift locks appear impassable to upstream movements. Our results suggest that these lockstations limit, but not entirely restrict, connectivity.

Movements and spatial distribution of an endangered fish (Sciaena umbra) within a marine protected area

The brown meagre (Sciaena umbra) is an endangered species, which requires specific protection measures to ensure its conservation. These measures need to be informed by high-quality scientific knowledge on their space use patterns. Here, we used acoustic telemetry to assess its seasonal movement patterns and habitat use within a marine protected area (MPA). Our results suggested that S. umbra is a highly sedentary species (home range < 1.0 km2) and, therefore, the MPA is extensive enough to protect the local population. Their population was discretely distributed in two main areas within the MPA, which was likely a result of habitat segregation and density-dependent movements. The temporal variability of their movements further uncovered when and where spawning occurs (mainly, but probably not only, in the fully protected area in June) and indicated that spillover of this species is limited but still possible. Overall, we highlight the importance of MPAs in the recovery of S. umbra, we advocate the need to perpetuate the current national fishing bans and extend it to other countries in the Mediterranean region, and we emphasize that considering the fine-scale movements of S. umbra in future management actions is key to achieving a successful recovery of their populations.

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

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. 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. 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. 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.

Spatiotemporal variations in reef manta ray (Mobula alfredi) residency at a remote meso‐scale habitat and its importance in future spatial planning

Abstract The Chagos Archipelago's vast no‐take marine protected area (MPA, 640,000 km2) provides refuge for elasmobranchs facing unsustainable depletion by fisheries. Nonetheless, illegal, unreported and unregulated (IUU) fishing poses a substantial threat, and potential future changes to the use of the MPA could render elasmobranchs increasingly vulnerable to exploitation, putting geographically isolated populations, such as reef manta rays (Mobula alfredi) at risk of local extinction. Therefore, the species' long‐term movements and habitat use must be identified to help prioritize current enforcement activity and inform future spatial planning. Passive acoustic telemetry and modelled environmental data were used to investigate variations in 42 tagged M. alfredi utilization of a meso‐scale aggregation hotspot, Egmont Atoll, between 2019 and 2022. Mobula alfredi displayed the highest levels of residency ever reported (77%), with prolonged absences (&gt;2 months) limited to seven individuals. Egmont atoll was used year‐round, with activity peaks during the southeast monsoon (April – November), particularly at sites on the southwest, while sites on the northwest were predominately frequented in the northwest monsoon (December–March). Tags were most likely to be detected when the Indian Ocean Dipole (IOD) was in a positive phase with a greater mixed layer depth, associated with a depression of chlorophyll α levels in the Indian Ocean. Thus, M. alfredi may be particularly reliant on Egmont Atoll, where they are predominantly observed feeding, when prey resources are limited elsewhere. In a region where the threat of fisheries is of increasing concern, the identification of crucial M. alfredi habitats is essential for conservation management planning. Given the significant role of Egmont Atoll for the local population, regular IUU enforcement patrols are crucial, particularly during the southeast monsoon. Any future changes to the MPA should prioritize preserving and actively enforcing no‐take regulations at Egmont Atoll.

TrackdAT, an acoustic telemetry metadata dataset to support aquatic animal tracking research

Data on the movement and space use of aquatic animals are crucial to understand complex interactions among biotic and abiotic components of ecosystems and facilitate effective conservation and management. Acoustic telemetry (AT) is a leading method for studying the movement ecology of aquatic animals worldwide, yet the ability to efficiently access study information from AT research is currently lacking, limiting advancements in its application. Here, we describe TrackdAT, an open-source metadata dataset where AT research parameters are catalogued to provide scientists, managers, and other stakeholders with the ability to efficiently identify and evaluate existing peer-reviewed research. Extracted metadata encompasses key information about biological and technical aspects of research, providing a comprehensive summary of existing AT research. TrackdAT currently hosts information from 2,412 journal articles published from 1969 to 2022 spanning 614 species and 380,289 tagged animals. TrackdAT has the potential to enable regional and global mobilization of knowledge, increased opportunities for collaboration, greater stakeholder engagement, and optimization of future ecological research.

Depredation rates and spatial overlap between Great Hammerheads and Tarpon in a recreational fishing hot spot

AbstractObjectiveShark depredation, the full or partial consumption of a hooked fish by a shark before it is landed, is an increasing source of human–wildlife conflict in recreational fisheries. Reports of shark depredation in the catch‐and‐release Tarpon (also known as Atlantic Tarpon) Megalops atlanticus fishery in the Florida Keys are increasing, specifically in Bahia Honda, a recreational fishing hot spot and a putative Tarpon prespawning aggregation site.MethodsUsing visual surveys of fishing in Bahia Honda, we quantified depredation rates and drivers of depredation. With acoustic telemetry, we simultaneously tracked 51 Tarpon and 14 Great Hammerheads (also known as Great Hammerhead Sharks) Sphyrna mokarran, the most common shark to depredate Tarpon, to quantify residency and spatial overlap in Bahia Honda.ResultDuring the visual survey, 394 Tarpon were hooked. The combined observed shark depredation and immediate postrelease predation rate was 15.3% for Tarpon that were fought longer than 5 min. Survival analysis and decision trees showed that depredation risk was highest in the first 5–12 min of the fight and on the outgoing current. During the spawning season, Great Hammerheads shifted their space use in Bahia Honda to overlap with Tarpon core use areas. Great Hammerheads restricted their space use on the outgoing current when compared to the incoming current, which could drive increased shark–angler interactions.ConclusionBahia Honda has clear ecological importance for both Tarpon and Great Hammerheads as a prespawning aggregation and feeding ground. The observed depredation mortality and postrelease predation mortality raise conservation concerns for the fishery. Efforts to educate anglers to improve best practices, including reducing fight times and ending a fight prematurely when sharks are present, will be essential to increase Tarpon survival and reduce shark–angler conflict.

Interannual variation in survival of wild Atlantic salmon smolts through a dynamic estuarine habitat

AbstractMigration timing is critical for diadromous fish, especially for survival. Migration through fresh water and estuarine transitional waterbodies may be an important early life survival bottleneck through potential exposure to anthropogenic pressures and predators. Monitoring smolt movement and survival through riverine and estuarine habitats is important for identifying causes of smolt mortality and the potential for directing focused, mitigating management actions. Acoustic telemetry was used to track 186 Atlantic salmon smolts during migration from the river Boyne to the Irish Sea on the east coast of Ireland during 2019–2021. Migration success from release to sea entry that ranged from 47% to 81% was linked to biotic (predation) and abiotic effects (water flow and water temperature). Predation was associated with interannual variation in smolt survival, with 5.4% of smolts consumed by marine mammals and 26.3% lost to unknown fates. Our findings contribute to the understanding of predation on smolts and also how water temperature, flow and tidal phase can influence this critical life stage of Atlantic Salmon, a species in decline across its range.

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.

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.

An Eulerian perspective on habitat models of striped bass occurrence in an offshore wind development area

Abstract Offshore wind energy development, including along the US Atlantic coast, frequently occurs within important multispecies migration corridors; however, assessing the regional factors influencing the local Eulerian occurrence of these species poses a significant challenge. We used generalized models incorporating lagged variables and hierarchical formulations to account for temporal dependencies and hierarchical structure that occur outside the narrower frame of a sampled project area. Acoustically tagged striped bass, the most frequently detected species regionally, were sampled using a gridded acoustic telemetry array in the Maryland Wind Energy Area of the US Mid-Atlantic Bight. The daily occurrence of striped bass was better explained by broad-scale sea surface temperature warming patterns than by local concurrent environmental conditions, demonstrating the importance of drivers that occur across the wider spatial scales of migration. Weekly residency patterns were similar between tagging origin groups, suggesting that Chesapeake Bay, Hudson River, Delaware Bay, and other Northwest Atlantic populations migrate synchronously through the Southern Mid-Atlantic Bight and are similarly influenced by sea surface temperature. Our study demonstrates that adapting an Eulerian approach to include lagged variables can improve regional assessments of fish on the move until richer Lagrangian insights become possible through future coordination of telemetry arrays throughout the Mid-Atlantic flyway.

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.

Comparing acoustic and satellite telemetry: an analysis quantifying the space use of Chelonia mydas in Bimini, Bahamas.

Passive acoustic and Argos satellite telemetry are common methods for tracking marine species and are often used similarly to quantify space use. However, data-driven comparisons of these methods and their associated ecological inferences are limited. To address this, we compared temporal durations, spatial resolutions, financial costs and estimates of occurrence and range distributions for each tracking approach using nine juvenile green turtles (Chelonia mydas) in Bimini, Bahamas. Tracking durations were similar, although acoustic tracking provided higher spatiotemporal resolution than satellite tracking. Occurrence distributions (95%) estimated from satellite telemetry were 12 times larger than those from acoustic telemetry, while satellite range distributions (95%) were 89 times larger. While individuals generally remained within the extent of the acoustic receiver array, gaps in coverage were identified. These gaps, combined with the lower accuracy of satellite telemetry, were likely drivers for the larger satellite distributions. Costs differed between telemetry methods, with acoustic telemetry being less expensive at larger sample sizes with a previously established array. Our results suggest that acoustic and satellite telemetry may not provide similar inferences of individual space use. As such, we provide recommendations to identify telemetry methods appropriate for specific study objectives and provide discussion on the biases of each.

Residency and movement patterns of juvenile yellowfin bream Acanthopagrus australis, stocked on estuarine artificial reefs

Context Successful fish stocking relies upon releasing cohorts directly into habitat containing suitable supporting resources. To facilitate this, an emerging trend has seen fish released onto artificial reefs. Aims We compared the residency and movement patterns of juvenile sparids (yellowfin bream – Acanthopagrus australis), stocked on artificial reefs and natural habitat, to determine the suitability of incorporating artificial structures as release sites in future stocking programs. Methods Forty hatchery-reared yellowfin bream were evenly divided and stocked on either artificial reef or natural habitat within an east-Australian estuary, and tracked using acoustic telemetry. Key results Artificial reefs provided greater habitat complexity than the natural habitat and fish stocked onto these structures showed greater fidelity to their release site and were detected for longer periods of time than those stocked into less complex natural habitat. Conclusions The high residency levels of fish around artificial reef sites indicate they contain resources to support stocked juvenile sparids. Implications Stocked yellowfin bream can tolerate a range of salinity and temperatures, allowing flexibility in stocking programs. Future stocking of bream or other sparids should consider releasing fish onto artificial reefs where available. Additionally, the deployment of reefs could aid existing stocking programs.

Repeatability of swimming activity of the Patagonian grouper Acanthistius patachonicus based on accelerometry.

The study of repeatability in behaviour and activity level can be used to evaluate inter-individual differences, which are fundamental to assess the resilience of populations to environmental variation. Previous work on repeatability in wild fish populations has largely been based on acoustic telemetry or mark-and-recapture and has revealed repeatable activity patterns over relatively long periods in a number of species. Although accelerometry is a promising tool for investigating the swimming activity of fish in the wild, little is known about the repeatability of accelerometry-based traits in wild fish. Here, we used external accelerometers to investigate the swimming activity of the Patagonian grouper Acanthistius patachonicus, a rocky-reef fish with high site fidelity, which ensures a high recapture rate of accelerometer tags. Accelerometry was used to investigate the short-term repeatability of a number of activity traits, including swimming, hovering, daily median tailbeat frequency, percentage of high tailbeat frequency and total number of tailbeats at different times of the year. We found that all of the variables are repeatable over the daily scale and four out of five variables are repeatable over weekly periods. Overall, our work suggests that these traits are individual-specific for the short time period investigated. In addition, the percentage of time spent in swimming and hovering was greater in the warm season compared to the cold season, suggesting higher activity levels related to higher temperatures. These results suggest that activity traits related to swimming are repeatable and likely related to the physiological state of each individual. Finally, our work shows that accelerometry can be considered a valuable tool to explore inter-individual differences with potential applications for assessing the resilience of wild populations.

Long-distance dispersal of the endangered Pacific nurse shark (Ginglymostoma unami, Orectolobiformes) in Costa Rica revealed through acoustic telemetry

Long-distance movements associated with environmental and ecological drivers are common in a wide array of mobile marine species. Understanding such movements and associated drivers is particularly important for management and conservation of threatened species, which may be exposed to anthropogenic threats throughout their range. Pacific nurse sharks (Ginglymostoma unami) are endangered because of their interactions with fisheries and a lack of biological knowledge as a recently described species. Little is known about this species’ movements throughout its broad distribution in the tropical Pacific coast of Latin America. Using passive acoustic telemetry, we report the longest ranging movement so far observed for Pacific nurse sharks. A male individual, measuring 176-cm total length at the time of tagging, travelled over 390 km within 46 days along the North Pacific coastline of Costa Rica. These findings highlight the importance of collaboration and data exchange among researchers to enable monitoring of broader geographical ranges and provide critical insight into the movement capabilities of this endangered species. Defining marine corridors and identifying essential habitats for Pacific nurse sharks may be necessary to mitigate fishing-related impacts if these longer distance movements are common across their population.

Spawning behaviour of lake whitefish in Lake Huron revealed by fine-scale acoustic telemetry and Indigenous ecological knowledge

Lake whitefish (Coregonus clupeaformis) are ecologically, economically, and culturally significant across their range, yet little is documented about the species’ spawning behaviour. Here, we used a VEMCO Positioning System (VPS) and the local ecological knowledge of Saugeen Ojibway Nation in a Two-Eyed Seeing approach to characterize sex-specific movement patterns of lake whitefish at a spawning shoal in Georgian Bay, Lake Huron (Ontario, Canada). In fall 2020, 50 lake whitefish were tagged with acoustic transmitters and 28 of those fish were detected during the deployment of the VPS between 15 October 2021 and 18 January 2022. Occupancy of lake whitefish on the VPS followed a bell-shaped pattern, with an increase, plateau that lasted ∼1 week, and subsequent decrease in activity over a 48-day period demarcating the 2021 spawning season. Apart from 2 resident females, males appeared in the area first. Both sexes moved into shallower lake depths at night and deeper depths during the day. Females moved 18.0% faster than males during the day whereas males moved 39.5% faster than females at night. Depths occupied became shallower over time until the peak activity phase, followed by a transition back to deeper waters. Both sexes used a variety of habitats over the spawning season, including the main spawning shoal, sandy substrates, and steep slopes. These insights on the habitat usage and spawning phenology of lake whitefish provided through our Two-Eyed Seeing approach can help inform stewardship initiatives aimed at increasing recruitment of depleted populations of this important species.

Determining early marine survival and predation by endothermic predators on acoustically tagged Atlantic salmon (Salmo salar) post-smolts

Many Atlantic salmon ( Salmo salar) populations have experienced significant declines for decades throughout North America and Europe. Mortality due to marine mammal predation during their early marine life could be an important factor contributing to these declines and limiting their population recoveries. However, quantifying predation events, and particularly the extent of marine mammal predation on Atlantic salmon, remains a challenge. In this study, we estimated the contribution of mesothermic and endothermic species predation to the mortality of Atlantic salmon post-smolts during their early marine life using acoustic telemetry. Predation events were inferred from changes in temperatures and depths experienced by acoustically tagged hatchery-reared Atlantic salmon smolts. No salmon were consumed by mesothermic predators, with most endothermic predation events being classified as marine mammals. Post-smolt mortality during the study period was low overall in both years (13.1%–16.7%), with endothermic predation accounting for 33.1%–42.9% of all marine mortality events (5.2%–5.6% mortality). Our results suggest that the current low return of adult Atlantic salmon observed in this area in recent years was not heavily influenced by endothermic predation on post-smolts in the first weeks at sea.

To hear or not to hear: selective tidal stream transport can interfere with the detectability of migrating silver eels in a Tidal River

Acoustic telemetry provides valuable insights into behavioural patterns of aquatic animals such as downstream migrating European eels (Anguilla anguilla), so called silver eels. The behaviour of silver eels during the migration is known to be influenced by environmental factors, yet so is the performance of acoustic telemetry networks. This study quantifies the impact of these environmental factors on both, migration behaviour and receiver performance to determine possible limiting conditions for detecting tagged eels in tidal areas. A dominance analysis of the selected models describing migration speed, activity and receiver performance was conducted following 234 silver eels that were tagged with acoustic transmitters and observed by a receiver network in the Ems River during two subsequent migration seasons. The results suggest a passive locomotion of silver eels during their downstream migration by taking advantage of selective tidal stream transport (STST). It is further shown that water temperature, salinity, turbidity, precipitation, and especially current velocity were major parameters influencing migration activity and speed. At the same time, analyses of the detection probability of tagged eels under varying environmental conditions indicated a decreased receiver performance during increased current velocities, meaning that high migration activity and -speed coincides with reduced detection probability. Consequently, there is a risk that particularly during phases of increased activity, migration activity may be underestimated due to reduced acoustic telemetry performance. To avoid bias in telemetry studies, it is, therefore, crucial to conduct range tests and adjust the receiver placement in areas and conditions of high current velocities.

Survivorship of discarded sole (Solea solea) characterised via telemetry, vitality, and physiology

While the discarding practices of commercial fisheries might greatly contribute to fish mortality, a better understanding of discard survival patterns and their drivers is crucial to assess fishing mortality and develop effective mitigation measures. Bottom trawling in particular is likely to induce multiple forms of stress in captured fish. In this study, we used three experimental methods to explore discard survivorship of the common sole (Solea solea). We focused on fish captured via a coastal bottom trawler of the Bay of Biscay and conducted sampling during commercial fishing operations. First, we used acoustic telemetry to determine the survival rate of 160 discarded soles. While this technique makes it possible to study discarded fish in their natural habitat, inferring fish fate from these data can be challenging, especially when acoustic receiver network coverage is fragmented. Second, we scored the vitality of the sole before their release. Third, we examined three plasma markers of fish physiological status over a 90-min period following capture. The method based on acoustic telemetry estimated a minimum discard survival rate of 41%. This metric was positively correlated to the vitality. Furthermore, both survival and vitality were associated to tow duration and the presence of algae in the catch consistently. The three physiological markers reflected the presence of various levels of stress and fatigue among discarded individuals, but also a fish’s ability to recover after being discarded. Individuals with excellent vitality (33% of discards in this study) suffered less stress, were less fatigued and, therefore, were greater equipped to react to other sources of stress than individuals in lower vitality. The consistency of the results demonstrates the reliability of our approach to estimate post-release survival from acoustic tagging even from fragmented network coverage, and that vitality can serve as a proxy for discard survivorship.

Spatial ecology of the Giant Sea Bass, Stereolepis gigas, in a southern California kelp forest as determined by acoustic telemetry.

The fisheries history of the Giant Sea Bass, Stereolepis gigas (Telostei: Polyprionidae), is closely linked to its spatial ecology. Its overharvest is directly associated with formation of spatially distinct spawning aggregations during summer, while its subsequent recovery is hypothesized to be the result of spatially explicit gear restrictions. Understanding the spatial ecology of Giant Sea Bass is a key part of efforts to assess contemporary threats such as commercial harvest and incidental catch by recreational fisheries. In this study, we used acoustic telemetry to characterize Giant Sea Bass space use in the La Jolla kelp forest using an acoustic array that encompasses two marine protected areas (MPAs) and heavily trafficked recreational fishing grounds. Five of the seven fish we tagged remained in the La Jolla array for at least 6 months. Two fish were resident across multiple years, with one fish consistently detected for 4 years. Only one fish was detected in the broader network of regional acoustic receivers, moving north approximately 8 km to Del Mar. Most tagged fish had home ranges and core use areas indicating they spend considerable time outside MPAs, particularly in areas with high recreational fishing activity. During spawning season we detected fish less frequently in the La Jolla array and recorded higher movement rates. While the current MPA network in La Jolla by no means offers complete protection to this fish, it does appear to support long-term persistence of some individuals in a region of exceptionally high recreational fishing pressure.