Modeling juvenile sea turtle bycatch risk in commercial and recreational fisheries.

Understanding the drivers of fisheries bycatch is essential for limiting its impacts on vulnerable species. Here we present a model to estimate the relative magnitude of sea turtle bycatch in major coastal fisheries across the southeastern US based on spatiotemporal variation in fishing effort and the simulated distributions of juvenile Kemp's ridley (Lepidochelys kempii) and green (Chelonia mydas) sea turtles recruiting from oceanic to nearshore habitats. Over the period modeled (1996-2017), bycatch in recreational fisheries was estimated to be greater than the sum of bycatch that occurred in commercial fisheries that have historically been considered high risks to turtles (e.g., those using trawls, gillnets, and bottom longlines). Prioritizing engagement with recreational anglers to reduce bycatch could be especially beneficial to sea turtle populations. Applying lessons learned from efforts to protect turtles in commercial fisheries may help meet the challenges that arise from the large, diffuse recreational fishing sector.

Map-like use of Earth's magnetic field in sharks.

Migration is common in marine animals,1-5 and use of the map-like information of Earth's magnetic field appears to play an important role.2,6-9 While sharks are iconic migrants10-12 and well known for their sensitivity to electromagnetic fields,13-20 whether this ability is used for navigation is unresolved.14,17,21,22 We conducted magnetic displacement experiments on wild-caught bonnetheads (Sphyrna tiburo) and show that magnetic map cues can elicit homeward orientation. We further show that use of a magnetic map to derive positional information may help explain aspects of the genetic structure of bonnethead populations in the northwest Atlantic.23-26 These results offer a compelling explanation for the puzzle of how migratory routes and population structure are maintained in marine environments, where few physical barriers limit movements of vagile species. VIDEO ABSTRACT.

Animal navigation: What is truth?

'True navigation' indicates that animals can move toward a destination without using familiar landmarks. Migratory birds apparently achieve this by extrapolating their position from geomagnetic cues. What this ability implies about the function and representation of animals' large-scale maps remains uncertain.

Identifying global favourable habitat for early juvenile loggerhead sea turtles.

Loggerhead sea turtles (Caretta caretta) nest globally on sandy beaches, with hatchlings dispersing into the open ocean. Where these juveniles go and what habitat they rely on remains a critical research question for informing conservation priorities. Here a high-resolution Earth system model is used to determine the biophysical geography of favourable ocean habitat for loggerhead sea turtles globally during their first year of life on the basis of ocean current transport, thermal constraints and food availability (defined here as the summed lower trophic level carbon biomass). Dispersal is simulated from eight major nesting sites distributed across the globe in four representative years using particle tracking. Dispersal densities are identified for all turtles, and for the top 15% 'best-fed' turtles that have not encountered metabolically unfavourable temperatures. We find that, globally, rookeries are positioned to disperse to regions where the lower trophic biomass is greatest within loggerheads' thermal range. Six out of the eight nesting sites are associated with strong coastal boundary currents that rapidly transport hatchlings to subtropical-subpolar gyre boundaries; narrow spatial migratory corridors exist for 'best-fed' turtles associated with these sites. Two other rookeries are located in exceptionally high-biomass tropical regions fuelled by natural iron fertilization. 'Best-fed' turtles tend to be associated with lower temperatures, highlighting the inverse relationship between temperature and lower trophic biomass. The annual mean isotherms between 20°C and the thermal tolerance of juvenile loggerheads are a rough proxy for favourable habitat for loggerheads from rookeries associated with boundary currents. Our results can be used to constrain regions for conservation efforts for each subpopulation, and better identify foraging habitat for this critical early life stage.

Managing fisheries in a world with more sea turtles.

For decades, fisheries have been managed to limit the accidental capture of vulnerable species and many of these populations are now rebounding. While encouraging from a conservation perspective, as populations of protected species increase so will bycatch, triggering management actions that limit fishing. Here, we show that despite extensive regulations to limit sea turtle bycatch in a coastal gillnet fishery on the eastern United States, the catch per trip of Kemp's ridley has increased by more than 300% and green turtles by more than 650% (2001-2016). These bycatch rates closely track regional indices of turtle abundance, which are a function of increased reproductive output at distant nesting sites and the oceanic dispersal of juveniles to near shore habitats. The regulations imposed to help protect turtles have decreased fishing effort and harvest by more than 50%. Given uncertainty in the population status of sea turtles, however, simply removing protections is unwarranted. Stock-assessment models for sea turtles must be developed to determine what level of mortality can be sustained while balancing continued turtle population growth and fishing opportunity. Implementation of management targets should involve federal and state managers partnering with specific fisheries to develop bycatch reduction plans that are proportional to their impact on turtles.

Animal Navigation: Seabirds Home to a Moving Magnetic Target.

Shifts in the return locations of juvenile seabirds migrating from the Irish Sea to Argentina can be accurately predicted by changes in Earth's magnetic field, suggesting that these birds rely on a geomagnetic map for navigation.

Asymmetrical gene flow in five co-distributed syngnathids explained by ocean currents and rafting propensity.

Ocean circulation driving macro-algal rafting is believed to serve as an important mode of dispersal for many marine organisms, leading to predictions on population-level genetic connectivity and the directionality of effective dispersal. Here, we use genome-wide single nucleotide polymorphism data to investigate whether gene flow directionality in two seahorses (Hippocampus) and three pipefishes (Syngnathus) follows the predominant ocean circulation patterns in the Gulf of Mexico and northwestern Atlantic. In addition, we explore whether gene flow magnitudes are predicted by traits related to active dispersal ability and habitat preference. We inferred demographic histories of these co-distributed syngnathid species, and coalescent model-based estimates indicate that gene flow directionality is in agreement with ocean circulation data that predicts eastward and northward macro-algal transport. However, the magnitude to which ocean currents influence this pattern appears strongly dependent on the species-specific traits related to rafting propensity and habitat preferences. Higher levels of gene flow and stronger directionality are observed in Hippocampus erectus, Syngnathus floridae and Syngnathus louisianae, which closely associated with the pelagic macro-algae Sargassum spp., compared to Hippocampus zosterae and the Syngnathus scovelli/Syngnathus fuscus sister-species pair, which prefer near shore habitats and are weakly associated with pelagic Sargassum. This study highlights how the combination of population genomic inference together with ocean circulation data can help explain patterns of population structure and diversity in marine ecosystems.

Changes in Shrimping Effort in the Gulf of Mexico and the Impacts to Red Snapper.

Despite a complex management landscape and decades of overfishing, Red Snapper (Lutjanus campechanus) stocks have grown substantially in the Gulf of Mexico and restrictions on the fisheries that catch them are being loosened. This year, annual shrimping effort was allowed to increase by 21% after National Marine Fisheries Service scientists concluded that the resulting bycatch of Red Snapper would only reduce the annual allowable catch in other fisheries by ∼1% and have no impact on population growth. Nonetheless, the recreational fishing sector intensely campaigned against this rule, fueled by wild mischaracterization of shrimp trawl bycatch in media outlets targeting anglers. Here, we aim to elevate the debates surrounding Red Snapper management by presenting scientific and historical context for the potential impacts from shrimping. We discuss our views of the current problems facing Red Snapper and key ecological questions to address for more effective management of this resource.

Magnetoreception in fishes: the effect of magnetic pulses on orientation of juvenile Pacific salmon.

A variety of animals sense Earth's magnetic field and use it to guide movements over a wide range of spatial scales. Little is known, however, about the mechanisms that underlie magnetic field detection. Among teleost fish, growing evidence suggests that crystals of the mineral magnetite provide the physical basis of the magnetic sense. In this study, juvenile Chinook salmon (Oncorhynchus tshawytscha) were exposed to a brief but strong magnetic pulse capable of altering the magnetic dipole moment of biogenic magnetite. Orientation behaviour of pulsed fish and untreated control fish was then compared in a magnetic coil system under two conditions: (1) the local magnetic field and (2) a magnetic field that exists near the southern boundary of the natural oceanic range of Chinook salmon. In the local field, no significant difference existed between the orientation of the control and pulsed groups. By contrast, orientation of the two groups was significantly different in the magnetic field from the distant site. These results demonstrate that a magnetic pulse can alter the magnetic orientation behaviour of a fish and are consistent with the hypothesis that salmon have magnetite-based magnetoreception.

A sense of place: pink salmon use a magnetic map for orientation.

The use of 'map-like' information from the Earth's magnetic field for orientation has been shown in diverse taxa, but questions remain regarding the function of such maps. We used a 'magnetic displacement' experiment to demonstrate that juvenile pink salmon (Oncorhynchus gorbuscha) use magnetic cues to orient. The experiment was designed to simultaneously explore whether their magnetic map is used to direct fish (i) homeward, (ii) toward the center of their broad oceanic range or (iii) along their oceanic migratory route. The headings adopted by these navigationally naive fish coincided remarkably well with the direction of the juveniles' migration inferred from historical tagging and catch data. This suggests that the large-scale movements of pink salmon across the North Pacific may be driven largely by their innate use of geomagnetic map cues. Key aspects of the oceanic ecology of pink salmon and other marine migrants might therefore be predicted from magnetic displacement experiments.

Magnetic map in nonanadromous Atlantic salmon.

Long-distance migrants, including Pacific salmon (Oncorhynchus spp), can use geomagnetic information to navigate. We tested the hypothesis that a "magnetic map" (i.e., an ability to extract positional information from Earth's magnetic field) also exists in a population of salmon that do not undertake oceanic migrations. This study examined juvenile Atlantic salmon (Salmo salar) originally from a nonanadromous population in Maine transferred ∼60 years ago to a lake in central Oregon. We exposed juveniles to magnetic displacements representative of locations at the latitudinal boundaries of the Pacific salmon oceanic range in the North Pacific and at the periphery of their ancestral oceanic range in the North Atlantic. Orientation differed among the magnetic treatments, indicating that Atlantic salmon detect map information from the geomagnetic field. Despite no recent history of ocean migration, these fish displayed adaptive orientation responses similar to those observed in native Pacific salmonids. These findings indicate that use of map information from the geomagnetic field is a shared ancestral character in the family Salmonidae and is not restricted to populations with anadromous life histories. Lastly, given that Atlantic salmon are transported throughout the world for capture fisheries and aquaculture, such a robust navigational system is of some concern. Escaped individuals may have greater potential to successfully navigate, and thus invade, introduced habitats than previously suspected.

Geomagnetic field influences upward movement of young Chinook salmon emerging from nests.

Organisms use a variety of environmental cues to orient their movements in three-dimensional space. Here, we show that the upward movement of young Chinook salmon (Oncorhynchus tshawytscha) emerging from gravel nests is influenced by the geomagnetic field. Fish in the ambient geomagnetic field travelled farther upwards through substrate than did fish tested in a field with the vertical component inverted. This suggests that the magnetic field is one of several factors that influences emergence from the gravel, possibly by serving as an orientation cue that helps fish determine which way is up. Moreover, our work indicates that the Oncorhynchus species are sensitive to the magnetic field throughout their life cycles, and that it guides their movements across a range of spatial scales and habitats.

First satellite tracks of South Atlantic sea turtle 'lost years': seasonal variation in trans-equatorial movement.

In the South Atlantic Ocean, few data exist regarding the dispersal of young oceanic sea turtles. We characterized the movements of laboratory-reared yearling loggerhead turtles from Brazilian rookeries using novel telemetry techniques, testing for differences in dispersal during different periods of the sea turtle hatching season that correspond to seasonal changes in ocean currents. Oceanographic drifters deployed alongside satellite-tagged turtles allowed us to explore the mechanisms of dispersal (passive drift or active swimming). Early in the hatching season turtles transited south with strong southward currents. Late in the hatching season, when currents flowed in the opposite direction, turtles uniformly moved northwards across the Equator. However, the movement of individuals differed from what was predicted by surface currents alone. Swimming velocity inferred from track data and an ocean circulation model strongly suggest that turtles' swimming plays a role in maintaining their position within frontal zones seaward of the continental shelf. The long nesting season of adults and behaviour of post-hatchlings exposes young turtles to seasonally varying ocean conditions that lead some individuals further into the South Atlantic and others into the Northern Hemisphere. Such migratory route diversity may ultimately buffer the population against environmental changes or anthropologic threats, fostering population resiliency.

Response to Durif et al.

Our recent study [1] in Current Biology used a magnetic displacement experiment and simulations in an ocean circulation model to provide evidence that young European eels possess a 'magnetic map' that can aid their marine migration. Our results support two major conclusions: first, young eels distinguish among magnetic fields corresponding to locations across their marine range; second, for the fields that elicited significantly non-random orientation, swimming in the experimentally observed direction from the corresponding locations would increase entrainment in the Gulf Stream system. In their critique, Durif et al.[2] seem to conflate the separate and potentially independent 'map step' and 'compass step' of animal navigation. In the map step, an animal derives positional information to select a direction, whereas in the compass step the animal maintains that heading [3,4]. Our experiment was designed such that differences in eel orientation among treatments would indicate an ability to use the magnetic field as a map; the compass cue(s) used by eels was not investigated.

A Magnetic Map Leads Juvenile European Eels to the Gulf Stream.

Migration allows animals to track the environmental conditions that maximize growth, survival, and reproduction [1-3]. Improved understanding of the mechanisms underlying migrations allows for improved management of species and ecosystems [1-4]. For centuries, the catadromous European eel (Anguilla anguilla) has provided one of Europe's most important fisheries and has sparked considerable scientific inquiry, most recently owing to the dramatic collapse of juvenile recruitment [5]. Larval eels are transported by ocean currents associated with the Gulf Stream System from Sargasso Sea breeding grounds to coastal and freshwater habitats from North Africa to Scandinavia [6, 7]. After a decade or more, maturing adults migrate back to the Sargasso Sea, spawn, and die [8]. However, the migratory mechanisms that bring juvenile eels to Europe and return adults to the Sargasso Sea remain equivocal [9, 10]. Here, we used a "magnetic displacement" experiment [11, 12] to show that the orientation of juvenile eels varies in response to subtle differences in magnetic field intensity and inclination angle along their marine migration route. Simulations using an ocean circulation model revealed that even weakly swimming in the experimentally observed directions at the locations corresponding to the magnetic displacements would increase entrainment of juvenile eels into the Gulf Stream System. These findings provide new insight into the migration ecology and recruitment dynamics of eels and suggest that an adaptive magnetic map, tuned to large-scale features of ocean circulation, facilitates the vast oceanic migrations of the Anguilla genus [7, 13, 14].

Earth-strength magnetic field affects the rheotactic threshold of zebrafish swimming in shoals.

Rheotaxis, the unconditioned orienting response to water currents, is a main component of fish behavior. Rheotaxis is achieved using multiple sensory systems, including visual and tactile cues. Rheotactic orientation in open or low-visibility waters might also benefit from the stable frame of reference provided by the geomagnetic field, but this possibility has not been explored before. Zebrafish (Danio rerio) form shoals living in freshwater systems with low visibility, show a robust positive rheotaxis, and respond to geomagnetic fields. Here, we investigated whether a static magnetic field in the Earth-strength range influenced the rheotactic threshold of zebrafish in a swimming tunnel. The direction of the horizontal component of the magnetic field relative to water flow influenced the rheotactic threshold of fish as part of a shoal, but not of fish tested alone. Results obtained after disabling the lateral line of shoaling individuals with Co2+ suggest that this organ system is involved in the observed magneto-rheotactic response. These findings constitute preliminary evidence that magnetic fields influence rheotaxis and suggest new avenues for further research.

Passive drift or active swimming in marine organisms?

Predictions of organismal movements in a fluid require knowing the fluid's velocity and potential contributions of the organism's behaviour (e.g. swimming or flying). While theoretical aspects of this work are reasonably well-developed, field-based validation is challenging. A much-needed study recently published by Briscoe and colleagues in Proceedings of the Royal Society B compared movements and distribution of satellite-tracked juvenile sea turtles to virtual particles released in a data-assimilating hindcast ocean circulation model. Substantial differences observed between turtles and particles were considered evidence for an important role of active swimming by turtles. However, the experimental design implicitly assumed that transport predictions were insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the model. Here, we show that the magnitude of variation in physical parameters between turtles and virtual particles can profoundly alter transport predictions, potentially sufficient to explain the reported differences without evoking swimming behaviour. We present a more robust method to derive the environmental contributions to individual movements, but caution that resolving the ocean velocities experienced by individual organisms remains a problem for assessing the role of behaviour in organismal movements and population distributions.

Multi-Modal Homing in Sea Turtles: Modeling Dual Use of Geomagnetic and Chemical Cues in Island-Finding.

Sea turtles are capable of navigating across large expanses of ocean to arrive at remote islands for nesting, but how they do so has remained enigmatic. An interesting example involves green turtles (Chelonia mydas) that nest on Ascension Island, a tiny land mass located approximately 2000 km from the turtles' foraging grounds along the coast of Brazil. Sensory cues that turtles are known to detect, and which might hypothetically be used to help locate Ascension Island, include the geomagnetic field, airborne odorants, and waterborne odorants. One possibility is that turtles use magnetic cues to arrive in the vicinity of the island, then use chemical cues to pinpoint its location. As a first step toward investigating this hypothesis, we used oceanic, atmospheric, and geomagnetic models to assess whether magnetic and chemical cues might plausibly be used by turtles to locate Ascension Island. Results suggest that waterborne and airborne odorants alone are insufficient to guide turtles from Brazil to Ascension, but might permit localization of the island once turtles arrive in its vicinity. By contrast, magnetic cues might lead turtles into the vicinity of the island, but would not typically permit its localization because the field shifts gradually over time. Simulations reveal, however, that the sequential use of magnetic and chemical cues can potentially provide a robust navigational strategy for locating Ascension Island. Specifically, one strategy that appears viable is following a magnetic isoline into the vicinity of Ascension Island until an odor plume emanating from the island is encountered, after which turtles might either: (1) initiate a search strategy; or (2) follow the plume to its island source. These findings are consistent with the hypothesis that sea turtles, and perhaps other marine animals, use a multi-modal navigational strategy for locating remote islands.