Route optimisation and solving Zermelo's navigation problem during long distance migration in cross flows

The optimum path to follow when subjected to cross flows was first considered over 80 years ago by the German mathematician Ernst Zermelo, in the context of a boat being displaced by ocean currents, and has become known as the ‘Zermelo navigation problem’. However, the ability of migrating animals to solve this problem has received limited consideration, even though wind and ocean currents cause the lateral displacement of flyers and swimmers, respectively, particularly during long‐distance journeys of 1000s of kilometres. Here, we examine this problem by combining long‐distance, open‐ocean marine turtle movements (obtained via long‐term GPS tracking of sea turtles moving 1000s of km), with a high resolution basin‐wide physical ocean model to estimate ocean currents. We provide a robust mathematical framework to demonstrate that, while turtles eventually arrive at their target site, they do not follow the optimum (Zermelo's) route. Even though adult marine turtles regularly complete incredible long‐distance migrations, these vertebrates primarily rely on course corrections when entering neritic waters during the final stages of migration. Our work introduces a new perspective in the analysis of wildlife tracking datasets, with different animal groups potentially exhibiting different levels of complexity in goal attainment during migration.     

Persistent leatherback turtle migrations present opportunities for conservation

Effective transboundary conservation of highly migratory marine animals requires international management cooperation as well as clear scientific information about habitat use by these species. Populations of leatherback turtles (Dermochelys coriacea) in the eastern Pacific have declined by >90% during the past two decades, primarily due to unsustainable egg harvest and fisheries bycatch mortality. While research and conservation efforts on nesting beaches are ongoing, relatively little is known about this population of leatherbacks' oceanic habitat use and migration pathways. We present the largest multi-year (2004-2005, 2005-2006, and 2007) satellite tracking dataset (12,095 cumulative satellite tracking days) collected for leatherback turtles. Forty-six females were electronically tagged during three field seasons at Playa Grande, Costa Rica, the largest extant nesting colony in the eastern Pacific. After completing nesting, the turtles headed southward, traversing the dynamic equatorial currents with rapid, directed movements. In contrast to the highly varied dispersal patterns seen in many other sea turtle populations, leatherbacks from Playa Grande traveled within a persistent migration corridor from Costa Rica, past the equator, and into the South Pacific Gyre, a vast, low-energy, low-productivity region. We describe the predictable effects of ocean currents on a leatherback migration corridor and characterize long-distance movements by the turtles in the eastern South Pacific. These data from high seas habitats will also elucidate potential areas for mitigating fisheries bycatch interactions. These findings directly inform existing multinational conservation frameworks and provide immediate regions in the migration corridor where conservation can be implemented. We identify high seas locations for focusing future conservation efforts within the leatherback dispersal zone in the South Pacific Gyre.     

Pheromones in marine algae: A technical approach

It is now well known that many marine organisms use low-molecular volatile substances as signals, in order to coordinate activities between different individuals. The study of such pheromones requires the isolation and enrichment of the secretions from undisturbed living cells or organisms over extended periods of time. The Grob-Hersch extraction device, which we describe here, avoids adverse factors for the biological materials such as strong water currents, rising gas bubbles or chemical solvents. Furthermore, the formation of sea-water spray is greatly reduced. The application of this technique for the isolation of pheromones of marine algae and animals is described.     

Towards the integration of animal‐borne instruments into global ocean observing systems

Marine animals are increasingly instrumented with environmental sensors that provide large volumes of oceanographic data. Here, we conduct an innovative and comprehensive global analysis to determine the potential contribution of animal‐borne instruments (ABI) into ocean observing systems (OOSs) and provide a foundation to establish future integrated ocean monitoring programmes. We analyse the current gaps of the long‐term Argo observing system (>1.5 million profiles) and assess its spatial overlap with the distribution of marine animals across eight major species groups (tuna and billfishes, sharks and rays, marine turtles, pinnipeds, cetaceans, sirenians, flying seabirds and penguins). We combine distribution ranges of 183 species and satellite tracking observations from >3,000 animals. Our analyses identify potential areas where ABI could complement OOS. Specifically, ABI have the potential to fill gaps in marginal seas, upwelling areas, the upper 10 m of the water column, shelf regions and polewards of 60° latitude. Our approach provides the global baseline required to plan the integration of ABI into global and regional OOS while integrating conservation and ocean monitoring priorities.     

Sea turtle nesting distributions and oceanographic constraints on hatchling migration

Patterns of abundance across a species''s reproductive range are influenced by ecological and environmental factors that affect the survival of offspring. For marine animals whose offspring must migrate long distances, natural selection may favour reproduction in areas near ocean currents that facilitate migratory movements. Similarly, selection may act against the use of potential reproductive areas from which offspring have difficulty emigrating. As a first step towards investigating this conceptual framework, we analysed loggerhead sea turtle (Caretta caretta) nest abundance along the southeastern US coast as a function of distance to the Gulf Stream System (GSS), the ocean current to which hatchlings in this region migrate. Results indicate that nest density increases as distance to the GSS decreases. Distance to the GSS can account for at least 90 per cent of spatial variation in regional nest density. Even at smaller spatial scales, where local beach conditions presumably exert strong effects, at least 38 per cent of the variance is explained by distance from the GSS. These findings suggest that proximity to favourable ocean currents strongly influences sea turtle nesting distributions. Similar factors may influence patterns of abundance across the reproductive ranges of diverse marine animals, such as penguins, eels, salmon and seals.     

Ultrasonic Telemetry, Tracking and Automated Monitoring Technology for Sharks

Sharks were among the first marine animals to carry telemetry systems because of their size and the need to understand their interactions with humans. Modern telemetry systems can gather many kinds of data (limited only by imagination, funding and sensor types), indicating which animals are near telemetry receivers and what they are doing. Receivers now range from simple autonomous detector units for deployment in mid-water in large-scale grids, to sophisticated automated benthic recorders, to triangulating radio-linked buoy systems (RAP), to ship-borne transponders. In addition, archival tags can now gather and store data even while the shark is away, to be downloaded later. Older types had to be recovered, but popup tags release from sharks automatically, surface and transfer data to satellites, while CHAT tags download whenever queried by a nearby transponding acoustic receiver. Sophisticated animal-borne tags dramatically increase the information gathered about sharks and their environment. The examples provided show the parallel progression of shark biology and acoustic biotelemetry illustrating that telemetry systems are tools for gathering data, which can often be honed to facilitate biological experiments. Future visions include sensors that directly measure shark swimming power and cardiac output, compressing the data so that it can be delivered to RAP systems tracking multiple animals with meter resolution in near real time. CHAT tags as small as 22mm diameter should be able to return similar data from trips of hundreds of kilometers. Continued communication between biologists and engineers is essential to develop these technologies.     

Tracking and data–logging devices attached to elephant seals do not affect individual mass gain or survival

Understanding the cryptic lives of wide–ranging wild animals such as seals can be challenging, but with the advent of miniaturised telemetry and data–logging devices this is now possible and relatively straightforward. However, because marine animals have streamline bodies to reduce drag in their aquatic habitats, attaching external devices to their back or head may affect swimming performance, prey capture efficiency and ultimately, fitness. Given this, and allied welfare concerns, we assessed the short- and long-term consequences of external devices attached to southern elephant seal juveniles and adults under varying environmental conditions. We also assessed the effects of multiple deployments on individuals. There was no evidence for short-term differences in at-sea mass gain (measured as mass on arrival from a foraging trip) or long-term survival rate. The number of times that a seal carried a tracking device (ranging from 1 to 8 times) did not affect mass or estimated survival. Further, there were no tracking device effects in years of contrasting environmental conditions measured as ENSO anomalies. Consequently, we conclude that the current tracking devices available to researchers are valuable conservation tools that do not adversely affect the performance of a large marine mammal in terms of mass gain or survival probability over short (seasonal) or long (years) temporal scales.     

Movements of foraging king penguins through marine mesoscale eddies

Despite increasing evidence that marine predators associate with mesoscale eddies, how these marine features influence foraging movements is still unclear. This study investigates the relationship of at-sea movements of king penguins to mesoscale eddies using oceanographic remote sensing and movement data from 43 individual trips over 4 years. Simultaneous satellite measurements provided information on gradients of sea surface temperature and currents associated with eddies determined from altimetry. Penguins tended to swim rapidly with currents as they travelled towards foraging zones. Swimming speed indicative of foraging occurred within mesoscale fronts and strong currents associated with eddies at the Polar Front. These results demonstrate the importance of mesoscale eddies in directing foraging efforts to allow predators to rapidly get to rich areas where high concentrations of prey are likely to be encountered. When returning to the colony to relieve the incubating partner or to feed the chick, the birds followed a direct and rapid path, seemingly ignoring currents.     

The influence of diet on foraging habitat models: a case study using nursing Antarctic fur seals

Predictable sources of food underpin lifetime reproductive output in long lived animals. The most important foraging areas of top marine predators are therefore likely to be related to environmental features that enhance productivity in predictable spatial and temporal patterns. Even so, although productive areas within the marine environment are distributed patchily in space and time, most studies assess the relationships between feeding activity and proximate, not long term, environmental characteristics. In addition, individuals within a population may exploit different prey types, and these are often associated with different hydrographic features. Until now, models attempting to associate core foraging areas (CFAs) of marine predators with the environmental characteristics of those areas have not considered the diet of individual animals, despite the influence this could have on these relationships. We used bathymetry and multi‐year (n=24) mean sea surface temperature and variability as predictors of CFAs of lactating Antarctic fur seals Arctocephalus gazella at Heard Island. The effect of prey types on the predictability of these models was explored by matching diet and foraging trip data of individual seals (n=40 seals, n=1 trip each). Differences in diet between seals were mirrored by their spatial behaviour. Foraging strategies differed both between and within groups of seals consuming different diets. Long‐term environmental parameters were useful for predicting the foraging activity of seals that consumed a single prey type with relatively specific habitat preferences, but not for those that consumed single or multiple prey types associated with more varied habitats. Ignoring individual variation in predator diet probably contributes to the poor performance of foraging habitat models. These findings highlight the importance of incorporating individual specialization in foraging behaviour into ecological models and management of predator populations.     

Protected areas in the North Sea: An absolute need for future marine research

There are many signals that different human activities affect the marine ecosystem on local and sometimes regional scales. There is evidence that in the Dutch sector of the North Sea at least 25 species have decreased tremendously in numbers or have totally disappeared. But what has caused their disappearance: fisheries, pollution, eutrophication, climatic changes, or a combination of causes? On the Dutch Continental Shelf, the fisheries are now so intensive that every square metre is trawled, on an average, once to twice a year. Furthermore, it has been shown that trawling causes direct damage to the marine ecosystem. This indicates that the “natural” North Sea ecosystem we are studying is already a heavily influenced system. And what is the value of data on the diversity and production of benthic animals, if the research area has been raked by beamtrawl gear an unknown amount of times before sampling? To be able to study the natural trends in the marine ecosystem, or to answer the question which human activity has most influenced the ecosystem, there is an absolute and immediate need for protected areas to be established. The size of the protected areas must be determined by the behaviour of that species characteristic for the area. In such areas, where fisheries and local pollution would be forbidden or very limited, scientific research into the species composition and age distribution of different populations should be carried out and trends should be established.     

IMOS National Reference Stations: A Continental-Wide Physical,Chemical and Biological Coastal Observing System

Sustained observations allow for the tracking of change in oceanography and ecosystems, however, these are rare, particularly for the Southern Hemisphere. To address this in part, the Australian Integrated Marine Observing System (IMOS) implemented a network of nine National Reference Stations (NRS). The network builds on one long-term location, where monthly water sampling has been sustained since the 1940s and two others that commenced in the 1950s. In-situ continuously moored sensors and an enhanced monthly water sampling regime now collect more than 50 data streams. Building on sampling for temperature, salinity and nutrients, the network now observes dissolved oxygen, carbon, turbidity, currents, chlorophyll a and both phytoplankton and zooplankton. Additional parameters for studies of ocean acidification and bio-optics are collected at a sub-set of sites and all data is made freely and publically available. Our preliminary results demonstrate increased utility to observe extreme events, such as marine heat waves and coastal flooding; rare events, such as plankton blooms; and have, for the first time, allowed for consistent continental scale sampling and analysis of coastal zooplankton and phytoplankton communities. Independent water sampling allows for cross validation of the deployed sensors for quality control of data that now continuously tracks daily, seasonal and annual variation. The NRS will provide multi-decadal time series, against which more spatially replicated short-term studies can be referenced, models and remote sensing products validated, and improvements made to our understanding of how large-scale, long-term change and variability in the global ocean are affecting Australia''s coastal seas and ecosystems. The NRS network provides an example of how a continental scaled observing systems can be developed to collect observations that integrate across physics, chemistry and biology.     

Pacific Coral-reef Fishes: The Implications of Behaviour and Ecology of Larvae for Biodiversity and Conservation, and a Reassessment of the Open Population Paradigm

The two-phase life history of most marine fishes and invertebrates has enormous implications for dispersal, population connectivity, and resource management. Pelagic dispersal larvae of marine animals traditionally thought to ensure that populations are widespread, that chances of local extinction are low, and that marine protected areas (MPA) can easily function to replenish both their own populations and those of unprotected areas. Traditionally, dispersal is considered to depend primarily on two variables: pelagic larva duration and far-field currents. These conclusions arise from the open population paradigm and are usually accompanied by a simplifying assumption: larvae are distributed passively by far-field currents. Unfortunately, they ignore the complex reality of circulation and hydrological connectivity of reefs, and do not consider newly-demonstrated behavioural capabilities of coral-reef fish larvae. Far-field circulation varies with depth and often excludes water bodies where propagules are released, and this has important implications for predicting trajectories of even passive larvae. However, larvae are not passive: late-stage larvae of coral-reef fishes can swim faster than currents for long periods, can probably detect reefs at some distance, and can actively find them. This behaviour is flexible, which greatly complicates modelling of larval fish trajectories. Populations at ecological (as opposed to evolutionary) scales are probably less open and more subdivided than previously assumed. All this means that dispersal predictions based solely on far-field water circulation are probably wrong. An emerging view of larval-fish dispersal is articulated that takes these new data and perspectives into account. This emerging view shows that re-evaluation of traditional views in several areas is required, including the contribution of larval-fish biology and dispersal to biodiversity patterns, the way reef fishes are managed, and the way in which MPA are thought to operate. At evolutionary and zoogeographic scales, reef-fish populations are best considered to be open.     

Identification of high-use habitat and threats to leatherback sea turtles in northern waters: new directions for conservation

Incidental capture in fisheries threatens many marine vertebrates, however, conservation cannot be effective without identifying major sources of mortality. For the critically endangered leatherback turtle (Dermochelys coriacea), a reliance on fisheries observer data and an absence of behavioural data sets corresponding to a large and diverse sample of animals have focused conservation efforts on a very limited part of the species marine habitat. Using the largest satellite telemetry data set for Atlantic leatherbacks, morphometrics from foraging animals and entanglement records, we show annual return migrations to key feeding areas by males, females and juveniles, and demonstrate the importance of northern latitudes to leatherbacks. We show that leatherbacks are vulnerable to entanglement in northern coastal and shelf waters, where turtle–fishery interactions represent a greater threat to this species than previously recognized. Unless conservation efforts expand to coastal and shelf areas, present efforts alone will not be sufficient to save the species.     

Navigational challenges in the oceanic migrations of leatherback sea turtles

The open-sea movements of marine animals are affected by the drifting action of currents that, if not compensated for, can produce non-negligible deviations from the correct route towards a given target. Marine turtles are paradigmatic skilful oceanic navigators that are able to reach remote goals at the end of long-distance migrations, apparently overcoming current drift effects. Particularly relevant is the case of leatherback turtles (Dermochelys coriacea), which spend entire years in the ocean, wandering in search of planktonic prey. Recent analyses have revealed how the movements of satellite-tracked leatherbacks in the Indian, Atlantic and Pacific Oceans are strongly dependent on the oceanic currents, up to the point that turtles are often passively transported over long distances. However, leatherbacks are known to return to specific areas to breed every 2–3 years, thus finding their way back home after long periods in the oceanic environment. Here we examine the navigational consequences of the leatherbacks'' close association with currents and discuss how the combined reliance on mechanisms of map-based navigation and local orientation cues close to the target may allow leatherbacks to accomplish the difficult task of returning to specific sites after years spent wandering in a moving medium.     

Perspectives and challenges for the use of radar in biological conservation

Radar is at the forefront for the study of broad‐scale aerial movements of birds, bats and insects and related issues in biological conservation. Radar techniques are especially useful for investigating species which fly at high altitudes, in darkness, or which are too small for applying electronic tags. Here, we present an overview of radar applications in biological conservation and highlight its future possibilities. Depending on the type of radar, information can be gathered on local‐ to continental‐scale movements of airborne organisms and their behaviour. Such data can quantify flyway usage, biomass and nutrient transport (bioflow), population sizes, dynamics and distributions, times and dimensions of movements, areas and times of mass emergence and swarming, habitat use and activity ranges. Radar also captures behavioural responses to anthropogenic disturbances, artificial light and man‐made structures. Weather surveillance and other long‐range radar networks allow spatially broad overviews of important stopover areas, songbird mass roosts and emergences from bat caves. Mobile radars, including repurposed marine radars and commercially dedicated ‘bird radars’, offer the ability to track and monitor the local movements of individuals or groups of flying animals. Harmonic radar techniques have been used for tracking short‐range movements of insects and other small animals of conservation interest. However, a major challenge in aeroecology is determining the taxonomic identity of the targets, which often requires ancillary data obtained from other methods. Radar data have become a global source of information on ecosystem structure, composition, services and function and will play an increasing role in the monitoring and conservation of flying animals and threatened habitats worldwide.     

Acquisition of fishes and aquatic invertebrates for zoological collections. Is there a future?

The majority of the freshwater fishes in the ornamental trade now originate from captive‐bred sources, as do a large proportion of the freshwater species exhibited in public aquariums. In contrast, commercial operators who also supply marine specimens to the ornamental trade remove directly from the wild approximately 98% of the marine fishes and invertebrates exhibited in public aquariums. The common perception prevails that captive propagation is inherently a better alternative to obtaining animals from the wild. Although captive propagation has been shown to have many benefits for terrestrial species, there are a number of features unique to marine species that challenge the idea that every species should be bred in captivity. Some of the key issues relating to the development of widespread conservation‐oriented captive propagation programs include: 1) the high taxonomic diversity in marine animals; 2) the resultant variety in their reproductive methods; 3) their ecological, behavioral, physiological, and nutritional needs; and 4) our general lack of knowledge on their husbandry and medical care. There are several characteristics of marine fish and invertebrate populations that make them suitable candidates for sustainable harvest. For instance, marine teleosts are “r‐selected,” meaning that they have an extremely high fecundity, and most marine teleosts have a wide distribution and the ability to disperse over long distances. In locations considered for fish collection, appropriate management techniques should be employed to ensure that fishes and invertebrates are collected with as little impact on the ecosystem as possible. The collection of marine fishes and invertebrates for public aquariums and the hobby trade should be managed like a fishery to ensure long‐term sustainability. The public aquarium community should support marine organism certification initiatives, such as the Marine Aquarium Council (MAC). Marine organism certification will create market incentives that encourage and support quality and sustainable practices by creating consumer demand and confidence for certified organisms, practices, and industry participants. The creation of refuges that supply propagules to harvested areas, the rotation of areas fished, species‐specific size limits and seasons, and standardization of collecting, handling, and transportation techniques should be used to manage these fisheries and harvest areas. Zoo Biol 22:519–527, 2003. © 2003 Wiley‐Liss, Inc.     

Habitat utilization by an invasive herbivorous fish (<Emphasis Type="Italic">Siganus rivulatus</Emphasis>) in its native and invaded range

Movement is essential for understanding the distribution and abundance of animals. While it has been suggested that invasion success can be facilitated by species’ ability to adapt to novel environments, direct comparisons of movement patterns between native and invaded ranges of animals in their natural habitat are rare. The rivulated rabbitfish Siganus rivulatus was introduced from the Red Sea into the Mediterranean, where it is now found in extremely high abundances, and has overgrazed the coastal marine ecosystem in many locations. Through a continuous acoustic tracking system, we found that the movement of S. rivulatus individuals at a Mediterranean site differed substantially from those at a Red Sea site, with individuals in the Mediterranean having larger overall home ranges and lower site fidelity. However, no variation between sites was found in daily home range sizes. Results show that at the Mediterranean site S. rivulatus individuals have a larger spatial footprint, which may contribute to their impact and ability to expand their distribution. This study demonstrates a potential shift in individual movement of a marine invasive species between its native and invaded range, and highlights the role of movement in understanding biological invasions.     

Markov models and network analysis reveal sex‐specific differences in the space‐use of a coastal apex predator

Understanding the links between external variables such as habitat and interactions with conspecifics and animal space‐use is fundamental to developing effective management measures. In the marine realm, automated acoustic tracking has become a widely used method for monitoring the movement of free‐ranging animals, yet researchers generally lack robust methods for analysing the resulting spatial‐usage data. In this study, acoustic tracking data from male and female broadnose sevengill sharks Notorynchus cepedianus, collected in a system of coastal embayments in southeast Tasmania were analyzed to examine sex‐specific differences in the sharks’ coastal space‐use and test novel methods for the analysis of acoustic telemetry data. Sex‐specific space‐use of the broadnose sevengill shark from acoustic telemetry data was analysed in two ways: The recently proposed spatial network analysis of between‐receiver movements was employed to identify sex‐specific space‐use patterns. To include the full breadth of temporal information held in the data, movements between receivers were furthermore considered as transitions between states of a Markov chain, with the resulting transition probability matrix allowing the ranking of the relative importance of different parts of the study area. Both spatial network and Markov chain analysis revealed sex‐specific preferences of different sites within the study area. The identification of priority areas differed for the methods, due to the fact that in contrast to network analysis, our Markov chain approach preserves the chronological sequence of detections and accounts for both residency periods and movements. In addition to adding to our knowledge of the ecology of a globally distributed apex predator, this study presents a promising new step towards condensing the vast amounts of information collected with acoustic tracking technology into straightforward results which are directly applicable to the management and conservation of any species that meet the assumptions of our model.     

New perspectives for research on antarctic birds and mammals

New technology, that is based on miniaturization of electronic equipment and progress in computers, now enables us to get data on the foraging behavior of Antarctic birds and mammals. This technical revolution is of a major scientific importance because until recently it was considered impossible to study where animals feed and how much they consume. Accordingly, feeding strategies may now be studied in relation to breeding status or availability in marine resources. This, in turn, facilitates the use of antarctic animals as indicators of the changes in these resources. Automatic indentification and weighing of breeding seabirds has also become possible. This means changes in their body condition, i.e. body fuel reserves and food stores at sea, can be monitored. Thus a short-term indication of the changes in the availability of some marine resources may now be obtained. These new methods represent a major advance because they open new research possibilities, whereby antarctic animals can be individually monitored or at a population level, ashore and at sea, while minimizing human disturbance.Paper presented at the Symposium on Polar regions: the challenge for biological and ecological research organized by the Swiss Committee for Polar Research, Basel on 2 October 1992     

The Torres Reefs,North Queensland,Australia —strong tidal flows a modern control on their growth

High resolution seismic data from Torres Strait off northern Australia provide the first insights into the development of the striking platform reefs of the region. In the southern part of Torres Strait, where the sea floor is shallow and featureless, the Torres Reefs have developed as long, narrow platforms parallel to very strong east-west flowing tidal currents. The reefs have expanded since the end of the post-glacial marine transgression (approximately 6000 years ago) through preferential growth at each end of their platforms. In these localized areas sheltered from the strong tidal currents, patch reefs have developed. Sediments swept from the reef margin and the inter-reef channels are deposited in these current lee areas at the ends of the reefs. The combination of the patch reefs and reefal sediments provides the essentials for extension of the platform reefs. The seismic data confirm that modern reef expansion is occurring without there being a high substrate for coral colonization. The evolutionary sequence from an uncolonized seafloor to a mature platform reef envisages a synchronous process of patch reef development and sediment accumulation.