Linking vertical movements of large pelagic predators with distribution patterns of biomass in the open ocean.

Many predator species make regular excursions from near-surface waters to the twilight (200 to 1,000 m) and midnight (1,000 to 3,000 m) zones of the deep pelagic ocean. While the occurrence of significant vertical movements into the deep ocean has evolved independently across taxonomic groups, the functional role(s) and ecological significance of these movements remain poorly understood. Here, we integrate results from satellite tagging efforts with model predictions of deep prey layers in the North Atlantic Ocean to determine whether prey distributions are correlated with vertical habitat use across 12 species of predators. Using 3D movement data for 344 individuals who traversed nearly 1.5 million km of pelagic ocean in [Formula: see text]42,000 d, we found that nearly every tagged predator frequented the twilight zone and many made regular trips to the midnight zone. Using a predictive model, we found clear alignment of predator depth use with the expected location of deep pelagic prey for at least half of the predator species. We compared high-resolution predator data with shipboard acoustics and selected representative matches that highlight the opportunities and challenges in the analysis and synthesis of these data. While not all observed behavior was consistent with estimated prey availability at depth, our results suggest that deep pelagic biomass likely has high ecological value for a suite of commercially important predators in the open ocean. Careful consideration of the disruption to ecosystem services provided by pelagic food webs is needed before the potential costs and benefits of proceeding with extractive activities in the deep ocean can be evaluated.

A vision for incorporating human mobility in the study of human-wildlife interactions.

As human activities increasingly shape land- and seascapes, understanding human-wildlife interactions is imperative for preserving biodiversity. Habitats are impacted not only by static modifications, such as roads, buildings and other infrastructure, but also by the dynamic movement of people and their vehicles occurring over shorter time scales. Although there is increasing realization that both components of human activity substantially affect wildlife, capturing more dynamic processes in ecological studies has proved challenging. Here we propose a conceptual framework for developing a 'dynamic human footprint' that explicitly incorporates human mobility, providing a key link between anthropogenic stressors and ecological impacts across spatiotemporal scales. Specifically, the dynamic human footprint integrates a range of metrics to fully acknowledge the time-varying nature of human activities and to enable scale-appropriate assessments of their impacts on wildlife behaviour, demography and distributions. We review existing terrestrial and marine human-mobility data products and provide a roadmap for how these could be integrated and extended to enable more comprehensive analyses of human impacts on biodiversity in the Anthropocene.

Direct measurement of cruising and burst swimming speeds of the shortfin mako shark (Isurus oxyrinchus) with estimates of field metabolic rate.

The shortfin mako shark is a large-bodied pursuit predator thought to be capable of the highest swimming speeds of any elasmobranch and potentially one of the highest energetic demands of any marine fish. Nonetheless, few direct speed measurements have been reported for this species. Here, animal-borne bio-loggers attached to two mako sharks were used to provide direct measurements of swimming speeds, kinematics and thermal physiology. Mean sustained (cruising) speed was 0.90 m s-1 (±0.07 s.d.) with a mean tail-beat frequency (TBF) of 0.51 Hz (±0.16 s.d.). The maximum burst speed recorded was 5.02 m s-1 (TBFmax = 3.65 Hz) from a 2 m long female. Burst swimming was sustained for 14 s (mean speed = 2.38 m s-1 ), leading to a 0.24°C increase in white muscle temperature in the 12.5 min after the burst. Routine field metabolic rate was estimated at 185.2 mg O2 kg-1 h-1 (at 18°C ambient temperature). Gliding behaviour (zero TBF) was more frequently observed after periods of high activity, especially after capture when internal (white muscle) temperature approached 21°C (ambient temperature: 18.3°C), indicating gliding probably functions as an energy recovery mechanism and limits further metabolic heat production. The results show shortfin mako sharks generally cruise at speeds similar to other endothermic fish - but faster than ectothermic sharks - with the maximum recorded burst speed being among the highest so far directly measured among sharks, tunas and billfishes. This newly recorded high-oxygen-demand performance of mako sharks suggests it may be particularly vulnerable to habitat loss due to climate-driven ocean deoxygenation.

Building use-inspired species distribution models: Using multiple data types to examine and improve model performance.

Species distribution models (SDMs) are becoming an important tool for marine conservation and management. Yet while there is an increasing diversity and volume of marine biodiversity data for training SDMs, little practical guidance is available on how to leverage distinct data types to build robust models. We explored the effect of different data types on the fit, performance and predictive ability of SDMs by comparing models trained with four data types for a heavily exploited pelagic fish, the blue shark (Prionace glauca), in the Northwest Atlantic: two fishery dependent (conventional mark-recapture tags, fisheries observer records) and two fishery independent (satellite-linked electronic tags, pop-up archival tags). We found that all four data types can result in robust models, but differences among spatial predictions highlighted the need to consider ecological realism in model selection and interpretation regardless of data type. Differences among models were primarily attributed to biases in how each data type, and the associated representation of absences, sampled the environment and summarized the resulting species distributions. Outputs from model ensembles and a model trained on all pooled data both proved effective for combining inferences across data types and provided more ecologically realistic predictions than individual models. Our results provide valuable guidance for practitioners developing SDMs. With increasing access to diverse data sources, future work should further develop truly integrative modeling approaches that can explicitly leverage the strengths of individual data types while statistically accounting for limitations, such as sampling biases.

Juvenile survival and movements of two threatened oceanic sharks in the North Atlantic Ocean inferred from tag-recovery data.

Understanding population dynamics, movements, and fishing mortality is critical to establish effective shark conservation measures across international boundaries in the ocean. There are few survival and dispersal estimates of juveniles of oceanic shark species in the North Atlantic despite it being one of the most fished regions in the world. Here we provide estimates of dispersal, survival, and proportion of fishing mortality in the North Atlantic for two threatened oceanic sharks: the blue shark (Prionace glauca) and the shortfin mako shark (Isurus oxyrinchus). Our results are based on multi-event models applied to tag-recovery data of 700 blue sharks and 132 shortfin makos tagged over a decade. A total of 60 blue sharks (8.57% of tagged) and 30 makos (22.73%) were recovered by the longline fishery between 2009 and 2017. Tag-reporting rate (percentage of returned information when a tagged shark was caught) was estimated to be high (0.794 ± 0.232 SE). Mean annual survival, as predicted from the models, was higher for blue shark (0.835 ± 0.040 SE) than for shortfin mako (0.618 ± 0.189 SE). Models predicted that fishing caused more than a half of total mortality in the study area for both species (0.576 ± 0.209), and more than a third of tagged individuals dispersed from the study area permanently (0.359 ± 0.073). Our findings, focused mainly on juveniles from oceanic areas, contribute to a better understanding of shark population dynamics in the North Atlantic and highlight the need for further conservation measures for both blue shark and shortfin mako, such as implementing efficient bycatch mitigation measures and static/dynamic time-area closures in the open ocean.

Circles in the sea: annual courtship "torus" behaviour of basking sharks Cetorhinus maximus identified in the eastern North Atlantic Ocean.

Groups of basking sharks engaged in circling behaviour are rarely observed, and their function remains enigmatic in the absence of detailed observations. Here, underwater and aerial video recordings of multiple circling groups of basking sharks during late summer (August and September 2016-2021) in the eastern North Atlantic Ocean showed groups numbering between 6 and 23 non-feeding individuals of both sexes. Sharks swam slowly in a rotating "torus" (diameter range: 17-39 m), with individuals layered vertically from the surface to a maximum depth of 16 m. Within a torus, sharks engaged in close-following, echelon, close-flank approach or parallel-swimming behaviours. Measured shark total body lengths were 5.4-9.5 m (mean LT : 7.3 m ± 0.9 s.d.; median: 7.2 m, n = 27), overlapping known lengths of sexually mature males and females. Males possessed large claspers with abrasions that were also observed on female pectoral fins. Female body colouration was paler than that of males, similar to colour changes observed during courtship and mating in other shark species. Individuals associated with most other members rapidly (within minutes), indicating toroidal behaviours facilitate multiple interactions. Sharks interacted through fin-fin and fin-body contacts, rolling to expose the ventral surfaces to following sharks, and breaching behaviour. Toruses formed in late summer when feeding aggregations in zooplankton-rich thermal fronts switched to non-feeding following and circling behaviours. Collectively, the observations explain a courtship function for toruses. This study highlights northeast Atlantic coastal waters as a critical habitat supporting courtship reproductive behaviour of endangered basking sharks, the first such habitat identified for this species globally.

Global collision-risk hotspots of marine traffic and the world's largest fish, the whale shark.

Marine traffic is increasing globally yet collisions with endangered megafauna such as whales, sea turtles, and planktivorous sharks go largely undetected or unreported. Collisions leading to mortality can have population-level consequences for endangered species. Hence, identifying simultaneous space use of megafauna and shipping throughout ranges may reveal as-yet-unknown spatial targets requiring conservation. However, global studies tracking megafauna and shipping occurrences are lacking. Here we combine satellite-tracked movements of the whale shark, Rhincodon typus, and vessel activity to show that 92% of sharks' horizontal space use and nearly 50% of vertical space use overlap with persistent large vessel (>300 gross tons) traffic. Collision-risk estimates correlated with reported whale shark mortality from ship strikes, indicating higher mortality in areas with greatest overlap. Hotspots of potential collision risk were evident in all major oceans, predominantly from overlap with cargo and tanker vessels, and were concentrated in gulf regions, where dense traffic co-occurred with seasonal shark movements. Nearly a third of whale shark hotspots overlapped with the highest collision-risk areas, with the last known locations of tracked sharks coinciding with busier shipping routes more often than expected. Depth-recording tags provided evidence for sinking, likely dead, whale sharks, suggesting substantial "cryptic" lethal ship strikes are possible, which could explain why whale shark population declines continue despite international protection and low fishing-induced mortality. Mitigation measures to reduce ship-strike risk should be considered to conserve this species and other ocean giants that are likely experiencing similar impacts from growing global vessel traffic.

Tissue and gender-related differences in the elemental composition of juvenile ocean sunfish (Mola spp.).

Trace elements are potentially critical contaminants of aquatic environments and fish, occupying upper trophic levels, are especially vulnerable to bioaccumulation. Due to public health concerns, however, data on the elemental composition of non-commercially important marine species are particularly lacking. Ocean sunfish (Mola spp.) attain a low commercial value worldwide and information on their elemental composition is limited. In this context, we examined the concentration of 11 trace elements (V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Cd and Pb) in eight tissues [brain, gills, gelatin (subcutaneous white gelatinous layer), gonads, spleen, liver, white muscle and red muscle] of 20 juvenile specimens (37.5-85.5 cm TL). Gender-related differences were solely found in the gonads and chiefly for essential elements possibly as a result of their importance in embryo development. Overall, Zn and As were the elements observed in greatest concentrations in body tissues. The considerably high presence of As should be related to the dietary preferences of juvenile ocean sunfish. Considerable inter-individual variability in the concentration of each element in any given tissue was observed, especially in the liver, likely originating from the inclusion of both benthic and pelagic prey in the diet of analysed fish. Greatest elemental loads were found in the liver and gills whereas lowest loads were observed in white muscle, brain and gelatin. Moreover, a clear distinction in elemental load and elemental composition was observed between white and red muscles, likely deriving from existing divergent metabolism-related physiological adaptations linked to their different roles in locomotion.

Climate-driven deoxygenation elevates fishing vulnerability for the ocean's widest ranging shark.

Climate-driven expansions of ocean hypoxic zones are predicted to concentrate pelagic fish in oxygenated surface layers, but how expanding hypoxia and fisheries will interact to affect threatened pelagic sharks remains unknown. Here, analysis of satellite-tracked blue sharks and environmental modelling in the eastern tropical Atlantic oxygen minimum zone (OMZ) shows shark maximum dive depths decreased due to combined effects of decreasing dissolved oxygen (DO) at depth, high sea surface temperatures, and increased surface-layer net primary production. Multiple factors associated with climate-driven deoxygenation contributed to blue shark vertical habitat compression, potentially increasing their vulnerability to surface fisheries. Greater intensity of longline fishing effort occurred above the OMZ compared to adjacent waters. Higher shark catches were associated with strong DO gradients, suggesting potential aggregation along suitable DO gradients contributed to habitat compression and higher fishing-induced mortality. Fisheries controls to counteract deoxygenation effects on shark catches will be needed as oceans continue warming.

Presence of plastic debris and retained fishing hooks in oceanic sharks.

In a context where the problem of plastic pollution is globally increasing, more studies are needed to assess the real impact in oceanic megafauna. Here, we reported on the incidence of plastic and also retained hooks in two species of commercially exploited pelagic sharks in two ocean basins, the North Atlantic and South Pacific. In the South Pacific, 1.18% of caught blue sharks were observed with plastic debris on their body and 4.82% and with retained hooks, while 0.00% of shortfin makos had plastic debris and 1.76% were recorded with retained hooks. In the North Atlantic, 0.21% of blue sharks had plastic debris and 0.37% of blue, and 0.78% of shortfin makos were observed with retained hooks.

Global spatial risk assessment of sharks under the footprint of fisheries.

Effective ocean management and the conservation of highly migratory species depend on resolving the overlap between animal movements and distributions, and fishing effort. However, this information is lacking at a global scale. Here we show, using a big-data approach that combines satellite-tracked movements of pelagic sharks and global fishing fleets, that 24% of the mean monthly space used by sharks falls under the footprint of pelagic longline fisheries. Space-use hotspots of commercially valuable sharks and of internationally protected species had the highest overlap with longlines (up to 76% and 64%, respectively), and were also associated with significant increases in fishing effort. We conclude that pelagic sharks have limited spatial refuge from current levels of fishing effort in marine areas beyond national jurisdictions (the high seas). Our results demonstrate an urgent need for conservation and management measures at high-seas hotspots of shark space use, and highlight the potential of simultaneous satellite surveillance of megafauna and fishers as a tool for near-real-time, dynamic management.

Body size and season influence elemental composition of tissues in ocean sunfish Mola mola juveniles.

The effects of body size and season on the elemental composition of ocean sunfish Mola mola were assessed for the first time. A total of 57 by-caught juvenile specimens measuring between 31.8 and 120.3 cm were sampled in spring and autumn. Concentrations of trace elements (three essential - Co, Cu, Zn, and three non-essential - As, Cd, Pb) were determined in five body tissues [gills, gelatin (subcutaneous white gelatinous layer), liver, white muscle and red muscle]. Elemental composition of M. mola tissues was found to vary with both body size and season. When an effect of size was verified, the most common trend was a decrease in elemental levels with increasing fish size, most likely deriving from the fast growth rate of this fish (i.e. dilution effect) and the occurrence of an ontogenetic shift in dietary preferences. Differently, Zn levels increased with fish size in both gills and red muscle, potentially deriving from a greater physiological need in those tissues as fish grow. Seasonal differences in trace element levels were observed for approximately half of the studied cases (tissue/trace element) with greater elemental concentrations being mostly found in autumn. Such pattern was most obvious in liver tissue, presumably resulting from a greater dietary elemental uptake in the end of summer/autumn. A general absence of seasonal differences was observed in the gills, white muscle and red muscle. Interestingly, a trend of enhanced concentrations in spring was observed for gelatin, potentially deriving from past long-term differences in exposure to trace elements.

World without borders-genetic population structure of a highly migratory marine predator, the blue shark (Prionace glauca).

Highly migratory, cosmopolitan oceanic sharks often exhibit complex movement patterns influenced by ontogeny, reproduction, and feeding. These elusive species are particularly challenging to population genetic studies, as representative samples suitable for inferring genetic structure are difficult to obtain. Our study provides insights into the genetic population structure one of the most abundant and wide-ranging oceanic shark species, the blue shark Prionace glauca, by sampling the least mobile component of the populations, i.e., young-of-year and small juveniles (<2 year; N = 348 individuals), at three reported nursery areas, namely, western Iberia, Azores, and South Africa. Samples were collected in two different time periods (2002-2008 and 2012-2015) and were screened at 12 nuclear microsatellites and at a 899-bp fragment of the mitochondrial control region. Our results show temporally stable genetic homogeneity among the three Atlantic nurseries at both nuclear and mitochondrial markers, suggesting basin-wide panmixia. In addition, comparison of mtDNA CR sequences from Atlantic and Indo-Pacific locations also indicated genetic homogeneity and unrestricted female-mediated gene flow between ocean basins. These results are discussed in light of the species' life history and ecology, but suggest that blue shark populations may be connected by gene flow at the global scale. The implications of the present findings to the management of this important fisheries resource are also discussed.

Oceanic adults, coastal juveniles: tracking the habitat use of whale sharks off the Pacific coast of Mexico.

Eight whale sharks tagged with pop-up satellite archival tags off the Gulf of California, Mexico, were tracked for periods of 14-134 days. Five of these sharks were adults, with four females visually assessed to be pregnant. At least for the periods they were tracked, juveniles remained in the Gulf of California while adults moved offshore into the eastern Pacific Ocean. We propose that parturition occurs in these offshore waters. Excluding two juveniles that remained in the shallow tagging area for the duration of tracking, all sharks spent 65 ± 20.7% (SD) of their time near the surface, even over deep water, often in association with frontal zones characterized by cool-water upwelling. While these six sharks all made dives into the meso- or bathypelagic zones, with two sharks reaching the maximum depth recordable by the tags (1285.8 m), time spent at these depths represented a small proportion of the overall tracks. Most deep dives (72.7%) took place during the day, particularly during the early morning and late afternoon. Pronounced habitat differences by ontogenetic stage suggest that adult whale sharks are less likely to frequent coastal waters after the onset of maturity.

Integrated Monitoring of Mola mola Behaviour in Space and Time.

Over the last decade, ocean sunfish movements have been monitored worldwide using various satellite tracking methods. This study reports the near-real time monitoring of fine-scale (< 10 m) behaviour of sunfish. The study was conducted in southern Portugal in May 2014 and involved satellite tags and underwater and surface robotic vehicles to measure both the movements and the contextual environment of the fish. A total of four individuals were tracked using custom-made GPS satellite tags providing geolocation estimates of fine-scale resolution. These accurate positions further informed sunfish areas of restricted search (ARS), which were directly correlated to steep thermal frontal zones. Simultaneously, and for two different occasions, an Autonomous Underwater Vehicle (AUV) video-recorded the path of the tracked fish and detected buoyant particles in the water column. Importantly, the densities of these particles were also directly correlated to steep thermal gradients. Thus, both sunfish foraging behaviour (ARS) and possibly prey densities, were found to be influenced by analogous environmental conditions. In addition, the dynamic structure of the water transited by the tracked individuals was described by a Lagrangian modelling approach. The model informed the distribution of zooplankton in the region, both horizontally and in the water column, and the resultant simulated densities positively correlated with sunfish ARS behaviour estimator (rs = 0.184, p<0.001). The model also revealed that tracked fish opportunistically displace with respect to subsurface current flow. Thus, we show how physical forcing and current structure provide a rationale for a predator's fine-scale behaviour observed over a two weeks in May 2014.