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Calculation of dietary niche characteristics using stable isotopes has become a popular approach to understand the functional role of taxa across food webs. An underlying assumption of this approach is that stable isotopes accurately reflect the dietary breadth of a species over a temporal duration defined by tissue-specific isotopic turnover rates. In theory, dietary niche estimates derived from fast turnover rate tissues (e.g., blood plasma and liver) may augment stomach content-derived estimates more agreeably than slower turnover rate tissues (e.g., muscle or fin). We tested this hypothesis by comparing commonly used dietary niche estimates derived from stomach contents (nicheSCA: Levins’, Shannon–Wiener’s, and Smith’s), with those estimated using stable isotopes [nicheSIA: standard ellipse area (SEA), convex hull total area (TA), theta (θ), and ellipse eccentricity (E)] of liver and muscle tissue. Model species were three large-bodied sharks: white (Carcharodon carcharias), dusky (Carcharhinus obscurus), and scalloped hammerhead (Sphyrna lewini). Within-technique comparisons for nicheSCA and nicheSIA metrics (i.e., SEA vs. TA) were often correlated; however, we did not observe any statistically significant correlations between nicheSCA and liver/muscle tissue nicheSIA (i.e., Levins’ vs. SEA). We conclude that nicheSCA and nicheSIA do not provide comparable estimates of dietary niche, at least for the three predator species examined. This fundamental discrepancy highlights technique-specific limitations to estimating organismal dietary niche and identifies a need for the use of clearly defined niche metrics, i.e., the standardized use and reporting of the term isotopic niche as proposed by Newsome et al. (Front Ecol Environ 5:429–436, 2007). Finally, further investigation into the factors underpinning nicheSIA is required to better contextualize this popular ecological metric when compared to nicheSCA.

Spatial management for highly migratory species (HMS) is difficult due to many species’ mobile habits and the dynamic nature of oceanic habitats. Current static spatial management areas for fisheries in the United States have been in place for extended periods of time with limited data collection inside the areas, making any analysis of their efficacy challenging. Spatial modeling approaches can be specifically designed to integrate species data from outside of closed areas to project species distributions inside and outside closed areas relative to the fishery. We developed HMS-PRedictive Spatial Modeling (PRiSM), which uses fishery-dependent observer data of species’ presence–absence, oceanographic covariates, and gear covariates in a generalized additive model (GAM) framework to produce fishery interaction spatial models. Species fishery interaction distributions were generated monthly within the domain of two HMS longline fisheries and used to produce a series of performance metrics for HMS closed areas. PRiSM was tested on bycatch species, including shortfin mako shark (Isurus oxyrinchus), billfish (Istiophoridae), and leatherback sea turtle (Dermochelys coriacea) in a pelagic longline fishery, and sandbar shark (Carcharhinus plumbeus), dusky shark (C. obscurus), and scalloped hammerhead shark (Sphyrna lewini) in a bottom longline fishery. Model validation procedures suggest PRiSM performed well for these species. The closed area performance metrics provided an objective and flexible framework to compare distributions between closed and open areas under recent environmental conditions. Fisheries managers can use the metrics generated by PRiSM to supplement other streams of information and guide spatial management decisions to support sustainable fisheries.

Studying the behavior of pelagic sharks can be challenging due to the logistical difficulties of locating migratory individuals in the open ocean. This is further compounded by their rarity in certain ocean regions due to overfishing. The accessibility of deep-water basins in The Bahamas provides a unique model system for describing how pelagic sharks connect surface and deep ocean habitats through vertical behavior. Using custom deep-sea landers in The Bahamas, we obtained novel depth data for two species of pelagic sharks: the bignose shark at 767 m and the silky shark at 470 m. These new observations support and corroborate previous vertical niche records for these two species, suggesting that deep diving behavior is likely more common than previously thought and likely important for ecosystem connectivity.

When we downscaled the approach of Queiroz et al.3, we found errors in the data used to evaluate fishing exposure in these waters that were derived using a machine learning approach applied to vessel automatic identification system (AIS) location data5. In Western Australian state waters-an area larger than the Bering Sea-99.8% of longline and 100% of purse seine AIS data were incorrectly classified by the machine learning algorithm (Table 1 and Fig. 1). Contemporary longlining by a domestic tuna and billfish fishery still occurs, although these vessels were absent from the AIS data used by Queiroz et al.3. Since 2005, the intensity of this fishery has decreased and its footprint shifted to the southwest9. In Western Australia, the findings of Queiroz et al.3 risk undermining confidence in the science-based management controls that are already implemented to protect the mature biomass of long-lived dusky shark (Carcharhinus obscurus) and sandbar shark (C. plumbeus) stocks in the region12.

The International Union for Conservation of Nature's (IUCN) Red List is the global standard for quantifying extinction risk but assessing population reduction (criterion A) of wide‐ranging, long‐lived marine taxa remains difficult and controversial. We show how Bayesian state–space models (BSSM), coupled with expert knowledge at IUCN Red List workshops, can combine regional abundance data into indices of global population change. To illustrate our approach, we provide examples of the process to assess four circumglobal sharks with differing temporal and spatial data‐deficiency: Blue Shark (Prionace glauca), Shortfin Mako (Isurus oxyrinchus), Dusky Shark (Carcharhinus obscurus), and Great Hammerhead (Sphyrna mokarran). For each species, the BSSM provided global population change estimates over three generation lengths bounded by uncertainty levels in intuitive outputs, enabling informed decisions on the status of each species. Integrating similar analyses into future workshops would help conservation practitioners ensure robust, consistent, and transparent Red List assessments for other long‐lived, wide‐ranging species.

The epidermal microbiome is a critical element of marine organismal immunity, but the epidermal virome of marine organisms remains largely unexplored. The epidermis of sharks represents a unique viromic ecosystem. Sharks secrete a thin layer of mucus which harbors a diverse microbiome, while their hydrodynamic dermal denticles simultaneously repel environmental microbes. Here, we sampled the virome from the epidermis of three shark species in the family Carcharhinidae: the genetically and morphologically similar Carcharhinus obscurus (n = 6) and Carcharhinus galapagensis (n = 10) and the outgroup Galeocerdo cuvier (n = 15). Virome taxonomy was characterized using shotgun metagenomics and compared with a suite of multivariate analyses. All three sharks retain species-specific but highly similar epidermal viromes dominated by uncharacterized bacteriophages which vary slightly in proportional abundance within and among shark species. Intraspecific variation was lower among C. galapagensis than among C. obscurus and G. cuvier. Using both the annotated and unannotated reads, we were able to determine that the Carcharhinus galapagensis viromes were more similar to that of G. cuvier than they were to that of C. obscurus, suggesting that behavioral niche may be a more prominent driver of virome than host phylogeny.

Social and non-social animals can aggregate at a specific site for various reasons such as reproduction, feeding, or other synchronized patterns of movements. While shark aggregations are well documented, mixed-species aggregations are less studied and therefore poorly understood. To overcome this, a combination of acoustic telemetry and social network analysis was used to investigate population structure and behavior based on the temporal overlap of 22 individuals of 57 tagged sharks of two species, the sandbar (Carcharhinus plumbeus) and the dusky (Carcharhinus obscurus) sharks, that form a winter mixed-species aggregation in front of an Israeli coastal power plant. The results suggested that if both species co-occurred and share the study site, their finer-scale associations revealed temporal partitioning between species and species assortment in sandbar sharks. The multi-species network was also structured by sex. The difference between species may indicate separate strategies and temporal niche partitioning at the aggregation site. The particularly warmer temperatures (~ 5–10 °C warmer) caused by the electric power plant suggest that female dusky sharks follow the thermal niche–fecundity hypothesis by selecting warmer waters to optimize gestation, while male sandbar sharks socialize at the site. This study represents the first attempt to examine the fine-scale structure of a mixed-species aggregation of sharks and provides new insights into the shark’s social structures through tolerance of each other and social-niche partitioning in this mixed-species aggregation.

Information about spatial and temporal variability in the distribution and abundance of shark-populations are required for their conservation, management and to update measures designed to mitigate human-shark interactions. However, because some species of sharks are mobile, migratory and occur in relatively small numbers, estimating their patterns of distribution and abundance can be very difficult. In this study, we used a hierarchical sampling design to examine differences in the composition of species, size- and sex-structures of sharks sampled with bottom-set longlines in three different areas with increasing distance from the entrance of Sydney Harbour, a large urbanised estuary. During two years of sampling, we obtained data for four species of sharks (Port Jackson, Heterodontus portusjacksoni; wobbegong, Orectolobus maculatus; dusky whaler, Carcharhinus obscurus and bull shark, Carcharhinus leucas). Only a few O. maculatus and C. obscurus were caught, all in the area closest to the entrance of the Harbour. O. maculatus were caught in all seasons, except summer, while C. obscurus was only caught in summer. Heterodontus portusjacksoni were the most abundant species, caught in the entrance location mostly between July to November, when water temperature was below 21.5°C. This pattern was consistent across both years. C. leucas, the second most abundant species, were captured in all areas of Sydney Harbour but only in summer and autumn when water temperatures were above 23°C. This study quantified, for this first time, how different species utilise different areas of Sydney Harbour, at different times of the year. This information has implications for the management of human-shark interactions, by enabling creation of education programs to modify human behaviour in times of increased risk of potentially dangerous sharks.

Estuaries function as important nursery and foraging habitats for many coastal species, including highly migratory sharks. Pamlico Sound, North Carolina, is one of the largest estuaries in the continental United States and provides a variety of potential habitats for sharks. In order to identify and spatially delineate shark habitats within Pamlico Sound, shark catch and environmental data were analyzed from the 2007-2014 North Carolina Division of Marine Fisheries (NCDMF) gillnet and longline surveys conducted within the estuary. Principal species were identified and environmental data recorded at survey sites (depth, temperature, salinity, dissolved oxygen, submerged aquatic vegetation (SAV) distance, and inlet distance) were interpolated across Pamlico Sound to create seasonal environmental grids with a 90-m2 cell size. Boosted Regression Tree (BRT) analysis was used to identify the most important environmental factors and ranges associated with presence of each principal species, and the resulting models were used to predict shark capture probability based on the environmental values within the grid cells. The Atlantic Sharpnose Shark (Rhizoprionodon terraenovae), Blacktip Shark (Carcharhinus limbatus), Bull Shark (Carcharhinus leucas), Sandbar Shark (Carcharhinus plumbeus), Smooth Dogfish (Mustelus canis), and Spiny Dogfish (Squalus acanthias) were the principal species in Pamlico Sound. Most species were associated with proximity to the inlet and/or high salinity, and warm temperatures, but the Bull Shark preferred greater inlet distances and the Spiny Dogfish preferred lower temperatures than the other species. Extensive Smooth Dogfish habitat overlap with seagrass beds suggests that seagrass may be a critical part of nursery habitat for this species. Spatial delineation of shark habitat within the estuary will allow for better protection of essential habitat and assessment of potential interactions with other species.

Highly mobile species can be challenging for fisheries management and conservation due to large home ranges combined with dependence on discrete habitat areas where they can be easily targeted or vulnerable to anthropogenic disturbances. Management of the Dusky Shark Carcharhinus obscurus in the northwest Atlantic Ocean has been particularly challenging due to the species’ inherent vulnerability to overfishing and poorly understood habitat associations. To better understand habitat associations and seasonal distributions, we combined telemetry and remotely sensed environmental data to spatially model juvenile Dusky Shark presence probability in the northwest Atlantic Ocean. To accomplish this, 22 juvenile Dusky Sharks (107–220 cm TL) that were tagged with acoustic transmitters at different locations within the U.S. Middle Atlantic Bight region were tracked through networked arrays of acoustic receivers. Tag detections were summarized as daily presence records, and data describing environmental conditions, including depth, chlorophyll‐a concentration, salinity, and sea surface temperature, were extracted at detection locations. These data were used in boosted regression tree models to predict juvenile Dusky Shark presence probability based on environmental parameters during fall 2017 and summer 2018. Telemetry observations and modeled presence probability showed consistent associations with temperatures between 16°C and 26°C and chlorophyll‐a concentrations between 2 and 7 mg/m3, which were associated with seasonal migration timing and monthly spatial distributions. Dusky Shark tag detections and predicted distributions during summer and early fall overlapped areas in the Middle Atlantic Bight that were affected by fisheries and potential offshore energy development. Our methodology provides a framework for assessing climate change effects on distribution.