Spatial and Ontogenetic Patterns in the Trophic Ecology of Juvenile Bull Sharks (Carcharhinus leucas) From the Northwest Gulf of Mexico

A combination of stomach content and stable isotope (δ13C, δ15N, and δ34S) analyses were used to characterize and examine spatiotemporal and ontogenetic trends in the feeding ecology of juvenile bull sharks (Carcharhinus leucas) captured in estuaries throughout the northwest Gulf of Mexico (GoM) between 2013 and 2016. Shark diets were dominated by fish prey taxa [>98% index of relative importance (%IRI)], and of those identified to the family level, two families comprised greater than 50% IRI, Mugilidae (mullets: ∼32%) and Sciaenidae (drums and croakers: 27%). Clupeidae (herrings: 14%) and Ariidae (sea catfishes: 15%) also contributed substantially to the diet of juvenile sharks, though consumption of Ariidae increased as consumption of Clupeidae decreased in juvenile sharks larger than 893 mm Fork Length (FL) (∼1 year old). Values of δ15N increased significantly with shark size, indicating a shift toward larger or higher trophic level prey with increasing shark size. Latitudinal and temporal trends in δ13C and δ34S suggest isotopic variation occurred in correspondence with shifts in primary producer assemblages and environmental drivers of sampled estuaries. These results highlight the importance of teleost prey resources along the freshwater-marine continuum in the diet of juvenile bull sharks, as well as the utility of natural tracers in tracking ontogenetic trends in feeding ecology.

North Pacific warming shifts the juvenile range of a marine apex predator

During the 2014–2016 North Pacific marine heatwave, unprecedented sightings of juvenile white sharks (Carcharodon carcharias) emerged in central California. These records contradicted the species established life history, where juveniles remain in warmer waters in the southern California Current. This spatial shift is significant as it creates potential conflicts with commercial fisheries, protected species conservation, and public safety concerns. Here, we integrate community science, photogrammetry, biologging, and mesoscale climate data to describe and explain this phenomenon. We find a dramatic increase in white sharks from 2014 to 2019 in Monterey Bay that was overwhelmingly comprised of juvenile sharks < 2.5 m in total body length. Next, we derived thermal preferences from 22 million tag measurements of 14 juvenile sharks and use this to map the cold limit of their range. Consistent with historical records, the position of this cold edge averaged 34° N from 1982 to 2013 but jumped to 38.5° during the 2014–2016 marine heat wave. In addition to a poleward shift, thermally suitable habitat for juvenile sharks declined 223.2 km2 year−1 from 1982 to 2019 and was lowest in 2015 at the peak of the heatwave. In addition to advancing the adaptive management of this apex marine predator, we discuss this opportunity to engage public on climate change through marine megafauna.

The Use of Baited Remote Underwater Video Systems (BRUVS) to Noninvasively Characterize a White Shark (Carcharodon carcharias) Nursery Area off Eastern Long Island, New York

Understanding if a discrete region fits three key criteria for shark nursery area designation is of utmost importance. Such a designation within United States territorial waters could result in the implementation of an Essential Fish Habitat (EFH) classification, a determination facilitating species protection. In this study, baited remote underwater video systems (BRUVS) were used to determine if they could effectively confirm a previously identified white shark (Carcharodon carcharias) nursery area south of Montauk, New York, so that the technique may be applied in other potential nursery areas. During experimentation, 1,348.32 h of video data were collected between Region A (Block Island Sound; 674.16 h), outside of the previously identified nursery area, and Region B (Atlantic Ocean; 674.16 h), within the identified nursery area. From the video data, 35 sightings of C. carcharias were recorded, with 31 unique individuals identified. Data associated with Region B fit all three nursery area criteria: (a) young-of-the-year (YOY) and juvenile sharks were more commonly found in this region over Region A (p < 0.001), (b) several YOY and juvenile sharks (n = 4) were resighted in this region within a given year (e.g., average resighting duration: 10.5 days), and (c) YOY and juvenile sharks exhibited repeated use of this region over multiple years. We conclude that BRUVS can be used for fine-scale analyses to help designate a discrete area(s) as a shark nursery, even in turbid New York waters. Therefore, using BRUVS to identify shark nursery areas is a viable and noninvasive approach that can contribute to species conservation.

Risky business for a juvenile marine predator? Testing the influence of foraging strategies on size and growth rate under natural conditions

Mechanisms driving selection of body size and growth rate in wild marine vertebrates are poorly understood, thus limiting knowledge of their fitness costs at ecological, physiological and genetic scales. Here, we indirectly tested whether selection for size-related traits of juvenile sharks that inhabit a nursery hosting two dichotomous habitats, protected mangroves (low predation risk) and exposed seagrass beds (high predation risk), is influenced by their foraging behaviour. Juvenile sharks displayed a continuum of foraging strategies between mangrove and seagrass areas, with some individuals preferentially feeding in one habitat over another. Foraging habitat was correlated with growth rate, whereby slower growing, smaller individuals fed predominantly in sheltered mangroves, whereas larger, faster growing animals fed over exposed seagrass. Concomitantly, tracked juveniles undertook variable movement behaviours across both the low and high predation risk habitat. These data provide supporting evidence for the hypothesis that directional selection favouring smaller size and slower growth rate, both heritable traits in this shark population, may be driven by variability in foraging behaviour and predation risk. Such evolutionary pathways may be critical to adaptation within predator-driven marine ecosystems.

PARAMETER POPULASI HIU MARTIL (Sphyrna lewini Griffith & Smith, 1834) DI PERAIRAN SELATAN NUSA TENGGARA

Hiu martil (Sphyrna lewini Griffith & Smith, 1834) merupakan salah satu target tangkapan bagi perikanan artisanal di Indonesia. Dengan status konservasi masuk dalam Appendix II CITES, pengelolaan terhadap hiu martil telah menjadi perhatian khusus di bidang perikanan tangkap. Penelitian ini bertujuan untuk mengkaji beberapa aspek parameter populasi hiu martil yang tertangkap di perairan selatan Nusa Tenggara pada periode Januari – Desember 2015. Data ukuran panjang dan jenis kelamin diperoleh di Tempat Pendaratan Ikan Tanjung Luar, Lombok Timur, Nusa Tenggara Barat. Analisis dilakukan secara deskriptif menggunakan perangkat lunak FiSAT II. Hasil penelitian menunjukkan bahwa dari 634 ekor hiu martil yang tertangkap didominasi oleh jenis kelamin betina dengan sebaran ukuran panjang total berkisar antara 81 – 320 cm (rerata 211,2 cm) dan jenis kelamin jantan berkisar antara 91 – 310 cm (rerata 176,9 cm). Dominasi kelompok hiu muda yang belum matang kelamin berpotensi terjadinya recruitment overfishing. Hiu martil mampu mencapai panjang asimtot 399 cm. Laju pertumbuhan dan mortalitas jenis hiu jantan lebih tinggi dibandingkan jenis betina. Populasi hiu martil telah mengalami kondisi tangkap lebih sehingga perlu adanya regulasi dan pengelolaan agar pemanfaatannya tetap lestari. The scalloped hammerhead sharks (Sphyrna lewini Griffith & Smith, 1834) is one of the main target fishing for artisanal shark fisheries in Indonesia. By the conservation status of Appendix II CITES, its management had been concerned in capture fisheries. This research aimed to assess some aspects of scalloped hammerhead shark population parameters caught in the southern of Nusa Tenggara in the period from January to December 2015. Fish length and sex was obtained in Tanjung Luar landing site, East Lombok, West Nusa Tenggara. The analysis was done descriptively used FiSAT II software. The results showed that 634 individuals of hammerhead shark caught dominated by female with a total length size distribution ranging between 81-320 cm (mean 211.2 cm) and male ranged between 91-310 cm (mean 176.9 cm). The dominance catch of juvenile sharks with immature was potential to recruitment overfishing. Sphyrna lewini was capable of reaching 399 cm asymptotic length. The growth rate and mortality of male sharks was higher than female. The population of S. lewini had been overfished so that the regulation and management are needed in order to maintain their sustainability of the population.

Wet-season effects on the distribution of juvenile pigeye sharks, Carcharhinus amboinensis, in tropical nearshore waters

Tropical nearshore environments are highly dynamic systems owing to extreme freshwater flow and flooding episodes that occur in wet-season months. We hypothesised that juvenile sharks in tropical nearshore waters respond to seasonal freshwater inflow by moving away from areas of strong flow. An array of fifty-eight acoustic receivers deployed in Cleveland Bay, north Queensland, Australia, passively tracked thirty-two juvenile pigeye sharks, Carcharhinus amboinensis, throughout two wet seasons from 2008 to 2010. Influences associated with wet seasons appeared to play a role in habitat use by juvenile C. amboinensis in this region. Home ranges and distribution of individuals showed distinct changes, with individuals moving north away from sources of freshwater inflow during high flows. The location of individuals within the bay was strongly influenced by freshwater inflow in both years. Although juvenile C. amboinensis moved in response to freshwater inflow, home-range sizes remained stable, and the amount of space individuals used did not change in relation to freshwater inflow. By defining the response of juvenile sharks to highly variable freshwater flow events, this research provides useful information for understanding species behaviour in a dynamic and changing climate, and contributes towards effective management of tropical river systems.