Carcharhinus Plumbeus Research Articles

Using historical and citizen science data to improve knowledge about the occurrence of the elusive sandbar shark Carcharhinus plumbeus (Chondrichthyes - Carcharhinidae) in the Adriatic Sea.

The Mediterranean Sea is an extinction hotspot for sharks, skates, and rays, with at least half of all species being threatened. The sandbar shark Carcharhinus plumbeus is among the most threatened species and is declared endangered (EN) in the Mediterranean according to the IUCN Red List of Threatened Species. The Adriatic Sea was previously proposed to be an important nursery area for this species based on sporadic sightings of neonates and juveniles. However, this species is generally considered to be rare in this region, and reports of its presence are scarce. Here, we present new records for the sandbar shark in the Adriatic Sea and review spatial and temporal data accumulated from literature, museum specimens, and citizen science initiatives in an attempt to assess the distribution and seasonality of this species. The majority of the records were from neonate and juvenile sandbar sharks, and three hotspots for their occurrence could be identified: (1) the Emilia-Romagna region, (2) the Veneto region, and (3) the Gulf of Trieste. Our data indicate a high concentration of sandbar sharks around the delta of the Po river, emphasising the need for scientific surveys in this confined area in order to confirm a possible nursery ground. Data collected by citizen science initiatives reveal that this species is still present in the Adriatic Sea and might be more abundant than previously thought, underlining the potential of citizen science as a valuable tool to monitor the distribution of rare and endangered species.

Sandbar shark aggregation in the central Mediterranean Sea and potential effects of tourism

Abstract An aggregation of sandbar sharks Carcharhinus plumbeus occurs every summer around the remote uninhabited islet of Lampione (Pelagie Islands Marine Protected Area, south‐western Mediterranean Sea), attracting an increasing number of tourists for a shark watching experience. Despite the ecological and socio‐economic importance of this rare occurrence in Mediterranean waters, there is a lack of scientific data and lack of information as to the potential impact of tourist activities on the presence and behaviour of this shark species. Using baited underwater videos, this study provides the first assessment of this shark aggregation, as well as a preliminary evaluation of the potential effects that boating and diving activities may have on sandbar sharks during two different periods within the tourism season (July and September 2019). Overall, 241 sandbar shark sightings (with up to five individuals together) were recorded in July, whereas there were only six sightings in September. The average MaxN was 1.18 hr−1 (±0.21 hr−1 SE) and 0.22 hr−1 (±0.10 hr−1 SE) individuals in July and September respectively. Higher frequentation levels of divers and boats significantly reduced the number of shark sightings at the aggregation site. This study emphasizes the need for further investigations on Mediterranean shark aggregations and the implementation of specific conservation measures, such as an increase of protection level in Lampione and better enforcement, aside from strategies that promote sustainable tourism, including restrictions on the number of boats and divers’ visits per day. In the meantime, a precautionary approach aimed at regulating the interactions between tourists and sharks should be implemented through the application of a self‐regulatory code of conduct for divers when sharks aggregate around the island.

Risk-based weight of evidence assessment of commercial sharks in western Australia

Sharks are generally taken in multispecies fisheries worldwide so in many cases it is logistically infeasible and cost prohibitive to assess the impact on all species caught. Risks of fishing impacts to the sustainability of shark species were assessed using a weight of evidence risk-based framework. Whiskery (Furgaleus macki), gummy (Mustelus antarcticus), dusky (Carcharhinus obscurus), and sandbar (C. plumbeus) sharks provide a good case study as they are the main (~80% of the catch) shark species taken in Western Australia (WA) and represent the range of life history strategies of other shark species typically caught. Based on the available lines of evidence for stock status (spatial and temporal time series of catch, effort, catch rates, size composition and population modelling), the 2015–16 risk level for the four species was scored as medium―i.e. it is unlikely (5–20% chance) that there is a high level of stock depletion. The status of each of the four species was scored as ‘acceptable’ with risk control measures in place, indicating that no additional restrictions are required.

Conventional tagging of sharks in Western Australia: the main commercial species exhibit contrasting movement patterns

Understanding movement patterns underlies effective management and conservation measures. The current study summarises the main findings from a tagging program of Western Australian sharks to provide insights into the movement patterns of the main commercial shark species: dusky (Carcharhinus obscurus), sandbar (C. plumbeus), gummy (Mustelus antarcticus) and whiskery (Furgaleus macki) sharks. Between 1993 and 2020, >12 000 individuals from 52 taxonomic groups were implanted with conventional tags in Western Australia, of which 8.5% were recaptured. Most of the tagged (74.5%) and recaptured (95.8%) individuals belong to the four main commercial shark species. Recaptured individuals of these species, as well as tiger (Galeocerdo cuvier) and bronze whaler (C. brachyurus) sharks showed displacements of >1000 km and rates of movement (ROMs) of >10 km day–1, with the exception of whiskery sharks, which showed much slower ROMs (<3 km day–1). Despite tagged dusky and sandbar sharks being predominately small individuals and gummy and whiskery sharks being large individuals, dusky and sandbar sharks had faster ROMs and a greater proportion of recaptures outside the release zone. Our study provided the information required for estimating movement rates across different fishing zones and therefore defining the spatial scale for managing these shark species.

The effects of elevated potassium, acidosis, reduced oxygen levels, and temperature on the functional properties of isolated myocardium from three elasmobranch fishes: clearnose skate (Rostroraja eglanteria), smooth dogfish (Mustelus canis), and sandbar shark (Carcharhinus plumbeus).

Elevated plasma potassium levels (hyperkalemia), reduced plasma pH (acidosis), reduced blood oxygen content, and elevated temperatures are associated with species-specific rates of at-vessel and post-release mortality in elasmobranch fishes. The mechanism linking these physiological disturbances to mortality remains undetermined however, and we hypothesize that the proximate cause is reduced myocardial function. We measured changes in the functional properties of isolated ventricular myocardial strips from clearnose skate (Rostroraja eglanteria), smooth dogfish (Mustelus canis), and sandbar shark (Carcharhinus plumbeus) when subjected to the following stressors (both in isolation and in combination): hyperkalemia (7.4mMK+), acidosis (from 7.9 to 7.1), and reduced oxygen (to 31% O2 saturation) applied at temperatures 5°C above and below holding temperatures. We selected these species based on phylogenetic distance, diverse routine activity levels, and their tolerance to capture and transport. Stressors had a few significant species-specific detrimental impacts on myocardial function (e.g., a 33-45% decrease in net force under acidosis + low O2). Net force production of myocardial strips from clearnose skate and smooth dogfish approximately doubled following exposure to isoproterenol, demonstrating that these species possess beta-adrenergic receptors and that their stimulation could provide a mechanism for preservation of cardiac function during stress. Our results suggest that disruption of physiological homeostasis associated with capture may fatally impair cardiac function in some elasmobranch species, although research with more severe stressors is needed.

Capture, processing and utilization of sharks in archaeological context: Its importance among fisher-hunter-gatherers from southern Brazil

The data obtained from the zooarchaeological remains of the Rio do Meio shallow site allowed for unprecedented discussions about the role of sharks in the lifestyle of fishermen-hunters-gatherers on the Southern Brazilian coast. These groups captured and processed sharks, using their body parts both as a food source and for other purposes, and the animals thus held great cultural significance for them. These data also provide a window for reflecting on the representativeness of different sampling methods and the difficulties of identification in the study of archaeological shark remains. In this respect, the difference between blue sharks (Prionace glauca) and the Sandbar shark (Carcharhinus plumbeus) is to be emphasized.

A scalable, satellite-transmitted data product for monitoring high-activity events in mobile aquatic animals

A growing number of studies are using accelerometers to examine activity level patterns in aquatic animals. However, given the amount of data generated from accelerometers, most of these studies use loggers that archive acceleration data, thus requiring physical recovery of the loggers or acoustic transmission from within a receiver array to obtain the data. These limitations have restricted the duration of tracking (ranging from hours to days) and/or type of species studied (e.g., relatively sessile species or those returning to predictable areas). To address these logistical challenges, we present and test a satellite-transmitted metric for the remote monitoring of changes in activity, measured via a pop-off satellite archival tag (PSAT) with an integrated accelerometer. Along with depth, temperature, and irradiance for geolocation, the PSAT transmits activity data as a time-series (ATS) with a user-programmable resolution. ATS is a count of high-activity events, relative to overall activity/mobility during a summary period. An algorithm is used to identify the high-activity events from accelerometer data and reports the data as a count per time-series interval. Summary statistics describing the data used to identify high-activity events accompany the activity time-series. In this study, we first tested the ATS activity metric through simulating PSAT output from accelerometer data logger archives, comparing ATS to vectorial dynamic body acceleration. Next, we deployed PSATs with ATS under captive conditions with cobia (Rachycentron canadum). Lastly, we deployed seven pop-off satellite archival tags (PSATs) able to collect and transmit ATS in the wild on adult sandbar sharks (Carcharhinus plumbeus). In the captive trials, we identified both resting and non-resting behavior for species and used logistic regression to compare ATS values with observed activity levels. In captive cobia, ATS was a significant predictor of observed activity levels. For 30-day wild deployments on sandbar sharks, satellites received 57.4–73.2% of the transmitted activity data. Of these ATS datapoints, between 21.9 and 41.2% of records had a concurrent set of temperature, depth, and light measurements. These results suggest that ATS is a practical metric for remotely monitoring and transmitting relative high-activity data in large-bodied aquatic species with variable activity levels, under changing environmental conditions, and across broad spatiotemporal scales.

Evidence of a second nursery area of the sandbar shar, Carcharhinus plumbeus (Nardo, 1827) in the Eastern Mediterranean Sea

The sandbar shark (Carcharhinus plumbeus) is a large coastal species with a cosmopolitan distribution that has been listed as Endangered in the Mediterranean Sea and as Vulnerable for the rest of the world on the IUCN Red List of threatened species. The Gökova's Boncuk Cove in south-western Turkey and the Gulf of Gabès in southern Tunisia are the only known nursery area for the Mediterranean population. On 24 – 27 July 2017, eight specimens, four males and four females, of sandbar sharks were accidentally caught at a depth of 7 to 9 m by a pelagic bluefish longline off the coast of Yumurtalik Bight in the Gulf of Iskenderun. The total length and weight of males and females were 54.5 – 61.0 cm, 930 – 1,484 g and 49.6 – 62.4 cm, 918 – 1,568 g, respectively. They all had unhealed umbilical scars that were still open as a narrow slit, measuring 4.35 and 5.39 mm in lengths. This study thus provides the records of the neonate sandbar sharks from the Yumurtalik Bight. Consequently, this occurrence represents that this area of the north-eastern Mediterranean Sea can be the second breeding and nursery grounds for this species after the Boncuk Cove in Gökova Bay in Turkey.

Sensitivity of a shark nursery habitat to a changing climate

Nursery area habitats such as estuaries are vital for the success of many fish populations. Climate change is altering conditions in these areas, which can thus impact the availability of suitable nursery habitat. The sandbar sharkCarcharhinus plumbeususes Chesapeake Bay (USA) as a nursery habitat during the summer months from birth up to 10 yr of age. To assess the impacts of climate change on juvenile sandbar sharks, we developed a habitat model using longline data collected from a fishery-independent survey within Chesapeake Bay. With this model, we projected contemporary and future distributions of suitable habitat for juvenile sandbar sharks in Chesapeake Bay under varying environmental regimes. Predicted suitable juvenile sandbar shark habitat was negatively impacted by future increases in temperature, but positively influenced by future decreases in dissolved oxygen. The latter trend was likely related to the habitat partitioning that occurs between different life stages. Changes in salinity had relatively small impacts. By end-of-century the projected amount of suitable bottom habitat decreased; however, when incorporating the entire water column, projected suitable habitat increased. This suggests that juvenile sandbar sharks may need to make a behavioral shift to avoid non-preferred conditions, which could alter their foraging ecology or refuge strategies. As nursery habitats change with climate change, it is crucial to understand how a species may be impacted during this vital life stage when trying to predict overall species success in the future.

Migration and Residency of Sandbar, Atlantic Sharpnose, Bull, and Nurse Sharks in the Northern Gulf of Mexico

AbstractAcoustic telemetry was used to monitor the movements and residency of Sandbar Sharks Carcharhinus plumbeus, Atlantic Sharpnose Sharks Rhizoprionodon terraenovae, Bull Sharks Carcharhinus leucas, and Nurse Sharks Ginglymostoma cirratum around artificial reefs in the northern Gulf of Mexico. Transmitter‐tagged sharks were monitored from November 2012 to July 2019. Residency comparisons were based on the percentage of days that an individual was present in the study area for each month. Sandbar Sharks had greater residencies in fall than spring and winter, Bull Sharks had greater residencies in summer, and Nurse Sharks had greater residencies in the fall. No seasonal patterns were detected for Atlantic Sharpnose Sharks, but one individual returned to the study area in all 5 years that the transmitter was active. One Sandbar Shark and one Nurse Shark had long‐term preferences (>3 years) for specific sites. Long‐distance migrations (n = 7) away from the artificial reef area in the northern Gulf of Mexico were observed for three Sandbar Sharks (588, 589, and 1,894 km), two Bull Sharks (927 and 968 km), and two Nurse Sharks (903 and 943 km), with six of these migrating sharks returning to their tagging areas. Specifically, two Bull Sharks and two Nurse Sharks made winter migrations to the southern end of Florida (Florida Keys), and two Sandbar Sharks migrated to the middle of the west coast of Florida. A third Sandbar Shark made the longest distance migration and moved out of the Gulf of Mexico, into the Atlantic Ocean, and up to the South Carolina coast. Importantly, this Sandbar Shark returned to the same artificial reef where it was tagged and released in the northern Gulf of Mexico after its 3,788‐km round trip migration. These long‐distance migrations and subsequent homing behaviors, coupled with local residencies, indicate that the artificial reefs in the northern Gulf of Mexico may have significance extending well beyond the localized areas of reef deployment.

Seasonal Presence of Atlantic Sturgeon and Sharks at Cape Hatteras, a Large Continental Shelf Constriction to Coastal Migration

AbstractCape Hatteras is a major topographic feature on the continental shelf of the U.S. eastern seaboard that changes the dynamics of nearshore large ocean currents, including the Labrador Current and Gulf Stream. Cape Hatteras constricts shelf habitat and restricts the migratory corridors of highly migratory species through this area. Our objective was to describe the seasonal patterns of presence for three species—the Spiny Dogfish Squalus acanthias, Atlantic Sturgeon Acipenser oxyrinchus oxyrinchus, and Sandbar Shark Carcharhinus plumbeus—and analyze environmental conditions associated with fish presence near this feature. These species are managed under the Magnuson–Stevens Act, and two of them are also listed as species of concern under the Endangered Species Act. Transmitter detections from tagged fish recorded by the Cape Hatteras acoustic array, which was deployed just south of the cape, indicated that these species are present year‐round. The greatest number of detections occurred from November through April. This simple baseline of seasonal presence can provide insights for regional offshore development activities, which have the potential to affect movement patterns of migratory species through the Cape Hatteras constriction. Our results show the value of strategically placed acoustic arrays for observing fish habitat use and provide presence/absence data to enhance our understanding of species ecology and distribution.

Seasonal occurrence, relative abundance, and migratory movements of juvenile sandbar sharks, Carcharhinus plumbeus, in Winyah Bay, South Carolina

In the past decade along the U.S. East coast, the overfished western North Atlantic stock of Carcharhinus plumbeus has been recovering; however, research investigating C. plumbeus habitats and seasonal habitat shifts is lacking. Accordingly, we studied the seasonal residency, catch abundance, and distribution of juvenile C. plumbeus in Winyah Bay, SC, as well as their migration patterns along the western North Atlantic. We set 303 bottom longlines from May through September in 2016 and 2017 and deployed 11 Vemco (V16-4H) acoustic transmitters in juvenile C. plumbeus. Catch abundance did not differ by month or year (p = 0.45) and was not significantly influenced by any tested water parameter (p = 0.58). C. plumbeus catches were dominated by individuals measuring 81–100 cm precaudal length, and mean size only significantly differed by year (p = 0.02) with slightly larger sharks (86.8 cm PCL) caught in 2017 than 2016 (81.4 cm PCL). Tidal stage and Bay region were positively correlated with catches (p = 0.02). From August 2016 through January 2019, juveniles were detected in Winyah Bay from April to November for 1–302 non-consecutive days (μ ± SE = 108.1 ± 32.6 days), with six juveniles exhibiting interannual return. Detection frequency and presence differed by Bay region, with most detections nearshore and in Lower Bay, fewest in Middle Bay, and none in Upper Bay. This study had two primary findings: monitored sharks utilized a previously unknown southern overwintering migration route, and Winyah Bay serves as a secondary nursery for C. plumbeus.

Are circle hooks effective management measures in the pelagic longline fishery for sharks in the Gulf of Gabès?

Abstract This study evaluated the circle hook use as a tool for shark management in the pelagic longline fishery in the Gulf of Gabès. The usual J‐hook No. 2 with 10° offset, which has been traditionally used by the fishery, was compared to the 18/0 non‐offset circle hook in an alternating fashion along the main line. In total, 22 experimental longline sets were deployed through the shark fishing seasons of 2016 and 2017 to examine the effects of hook types on the catch composition, the catch rates, the hooking location, and the status at haulback. The catch composition differed significantly among hook types. Moreover, an overall increase in catch rates for the main species, the sandbar shark Carcharhinus plumbeus, and the shortfin mako shark Isurus oxyrinchus, was observed when using circle hooks. There was no size‐selective effect of circle hooks for the common species. The circle hooks were not effective at reducing at‐haulback mortality of sandbar shark. Conversely, shortfin mako and smooth‐hound shark Mustelus mustelus showed significantly lower relative mortality at haulback with circle hooks than with J‐hooks. Furthermore, circle hooks were more frequently hooked externally than the J‐hooks for the three shark species. Results demonstrated that the use of 18/0 non‐offset circle hooks in the pelagic shark longline fishery can reduce mortality at haulback for some species without any benefit for the dominant species, the sandbar shark. Overall, it is difficult to promote the adoption of the use of circle hooks as a management measure in this specialized fishery. Management measures focusing on fishing effort controls, fishing closures in critical habitats, and size limits could have significant benefits for the conservation of shark species and may help to improve the sustainability of the shark fishery in the Gulf of Gabès.

Trends in Chondrichthyan Research: An Analysis of Three Decades of Conference Abstracts

Given the conservation status and ecological, cultural, and commercial importance of chondrichthyan fishes, it is valuable to evaluate the extent to which research attention is spread across taxa and geographic locations and to assess the degree to which scientific research is appropriately addressing the challenges they face. Here we review trends in research effort over three decades (1985–2016) through content analysis of every abstract (n = 2,701) presented at the annual conference of the American Elasmobranch Society (AES), the oldest and largest professional society focused on the scientific study and management of these fishes. The most common research areas of AES abstracts were reproductive biology, movement/telemetry, age and growth, population genetics, and diet/feeding ecology, with different areas of focus for different study species or families. The most commonly studied species were large and charismatic (e.g., White Shark, Carcharodon carcharias), easily accessible to long-term established field research programs (e.g., Lemon Shark, Negaprion brevirostris, and Sandbar Shark, Carcharhinus plumbeus), or easily kept in aquaria for lab-based research (e.g., Bonnethead Shark, Sphyrna tiburo). Nearly 90% of all described chondrichthyan species have never been mentioned in an AES abstract, including some of the most threatened species in the Americas. The proportion of female* first authors has increased over time, though many current female* Society members are graduate students. Nearly half of all research presented at AES occurred in the waters of the United States rather than in the waters of developing nations where there are more threatened species and few resources for research or management. Presentations based on research areas such as paleontology and aquarium-based research have declined in frequency over time, and identified research priorities such as social science and interdisciplinary research are poorly represented. Possible research gaps and future research priorities for the study of chondrichthyan fishes are also discussed.

Fatal Infection in a Wild Sandbar Shark (Carcharhinus plumbeus), Caused by Streptococcus agalactiae, Type Ia-ST7.

Simple SummaryStreptococcus agalactiae (group B Streptococcus, GBS) is a major fish pathogenic bacterium. In this study, we describe a fatal infection of a stranded wild sandbar shark (Carcharhinus plumbeus) by a post-mortem examination, histopathology, classical bacteriology and advanced molecular methods. The bacterial agent was characterized as S. agalactiae, type Ia-ST7.Streptococcus agalactiae is one of the most important fish pathogenic bacteria as it is responsible for epizootic mortalities in both wild and farmed species. S. agalactiae is also known as a zoonotic agent. In July 2018, a stranded wild sandbar shark (Carcharhinus plumbeus), one of the most common shark species in the Mediterranean Sea, was found moribund on the seashore next to Netanya, Israel, and died a few hours later. A post-mortem examination, histopathology, classical bacteriology and advanced molecular techniques revealed a bacterial infection caused by S. agalactiae, type Ia-ST7. Available sequences publicly accessible databases and phylogenetic analysis suggest that the S. agalactiae isolated in this case is closely related to fish and human isolates. To the best of our knowledge, this is the first description of a fatal streptococcosis in sandbar sharks.

Defining Sex-Specific Habitat Suitability for a Northern Gulf of Mexico Shark Assemblage

Understanding the factors that influence species’ distributions is crucial for implementing effective management and conservation practices, yet difficult for highly vagile species like sharks. Many shark species demonstrate either spatial and/or temporal sexual segregation, further confounding accurate quantification of habitat suitability. Given the importance of understanding spatiotemporal patterns in the distribution of coastal shark assemblages, we sought to quantify sex-specific abiotic factors that influence seasonal variation in a coastal shark assemblage using data from a long-term fisheries-independent bottom longline program in the northern Gulf of Mexico. Catch data (individuals/100 hooks/hour) were coupled with a suite of potentially predictive variables: surface and bottom values for temperature (C) and salinity (psu), sea surface height (m), three-dimensional surface and bottom current velocity (u, v, w, in m/s), bottom dissolved oxygen (mg/l), depth (m), substrate grain size (mm), daylength (min), and distance from shore (km). Data were analyzed using boosted regression trees (BRT) to describe the relationships between catch data and environmental factors potentially influencing sex-specific species distribution and abundance. Between May 2006 and November 2018, we conducted 1,226 bottom longline sets and caught 13,742 individuals encompassing 67 species. The majority of the animals captured (74%) were elasmobranchs, primarily sharks. Two species from each of the following three categories were selected for further analyses: small coastal sharks (Atlantic sharpnose shark Rhizoprionodon terraenovae and blacknose shark Carcharhinus acronotus), large coastal sharks (blacktip shark C. limbatus and sandbar shark C. plumbeus), and shelf-associated sharks (smoothhound sharks, Mustelus spp. and scalloped hammerhead Sphyrna lewini). Depth and distance from shore were the strongest predictors of distribution and relative abundance, followed by longitude and bottom salinity; other factors (e.g. temperature, daylength, substrate grain size) were less predictive. For the six species examined, predictive factors were often the same for males and females, although the range of preferred values varied. Surprisingly, the importance of these predictors varied little across seasons. Collectively, our findings demonstrate that sexual segregation is the norm for sharks in the north-central Gulf of Mexico. Long-term fishery-independent monitoring to further quantify these sex-based differences in habitat use should be prioritized, particularly in light of impending climate change.

Resource-use dynamics of co-occurring chondrichthyans from the First Coast, North Florida, USA.

Recent studies on shark assemblages on the northeast Florida and southeast Georgia coast (hereafter referred to collectively as the "First Coast") have demonstrated differences in species and age-class composition of catch from previously characterized estuaries and newly surveyed area beaches, demonstrating that these regions may provide a critical habitat to different segments (i.e., life stages) of local shark populations. In this study, carbon and nitrogen stable isotopes (δ13 C and δ15 N) from muscle tissue and blood plasma were used to examine trophic dynamics (and temporal variability thereof) of the three dominant co-occurring species found along First Coast beaches (the Atlantic Sharpnose shark Rhizoprionodon terraenovae, Blacknose shark Carcharhinus acronotus and Blacktip shark Carcharhinus limbatus) to determine if they exhibit overlap in resource use along with spatial and temporal habitat use. Although considered spatially segregated from the beach species, a dominant, age-class species found in First Coast estuaries (juvenile Sandbar sharks Carcharhinus plumbeus) was also included in this analysis for comparison. Temporal variability of resource-use characteristics was detected at the species level. Resource-use overlap among species varied by tissue type and was generally higher for blood plasma, suggesting greater resource sharing over more recent time periods. Over longer time periods Atlantic Sharpnose and Blacktip sharks exhibited resource-use expansion, whereas Blacknose sharks exhibited a narrowing in resource use, suggesting a more specialized foraging strategy compared to the other species. The resource-use breadth of Sandbar sharks also expanded between blood plasma and muscle tissue. Significant size relationships were detected in Blacktip and Sandbar sharks, indicating ontogenetic resource shifts for both species. A diversity of highly productive resource pools likely support shark populations along the First Coast such that resource-use differentiation is not required to facilitate species co-occurrence. This work may shed light on understanding patterns of species co-occurrence as well as aid in future conservation efforts.

Localization of Acoustically Tagged Marine Animals in Under-Ranked Conditions

A key technology in the movement tracking of marine animals is localization using acoustic transmitters. These are attached to marine animals and are detected by an array of receivers. Then, offline localization is performed by multilateration. However, due to the transmitter's low power and environmental conditions, emissions may be detected by only a limited number of receivers, causing localization ambiguities to arise. This work proposes a solution for such localization ambiguities. The proposed method assumes that the position of acoustically-tagged marine animals follows a hidden Markov model, such that localization ambiguities can probabilistically be resolved using a Forward-Backward algorithm. Our method is able to extrapolate the positions in a data series, as long as one sample in that series is picked up by three receivers, or if the identity of the receivers changes during tracking. Performance analysis shows that the localization accuracy of our method approaches the Cramér-Rao lower bound. Furthermore, to demonstrate the suitability of our method in a real sea environment, we have established a testbed that operated for three months, demonstrating localization of 20 acoustically-tagged sandbar sharks. Compared to the available solutions, roughly 20 times more location estimates were made; thereby, significantly increasing the impact of the test-site.

Effects of fishing practice changes on pelagic shark longline captures in the Gulf of Gabes, Tunisia

Abstract In order to increase their catches, longliners targeting sharks in the Gulf of Gabes, Tunisia, have increased the number of hooks per basket and have included new bait types instead of mackerel (Scomber scombrus). This paper reports the effects of these changes on catch composition, catch rates, size distribution, life‐stage captures, and mortality at haulback. Data from 48 and 96 longline sets, carried out during the shark fishing seasons of 2007 and 2008, with two hooks per basket, and 2016 and 2017, with five hooks per basket, respectively, were examined to assess the effect of gear change. Moreover, the effects of bait types were inspected based on 33 fishing sets using whole mackerel, 19 using salema (Sarpa salpa) halves, and 27 using pieces of stingray (Dasyatis spp.), sampled during 2016 and 2017. The species composition indicated that longliners expand the vertical distribution of their hooks to operate from the surface to the bottom. The catches with both longline designs were dominated by sandbar shark (Carcharhinus plumbeus). Despite gear changes, the catch rate of C. plumbeus decreased by 42.21%, suggesting population decline. The size composition indicated a shift towards larger specimens, mainly adult females, which could jeopardize the shark population. Moreover, the mortality of the main species at haulback seems to be affected by the design of the longline. The bait used, mainly pieces of stingray, significantly increased the catch rate of sandbar shark, which suggests a greater attractiveness of the new bait; however, fish size and mortality rates at haulback were unchanged in response to bait variation. The fishery operates in shark nursery grounds, which exposes these fish (principally C. plumbeus) to considerable exploitation pressure. The new fishing practices intensified the pressure and thus the risk of a rapid depletion of populations. To preserve the shark species in the area, the use of new gear and attractive bait should be banned.

Patterns in abundance and size of sharks in northwestern Australia: cause for optimism

Abstract Reliable information for population assessments is rare for sharks. We quantified patterns in catch rates and mean size for numerous tropical and subtropical species from 15 years of fishery-independent surveys (2002–2017) in northwestern Australia. This study region represents an area of ~0.8 million km2 which was closed to commercial shark fishing from 1993 or 2005 onward due to the very high State-wide catches of sandbar (Carcharhinus plumbeus) and dusky (Carcharhinus obscurus) sharks. A total of 43 shark and ray species were sampled, with sandbar shark being the most commonly caught species, followed by milk (Rhizoprionodon acutus), spot-tail (Carcharhinus sorrah), tiger (Galeocerdo cuvier), blacktip (Carcharhinus limbatus and Carcharhinus tilstoni), dusky and sliteye (Loxodon macrorhinus) sharks, and scalloped hammerhead (Sphyrna lewini). For sandbar shark, catch rates increased between 2008 and 2017 whereas for other taxa catch rates were mostly stable (albeit fluctuating). Mean size at capture fluctuated across years with no particular trends. Unlike for other parts of the world, catch rates and mean size of northwestern Australian sharks have been stable or increased in recent years. Though most shark species have conservative life histories, when science, management and enforcement work synergistically, sustainable resource use, recovery and conservation outcomes can all be achieved.