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Global warming raises seawater temperatures and creates changes which have been found to affect the movement of large migrating marine species. Understanding the thermal niches of marine species could prove essential to anticipate how the future climate will alter migrations, and how conservation efforts will have to change accordingly. Orot Rabin power station in Hadera, Israel uses seawater to cool its turbine and releases the warm water back into the Mediterranean Sea. As a result, a marine area with artificially elevated temperatures is created around the effluent. Every winter in the past two decades, this area attracts sharks of two species, Carcharhinus obscurus and Carcharhinus plumbeus, presumably to spend the cold months at a higher temperature. This study concentrated on this point of artificial heat dissipation, which maintains a wide gradient of surface temperatures and allowed us to examine the temperature preferences of these species when given a larger range than what is naturally found in the sea. Between 2016 and 2018, 16 sharks were tagged with acoustic tags, 3 of which had temperature sensors, and 2 were additionally tagged with pop-up archival tags also logging temperature data. Results show that the sharks stayed in the elevated temperature, while the ambient sea was cold during the winter, spending several months in the heated area. Both species displayed a similar preferred range, spending 90 percent of their time at a temperature between 21.8 °C and 26.1 °C while the surrounding sea was 15.5–25.5 °C. Considering this chosen thermal niche and the rise in water temperature, it appears that for the past 40 years, the Eastern shores of the Mediterranean have become more suitable for these species, especially during transitional seasons. The question that arises, however, is whether these shark populations will benefit from the expanding range of preferable temperatures, or whether their proximity to shorelines will put them at greater risk in terms of human activities such as fishing and pollution. [ABSTRACT FROM AUTHOR]

Two species of carcharhinid sharks aggregate every winter at the warm water effluent of a coastal power plant on the Israeli Mediterranean coast. The two species (Carcharhinus obscurus and Carcharhinus plumbeus) cooccur in a highly confined area for several months every year and are highly associated with the area in and around the hot water effluent. Niche partitioning has recently been suggested as a mechanism that enables the coexistence of similar shark species by resource partitioning, spatial partitioning, and temporal partitioning. In this study, we used acoustic telemetry to study the individual diel movement and activity patterns within this enclosed area and examined the differences between the two species sharing it. Although this location only reaches a maximum depth of 7.5 m, we found both species perform a diel vertical movement, rising closer to the surface at night and moving deeper during daytime. Furthermore, the two shark species swam at different depths both day and night, with C. obscurus swimming in the upper column, about 2 m shallower than C. plumbeus. The very small scale of movement, which nearly equals the sharks' body length, suggests movement patterns might be conserved at the species level. Moreover, spatiotemporal differences between the two species may reflect a mean of interspecific partitioning that occurs even in a highly confined and shallow habitat. [ABSTRACT FROM AUTHOR]

Shark assessments in the Mediterranean Sea are still scarce, and serum chemistry and haematological data have yet to be reported for wild dusky (Carcharhinus obscurus) or sandbar (Carcharhinus plumbeus) shark populations in the Mediterranean Sea. Herein, blood samples were obtained from adult dusky (n  = 23) and sandbar (n  = 14) sharks from an aggregation site near the Hadera power and desalination plants in Israel in the winters of 2016–20. Several serum chemistry analytes were characterized with relation to stress, body size and environmental conditions. Glucose concentrations were higher, while total cholesterol concentrations were lower in dusky sharks than in sandbar sharks, potentially due to distinct metabolic pathways utilized during the capture-related activity by both species. However, differences in sex and size are noted and should be considered. The blood cell morphology of both species was consistent with previous findings for sandbar sharks. Atypical monocytes were noted in one dusky shark. Preliminary and exploratory reference intervals for female dusky sharks were calculated for glucose, triglycerides, total cholesterol, total protein and creatine kinase. These data must be viewed with caution due to the potential influence of capture-related stress on analyte concentrations and activities and the fact that only females were employed in the calculations. Moreover, the sampling site is adjacent to coastal power and desalination plants, which may significantly affect shark physiology. Although limited, this novel database on dusky and sandbar shark serum chemistry and haematology aspects is essential as a first attempt to obtain data on these species in the eastern Mediterranean Sea and for future conservation and long-term biomonitoring efforts. [ABSTRACT FROM AUTHOR]

There are increasing concerns over the possible effects of aquaculture pens on the local abundance and residency of sharks, and its associated risk for shark bites at nearby beaches and surf breaks. We used acoustic tracking and a before–during–after–control–impact design to assess the residency and local abundance of 117 bronze whalers and 843 white sharks around a 45-m-diameter aquaculture pen installed in South Australia for tourism purposes. Only 14 bronze whalers (12.0% of individuals tagged) and nine white sharks (1.1% of individuals tagged) were detected throughout the 5-year monitoring period and there was no evidence of the pen affecting these sharks, in either the number of sharks detected or number of detections (proxy for length of time detected). The low amount of interactions with the pen was likely to be due to a combination of low fish biomass, pen installed in a shallow location, local shark species being migratory species, and good husbandry practices. Adequate planning and management, and good husbandry practices, such as removal of dead fish from pens and avoiding over-feeding to ensure the feed is entirely consumed, are key to reducing shark interactions with cage fish and associated infrastructure. Sharks are often observed interacting with caged fish and aquaculture industries. We tracked ∼900 bronze whalers and white sharks to determine whether local shark populations are affected by an aquaculture pen installed for tourism purposes. We found no evidence that sharks were attracted by the pen or stayed in the region for extended periods because of the pen. Good husbandry practices, shallow location, and shark species being migratory likely contributed to the low number of interactions with the pen. [ABSTRACT FROM AUTHOR]

The high of catching intensity or overfishing of dusky whaler shark, Carcharhinus obscurus (Lesueur, 1818), has threatened its sustainability. Even though fisheries and biological aspects of C. obscurus are important as a basic assessment for making regulation of shark fisheries management. However, those studies never been reported in Indonesia. The aim of this study is to identify the number of catches and biological aspects of C. obscurus. This study was conducted from January to March 2018 at Fishery Port Technical Implementation Unit (FP-TIU) Muncar, Banyuwangi, Indonesia. 54 C. obscurus individuals were obtained using random sampling survey method. Here, we found that the number of C. obscurus landed in 2018 was higher compared to 2014 and 2017. The shark length ranged from 172 to 394 cm. Percentages of male and female adults of C. obscurus were 22.22 and 46.30%, respectively. Whilst in the juveniles were 20.37% males and 11.11% females. The maturity rate based on clasper length, male landed sharks was 82.61% classified within full calcification and 17.39% non-full calcification level. The sex ratio of C. obscurus sharks that are landed was 1:1.34 (M: F). [ABSTRACT FROM AUTHOR]

Understanding the movement ecology of marine species and connectivity of populations is required for effective fisheries management. This is especially the case for species with wide-ranging distributions for which movement can span across several jurisdictions with different management regulations. We used the Australian national network of acoustic receivers facilitated by the Integrated Marine Observing System (IMOS) to describe the extent and frequency of movements for two large epipelagic shark species, the bronze whaler (Carcharhinus brachyurus) and dusky shark (Carcharhinus obscurus). A total of 210 sharks (117 bronze whalers and 93 dusky sharks) were tracked for a 10-year period during which 21% and 9% of detected bronze whalers and dusky sharks, respectively, moved between Australian states. Bronze whalers showed more variable inter-state movements, mostly between Western Australia and South Australia but also eastwards to New South Wales (NSW). Although no dusky sharks tagged in Western Australia undertook inter-state movements, ∼50% of the sharks tagged in South Australia went to Western Australia. Five of the 14 dusky sharks tagged in NSW (36%) were detected across different states but remained on the east and southeast coasts (Queensland, NSW, Victoria, and Tasmania). The IMOS receivers also detected six bronze whalers in Ningaloo Reef, representing an extension of the previously known Australian distribution. Our findings highlight the value of collaboration between researchers and the value of national infrastructure, by providing a more accurate understanding of inter-state movements. This new information will allow the development of more adequate population dynamic models for stock assessment and management advice, requiring collaboration among state agencies for coordinating research activities, sharing data and resources, and establishing appropriate cross-jurisdictional policies. This is essential to achieve successful management and conservation outcomes for highly migratory species.

The non-model shark species, dusky shark Carcharhinus obscurus, is a bio-economically and recreationally important shark in many areas of its range. Despite of the fishery importance of C. obscurus few genetic resources are currently available for the species. Here, we report on the isolation of eight novel microsatellite loci from C. obscurus using a double-digest restriction site associated DNA (RAD) sequencing approach on the Ion Proton semiconductor platform (ddRADseq-ion). We characterised the loci in 26 individuals and all loci were polymorphic, exhibiting 5-10 alleles (average 6.6), and observed and expected heterozygosities of 0.385-0.962 and 0.479-0.847, respectively. We found that all pairs of loci were in linkage equilibrium and conformed to Hardy-Weinberg expectations. The loci reported in this study are only the second set of microsatellite loci ever characterized for C. obscurus and will be valuable for molecular ecology studies for this vulnerable species. [ABSTRACT FROM AUTHOR]

Understanding the large-scale migrations of marine predators can allow better representation of their population dynamics. The migration biology of dusky sharks (Carcharhinus obscurus), a cosmopolitan large marine predator with very low resilience to fishing, was quantified using a large-scale network of acoustic receivers deployed across Western Australia. Time-series plotting of individual shark detections and modified logistic modelling were used to determine the timing of acoustically tagged sharks’ seasonal migration, the proportion of the population migrating, and the size at which sharks start to migrate. Large (>200 cm fork length) dusky sharks migrated between areas closed (north) and open (south) to commercial shark fishing. There was limited evidence that smaller sharks occurred in the northern study area, whereas several larger individuals of both sexes undertook repeated north–south displacements, moving between disparate ecosystems within the Indian Ocean (21.7°S–35.4°S) and covering round-trip distances of 2000–3000 km per migratory event. For migrating individuals, the probability of occurring in the north was high in the austral winter–spring and low (males) to moderate (females) during the austral summer–autumn. [ABSTRACT FROM AUTHOR]

Polychlorinated biphenyls (PCBs) are ubiquitous pollutants in the marine environment that are known to accumulate in apex predators such as sharks. Liver samples from dusky Carcharhinus obscurus , sandbar Carcharhinus plumbeus , and white Carcharodon carcharias sharks from south-eastern Australian waters were analysed for the seven indicator PCBs 28, 52, 101, 118, 138, 153 and 180. Median ∑ PCBs were significantly higher in white than sandbar sharks (3.35 and 0.36 μg g − 1 lipid, respectively, p = 0.05) but there were no significant differences between dusky sharks (1.31 μg g − 1 lipid) and the other two species. Congener concentrations were also significantly higher in white sharks. Significant differences in PCB concentrations between mature and immature dusky (3.78 and 0.76 μg g − 1 lipid, respectively) and sandbar (1.94 and 0.18 μg g − 1 lipid, respectively) sharks indicated that PCB concentrations in these species increased with age/growth. Higher-chlorinated congeners (hexa and heptachlorobiphenyls) dominated results, accounting for ~ 90% of ∑ PCBs. [ABSTRACT FROM AUTHOR]

Since the mid 1990’s, stock assessments of shark populations in the northwest Atlantic by the National Marine Fisheries Service (NMFS) have indicated that sandbar ( Carcharhinus plumbeus ) and dusky ( C. obscurus ) populations are overfished. In response to these assessments, the dusky shark bottom longline fishery was closed in 2000, followed by the sandbar fishery in 2005. A common management strategy is to mandate the release of prohibited species if caught as bycatch. However, a major assumption is that most released sharks survive, which may not be the case. Longline operations were conducted during 2011 and 2012 to capture and tag sandbar and dusky sharks, in order to assess (1) at-vessel mortality, (2) post-release mortality, and (3) investigate the effects of soak-time on each of these variables for each species. Our findings show that dusky sharks experience high at-vessel and post-release mortality, with mortality occurring more frequently after 3–5 h on the line. While time-on-the-line (TOL) increased dusky shark at-vessel (16% when less than 3 h TOL; 27.5% when more than 3 h TOL) and post-release mortality (11% when less than 3 h TOL; 42% when more than 3 h TOL), sandbar shark at-vessel mortality was low (0% when less than 3 h TOL). Nonetheless, the apparently hardier sandbar sharks did exhibit 29% post-release mortality after 3 h TOL. The total mortality (at-vessel + post-release, including moribund individuals) for dusky sharks was 97% after being captured on a demersal longline when TOL surpassed 3 h. These data clearly elucidate an interspecific difference in vulnerability to longline gear between these species. [ABSTRACT FROM AUTHOR]

The dusky shark Carcharhinus obscurus risks excessive fisheries exploitation worldwide due to its low productivity. Genetic monitoring is an effective way of resolving species stock structure, genetic diversity, and forensically identifying processed animals. Here we present the first C. obscurus species-specific microsatellite loci. Twenty-one di- to tetra-nucleotide loci with between 2 and 20 alleles per locus were developed. Observed heterozygosity ranged from 0.28 to 0.91 with only one locus slightly deviating from Hardy-Weinberg equilibrium. No significant evidence for null alleles or linkage disequilibrium was detected. These loci were cross-amplified in three related species. Seventeen, twelve, and eighteen loci exhibited polymorphism in sandbar shark Carcharhinus plumbeus, spinner shark Carcharhinus brevipinna, galapagos shark Carcharhinus galapagensis, respectively. Investigations into C. obscurus will benefit from these loci which possess attributes suitable for population-scale and individual-scale analyses. Additionally locus cross-amplification will facilitate research for these species with few existing microsatellites and similarly vulnerable life-histories. [ABSTRACT FROM AUTHOR]

Trophic position and body mass are traits commonly used to predict organochlorine burdens. Sharks, however, have a variety of feeding and life history strategies and metabolize lipid uniquely. Because of this diversity, and the lipid-association of organochlorines, the dynamics of organochlorine accumulation in sharks may be predicted ineffectively by stable isotope-derived trophic position and body mass, as is typical for other taxa. The present study compared ontogenetic organochlorine profiles in the dusky shark ( Carcharhinus obscurus) and white shark ( Carcharodon carcharias), which differ in metabolic thermoregulation and trophic position throughout their ontogeny. Although greater organochlorine concentrations were observed in the larger bodied and higher trophic position white shark (e.g., p,p′-dichlorodiphenyldichloroethylene: 20.2 ± 2.7 ng/g vs 9.3 ± 2.2 ng/g in the dusky shark), slopes of growth-dilution corrected concentrations with age were equal to those of the dusky shark. Similar ontogenetic trophic position increases in both species, less frequent white shark seal predation than previously assumed, or inaccurate species-specific growth parameters are possible explanations. Inshore habitat use (indicated by δ13C values) and mass were important predictors in white and dusky sharks, respectively, of both overall compound profiles and select organochlorine concentrations. The present study clarified understanding of trophic position and body mass as reliable predictors of interspecific organochlorine accumulation in sharks, whereas regional endothermy and diet shifting were shown to have less impact on overall rates of accumulation. Environ Toxicol Chem 2015;34:2051-2060. © 2015 SETAC [ABSTRACT FROM AUTHOR]

Age and growth of the dusky shark, Carcharhinus obscurus, in the western North Pacific Ocean was estimated based on 434 specimens (213 females and 256 males) collected between September 2002 and November 2003 at the Nanfanao fish market, in northeastern Taiwan. Sex-specific relationships between body weight (W) and total length (TL) were estimated as follows: W = 2.03 × 10-6TL3.216 (n = 209, p < 0.01) for females and W = 8.01 × 10-6TL2.957 (n = 294,p < 0.01) for males. The relationship between TL and the vertebral centrum radius (R) for both sexes ("sexes-combined" data) was described using the following equation TL = 14.74 + 20.31R (n = 399,p < 0.01). Growth band pairs (including translucent and opaque bands) in precaudal vertebrae were determined to form once annually, based on centrum edge analysis and verified by statistical analysis. The maximum number of growth band pairs was 29 for females and 24 for males. The Robertson growth function best fit the observed length-at-age data. Sex-specific growth equations were not significantly different; thus, the combined growth parameters for both sexes were estimated as: asymptotic length (L∞) = 369.4 cm TL, b = 0.912, growth coefficient (kR) = 0.126 y r¹ (n = 385, p < 0.01). Age at maturity was estimated to be 16.6 yrs for females and 15.5 yrs for males. [ABSTRACT FROM AUTHOR]

Shark fisheries have expanded due to increased demand for shark products. As long-lived apex predators, sharks are susceptible to bioaccumulation of metals and metalloids, and biomagnification of some such as Hg, primarily through diet. This may have negative health implications for human consumers. Concentrations of Hg, As, Cd, Cu, Fe, Se and Zn were analysed in muscle, liver and fin fibres (ceratotrichia) from dusky Carcharhinus obscurus , sandbar Carcharhinus plumbeus , and white Carcharodon carcharias sharks from south-eastern Australian waters. Concentrations of analytes were generally higher in liver than in muscle and lowest in fin fibres. Muscle tissue concentrations of Hg were significantly correlated with total length, and >50% of sampled individuals had concentrations above Food Standards Australia New Zealand’s maximum limit (1 mg kg −1 ww). Arsenic concentrations were also of concern, particularly in fins. Results warrant further investigation to accurately assess health risks for regular consumption of shark products. [ABSTRACT FROM AUTHOR]

With the development of genetics methods, it becomes possible to study the population structure and some aspects of the reproductive behaviour of endangered sharks. Here we describe the isolation of 20 polymorphic microsatellite markers in the bull shark Carcharhinus leucas (Carcharhinidae) and their characteristics. Two to 10 alleles per locus were detected. Observed and expected heterozygosities ranged from 0.00 to 0.78 and from 0.05 to 0.80, respectively. Four markers showed deviations from Hardy-Weinberg equilibrium; among them, three showed presence of null alleles. No linkage disequilibrium was detected among any of the loci. Moreover, four, 11 and 19 of these 20 markers successfully cross-amplified in the tiger shark Galeocerdo cuvier, the sandbar shark Carcharhinus plumbeus and the dusky shark Carcharhinus obscurus, respectively. [ABSTRACT FROM AUTHOR]

Age and growth estimates for the dusky shark, Carcharhinus obscurus, were derived from vertebral centra collected in the northwestern Atlantic Ocean. Sample collection spanned the years prior to and following the implementation of management measures (1963-2010). Growth was compared pre- and post- population depletion and pre- and post- management to investigate the possibility of density-mediated shifts in age and growth parameters over time. There was no evidence of difference between periods for either sex. Additionally, bomb radiocarbon dating was used to determine the periodicity of band pair formation. Results support the traditional interpretation of annual band pairs up to approximately 11 years of age. After this time, vertebral counts considerably underestimate true age. Maximum validated ages were estimated to be between 38 and 42 years of age (an increase of 15 to 19 years over the band count estimates), confirming longevity to at least 42 years of age. Growth curves estimated using only validated data were compared to those generated using band pair counts. Logistic growth parameters derived from validated vertebral length-at-age data were L = 261.5 cm FL, L = 85.5 cm, t = 4.89 year and g = 0.15 year for the sexes combined. Revised estimates of age at maturity were 17.4 years for males and 17.6 years for females. [ABSTRACT FROM AUTHOR]

The dusky shark ( Carcharhinus obscurus) is the largest member of the genus Carcharhinus and inhabits coastal and pelagic ecosystems circumglobally in temperate, subtropical and tropical marine waters. In the western North Atlantic Ocean (WNA), dusky sharks are overfished and considered vulnerable by the International Union for the Conservation of Nature. As a result, retention of dusky sharks in commercial and recreational fisheries off the east coast of the United States (US) and in the northern Gulf of Mexico is prohibited. Despite the concerns regarding the status of dusky sharks in the WNA, little is known about their habitat utilization. During the summers of 2008-2009, pop-up satellite archival tags were attached to ten dusky sharks (one male, nine females) at a location where they have been observed to aggregate in the north central Gulf of Mexico southwest of the Mississippi River Delta to examine their movement patterns and habitat utilization. All tags successfully transmitted data with deployment durations ranging from 6 to 124 days. Tag data revealed shark movements in excess of 200 km from initial tagging locations, with sharks primarily utilizing offshore waters associated with the continental shelf edge from Desoto Canyon to the Texas/Mexican border. While most sharks remained in US waters, one individual moved from the northern Gulf of Mexico into the Bay of Campeche off the coast of Mexico. Sharks spent 87 % of their time between 20 and 125 m and 83 % of their time in waters between 23 and 30 °C. Since dusky sharks are among the most vulnerable shark species to fishing mortality, there is a recovery plan in place for US waters; however, since they have been shown to make long-distance migrations, a multi-national management plan within the WNA may be needed to ensure the successful recovery of this population. [ABSTRACT FROM AUTHOR]

The removal of large predatory sharks from the world's oceans poses profound threats to marine community structure and species conservation. Effective management of exploited shark stocks requires a sound understanding of the life histories of target species. Here we provide the first assessment of age and growth for Carcharhinus brevipinna in Australian waters, and for C. obscurus and C. plumbeus in eastern Australian waters, based on interpretations of vertebral growth bands. In doing so, we provide arguably among the most robust growth parameters to date for the abovementioned taxa on the bases of genetic validation and sample size and distribution, but acknowledge equally a range of limitations—most notably those associated with vertebral ageing and our lack of age validation. Comparatively, the three species displayed both contrasts and consistencies in their growth characteristics off Australia's southeast coast. For all three sharks, rates of growth were greatest in the years immediately after birth, males grew more rapidly than females in the juvenile phase, and females were observed to grow larger, live longer and were generally larger at any given age. Longevity and all modelled growth parameters (L∞, k and L0), however, differed among the three species, and appeared to challenge the findings for conspecific populations in other parts of the world. The validity of these latter comparisons is, however, compromised by a range of confounding factors. Nevertheless, we provide the least conservative k estimates for C. obscurus and C. plumbeus of those previously reported, and extend maximum age estimates for C. brevipinna. In this way, our results have important implications for the assessment of natural mortality, productivity, and hence resilience to stock depletion, in these species in southeastern Australian waters. [ABSTRACT FROM PUBLISHER]

We investigated the broad-scale movements and pelagic habitats of large juvenile dusky sharks ( Carcharhinus obscurus) off Southern Australia. Pop-up satellite archival tags ( PSATs) were deployed on three large juvenile dusky sharks (∼2.2-2.6 m total length) for 6 months in Spencer Gulf during 2010. Tagged dusky sharks all migrated westward and across the Great Australian Bight ( GAB) during autumn to offshore shelf waters off Western Australia. Estimated minimum distances travelled ranged from 1760 to 2736 km. Depths occupied by tagged dusky sharks ranged from the surface to 355 m. The most common thermal ranges occupied were 19-22°C. Broad-scale movements of large juvenile dusky sharks across the continental shelves combined with periods of residency in semi-protected gulf waters indicated that a multi-jurisdictional management approach may be appropriate for this species. [ABSTRACT FROM AUTHOR]

Size, sex and seasonal patterns among Carcharhiniformes were examined in shallow regions of Moreton Bay, Queensland, Australia. A total of 1259 sharks were caught, comprising 13 species. The Australian sharpnose shark Rhizoprionodon taylori and the blacktip complex Carcharhinus limbatus- Carcharhinus tilstoni comprised 55% of all shark individuals. Neonates were observed for five species including the dusky shark Carcharhinus obscurus, which contrary to previous reports was relatively abundant in shallow, predominantly estuarine waters. Three contrasting patterns of occurrence were observed: smaller species were abundant and present throughout much of their ontogeny, larger species were mainly caught as neonates or juveniles and vagrant species were only caught during the warmer months. The shark assemblage differed significantly among seasons. While many species were observed during the warmer months, species diversity was lower in winter when C. obscurus comprised 43% of the catch. Overall, the results indicated that spatial and temporal distribution patterns were not synchronous for all species. The capture of small numbers of neonate C. obscurus in late autumn and winter demonstrates that parturition among Carcharhiniformes is not confined to spring and summer in sub-tropical waters. [ABSTRACT FROM AUTHOR]

The article presents a research study regarding the non-standard fishing of large sharks in Paraná, Brazil. Research shows the shark species that are being fished in the state including Elasmobranch, Trichiuridae, Rhinopteridae, and rays and 60 specimens have already recorded. Furthermore, dusky sharks are commonly caught by Brazilian fishermen in the coastal area of the state.

1. Life-history theory predicts that organisms will provide an optimal level of parental investment for offspring survival balanced against the effects on their own survival and future reproductive potential. 2. Optimal resource allocation models also predict an increase in reproductive output with age as expected future reproductive effort decreases. To date, maternal investment in sharks has received limited attention. 3. We found that neonatal dusky sharks ( Carcharhinus obscurus) are not independent from maternal resource allocation at the point of parturition but instead are provisioned with energy reserves in the form of an enlarged liver that constitutes approximately 20% of total body mass. 4. Analysis of long-term archived data sets showed that a large proportion of this enlarged liver is utilized during the first weeks or months of life suggesting that the reported weight loss of newborn sharks signifies a natural orientation process and is not necessarily related to prey abundance and/or indicative of high mortality rates. 5. Interrogation of near-term pup mass in two carcharhinids, the dusky and spinner shark ( Carcharhinus brevipinna), further revealed an increase in reproductive output with maternal size, with evidence for a moderate decline in the largest mothers. 6. For the dusky shark, there was a trade-off between increasing litter size and near-term pup mass in support of optimal offspring size theory. 7. For both the dusky and spinner shark, there was a linear increase in near-term pup mass with month, which may indicate variable parturition strategies and/or that carcharhinids are able to adjust the length of the gestation period. 8. The identification of optimal size-specific reproductive output has direct implications for improving the reproductive potential of exploited shark populations and for structuring future management strategies. [ABSTRACT FROM AUTHOR]

The onchobothriid Triloculatum n. gen. is erected to house species formerly considered members of Phoreiobothrium, but that bear 3, rather than 5 or more, bothridial subloculi. The species formerly known as Phoreiobothrium triloculatum is designated as its type. This species is redescribed based on 2 syntypes and on voucher material consistent with type material taken from the type host, Carcharhinus obscurus, from the type and 1 additional locality. Examination of the cestode fauna of a diversity of carcharhinid sharks resulted in the discovery of an additional 5 new tri-subloculate species belonging to the genus. These include: Triloculatum andersonorum n. sp. from Negaprion acutidens; Triloculatum bullardi n. sp. from Carcharhinus brevipinna; Triloculatum geeceearelensis n. sp. from Carcharhinus isodon; Triloculatum jodyi n. sp. from Carcharhinus acronotus; and Triloculatum oregontwoae n. sp. from Carcharhinus plumbeus. The new species differ from one another in the number of proglottids, scolex size, number of testes, number of lateral columns of vitelline follicles, and whether they possess craspedote or acraspedote proglottids. Histology and scanning electron microscopy suggest that the new genus differs further from Phoreiobothrium in that its species possess an anterior margin of the posterior loculus that is fused to the anterior loculus, rather than free. The diagnosis of Phoreiobothrium is emended to accommodate the removal of tri-subloculate species and the new information on the condition of the locular interface, and the symmetrical ovary and vitelline follicles arranged in 2 lateral bands; each band consisting of 2 to many columns of follicles. Existing host data indicate that the new genus is restricted to only a subset of the sharks parasitized by species of Phoreiobothrium. Results from the examination of a diversity of carcharhinid species suggest that species in the new genus may have an affinity for the larger, i.e., >200 cm in total length, species of Carcharhinus and Negaprion. One of the new Triloculatum species was found to attach in the crypts lying between the larger ridges found on the mucosal surface of the posterior inner region of the scroll-type spiral intestine of C. brevipinna. [ABSTRACT FROM AUTHOR]

Abstract: The effects of gear selectivity and sample size on growth model selection for shark length-at-age data was investigated using simulated data sets for dusky (Carcharhinus obscurus) and Atlantic sharpnose (Rhizoprionodon terraenovae) sharks. Simulated data sets were generated using four different individual growth functions (von Bertalanffy, Gompertz, logistic and Schnute) with five different sample sizes and seven different sampling gear selectivity functions. Five growth models were fit to each data set and the Akaike information criterion (AIC) and AIC weights used to evaluate which fit best to the data. The accuracy of each of the fitting models was also determined and compared with estimates derived from AIC multi-model inference (MMI). The results demonstrated that no one growth model outperformed all other models in all situations, with the two-parameter von Bertalanffy model being selected most often by AIC. There were clear differences between the fitting models that were ranked the highest based on AIC selection or AIC weights and those based on models accuracy of length and growth parameter estimates. In most situations, the two-parameter von Bertalanffy model provided the least accuracy in parameter recovery, while the Schnute model was the most accurate individual model in most situations. The AIC MMI approach improved estimation accuracy over most individual models, except at very low sample sizes. These simulation results suggest that sample sizes of 200 are required to consistently achieve good accuracy for growth parameters, and that sampling gear selectivity has clear effects on which growth models fit best to the data. The use of the AIC MMI approach is recommended, as it provides the most robust estimates of growth parameters. [Copyright &y& Elsevier]

The article provides information related to a study on measuring fish conditions. It states that data for 2120 dusky sharks were used in calculating seasonal trends in condition and in facilitating its index comparison. The study uses four condition measures which include somatic measure, hepatosomatic index (HSI), morphometric measures and residual condition. It notes that HSI has the most sensitive index and rapid condition indicator but requires the disaggregation of data. The increasing size of the animal affects the HSI which may require division of life-stage data into additional size classes. One possible complicating factor in interpreting condition indices is the large consumption of food by predators.

Demographic analyses were conducted for the dusky shark, Carcharhinus obscurus, in the Northwest Atlantic. Fishery-independent, fishery-dependent, and previously published data were used to estimate age specific by-catch mortality and reproductive cycle for incorporation into the analyses. Correspondence analyses were conducted for viability (dead or alive), shark length, month, soaktime, and region. Dead sharks were associated with small size, long soak times, and the spring season, while live sharks were associated with large size and shorter soak times. Data on reproduction suggested a 3-year reproductive cycle consisting of a 2-year gestation period and a 1-year resting period. Litter sizes ranged from 3–12 embryos. Decreasing age-specific fishing mortality and a 3-year reproductive cycle were used in age structured life tables which incorporated stochasticity in life history parameters through Monte Carlo simulation to estimate annual population growth. Demographic analyses showed that the dusky shark population would decline even at low levels of fishing mortality despite low natural mortality for neonates. The demographics of this species make it among the most vulnerable to excessive fishing mortality, and suggest stringent regulatory measures may be required to recover collapsed populations. [ABSTRACT FROM AUTHOR]

McAuley, R. B., Simpfendorfer, C. A., and Hall, N. G. 2007. A method for evaluating the impacts of fishing mortality and stochastic influences on the demography of two long-lived shark stocks. — ICES Journal of Marine Science, 64. [ABSTRACT FROM PUBLISHER]

It is not known whether sharksuckers have positive or negative effects on their hosts, partly because this association is difficult to study in free-ranging fish. I observed the behaviour of sharks with and without sharksuckers, to determine whether the hosts actively avoid sharksuckers. Wild blacktip sharks, Carcharhinus limbatus, took evasive actions when sharksuckers, Echeneis naucrates, attached to them, presumably to escape from skin irritation or hydrodynamical drag caused by the sharksuckers. Sharksuckers were most often attached to the belly or back of the shark, and sharks reacted most strongly to sharksuckers on their heads, sides, and dorsal fins. Observations of two captive bull sharks, Carcharhinus leucas, indicated that swimming speed increased when sharksuckers were attached. This paper supports the hypothesis that sharksucker attachment irritates sharks, and that the relationship between the two is best viewed as a subtle host–parasite interaction. [ABSTRACT FROM AUTHOR]

The spinner shark, Carcharhinus brevipinna, is a Lessepsian immigrant formerly common in the Gulf of Gabès (southeastern Tunisia, central Mediterranean). It migrated northward into the Gulf of Tunis as a consequence of competition pressure from its sympatric species, the sandbar shark, C. plumbeus. Adult males and females reach over 1720 mm and 1960 mm total length (TL), respectively. The largest known male and female are 2630 mm and 2750 mm TL, respectively. There is no significant relationship of mass versus TL between males and females. C. brevipinna is a placental viviparous elasmobranch. Adult females have a single functional ovary and two functional uteri, in which encapsulated eggs and embryos are equally distributed. Mating occurs in spring or early summer; parturition, in August. Gestation lasts 13-14 months or longer, and there is a seasonal reproductive cycle. Vitellogenesis proceeds in parallel with gestation. Diameter of the largest yellow-yolked oocytes ranges from 31 to 36 mm (33.3 mm ± 1.4); and weight from 9.1 to 9.9 g (9.5 g ± 1.6). The placenta is definitively established after the embryos reach 185 mm TL. Both uteri are compartmentalized in chambers, and a single embryo develops in each chamber. Size at birth and mass at birth, based on fully developed fetuses and the smallest free-swimming specimens (neonates) are 610-690 mm TL and 1060-1850 g, respectively. A chemical balance of development based on mean dry masses of the largest yellow-yolked oocytes and the fully developed embryos is 65.8. This is the highest value ever computed in a viviparous elasmobranch, indicating that C. brevipinna is a matrotrophic species. Ovarian fecundity is higher than uterine fecundity. There is no significant relationship between the two categories of fecundity and female TL. Litter size ranges from 6 to 10. The sex ratio of embryos is 1:1. Male neonates are significantly more numerous than female, in contrast with juveniles and adults. The relative abundance of pregnant females and neonates suggests that the Tunisian coasts could be considered a nursery area for the spinner shark. [ABSTRACT FROM AUTHOR]

Conservation of vulnerable shark species that are also heavily exploited, such as the sandbar shark Carcharhinus plumbeus is a pressing issue, and molecular genetics is an increasingly important population monitoring and species identification tool. Here we present 19 C. plumbeus microsatellite loci with a broad range of alleles per locus (3-22), and observed heterozygosities (0.17-0.95). Null alleles were not apparent and if present, were at frequencies <0.041. None of the loci showed significant deviation from Hardy-Weinberg equilibrium or indications of linkage disequilibrium. Cross-species amplification of these loci in three other carcharinids, dusky shark Carcharhinus obscurus, spinner shark Carcharhinus brevipinna, and galapagos shark Carcharhinus galapagensis showed polymorphism in 13, 13, and 14 loci respectively. These highly diverse loci will assist both individual-specific and population-wide genetic investigations for these species. [ABSTRACT FROM AUTHOR]

The dusky shark Carcharhinus obscurus is economically important but vulnerable to overharvesting. The complete C. obscurus mitogenome was assembled from approximately 1 million whole genome shotgun sequences using a combination of reference mapping and de novo assembly (mean coverage 59x). This resulted in a 16,706 bp double-stranded circular mitochondrial sequence. Following the consensus vertebrate mtDNA genome, it comprises 13 protein-coding genes, 22 transfer RNAs, two ribosomal RNAs and has 2 non-coding areas. The A+T (56.9%) versus G+C (43.1%) composition confirmed an A+T bias previously noted for sharks. This genome is the first for the speciose Carcharhinus genus and provides a valuable resource for studies of shark molecular systematics, phylogeography, conservation genetics, and stock structure. [ABSTRACT FROM AUTHOR]

Many shark species are at risk of overexploitation due to their high economic value, slow maturation, and low recruitment compared to most teleosts. However, there is insufficient knowledge about population structure at different spatial scales necessary to optimise fisheries models. We used single-nucleotide polymorphisms (SNPs) obtained through complexity-reduction genome sequencing to quantify the population structure of two highly mobile and commercially fished shark species: bronze whalers (Carcharhinus brachyurus) and dusky sharks (C. obscurus). We applied a comprehensive approach to test several population-structure hypotheses and signal consistency across methods and marker type. We found that C. obscurus was panmictic across Australia and Indonesia and across the Indian Ocean to South Africa based on neutral loci, whereas for C. brachyurus, the westernmost Australian samples appeared to be separate from the rest. The southernmost east Australian samples indicated some difference from the rest of Australia and New Zealand based on candidate loci for C. brachyurus, and potentially also C. obscurus; however, the lack of a reference genome makes the interpretation difficult. Despite similar patterns in both species, subtle and potentially important structure differences emphasise the importance of studying each target species independently rather than assuming similar patterns from closely related species with similar dispersal abilities, as well as considering different marker types in future studies. We found evidence of connectivity across the regions sampled, suggesting that the cumulative effects of regional fisheries and the potential for cross-jurisdictional fishery assessments and management should be considered for Australian, Indonesian, and New Zealand populations. [ABSTRACT FROM AUTHOR]