Horizontal and vertical movements of immature dusky sharks Carcharhinus obscurus in relation to commercial longline fisheries in the western North Atlantic Ocean

Background Many species of sharks, including the dusky shark Carcharhinus obscurus, are often incidentally captured in commercial pelagic and bottom longline fisheries. Incidental capture can lead to at-vessel or post-release mortality and can be detrimental to populations of threatened, endangered, or prohibited species. The estimated at-vessel mortality for dusky sharks can be high in longline fisheries and, to minimize mortality, time–area closures have been designated in the western North Atlantic Ocean to mitigate interactions with longline fisheries, yet incidental capture of dusky sharks is still common. We compared the vertical and horizontal movements of dusky sharks to the overall fishing effort and depth fished of commercial pelagic and bottom longline fisheries to determine when and where overlap is present that could lead to incidental capture. Results Twenty-one (n = 21) dusky sharks were tagged with pop-up archival transmitting satellite tags and all were immature animals (123–200 cm FL) apart from two individuals (230 and 300 cm FL). Sharks were tagged off the coasts of North Carolina (71%) and Florida (29%). Twenty tags (95%) reported and provided tracks between 1 and 107 days (median 15 days) and 10 tags (50%) remained on sharks for > 4 days. Most individuals remained within the bottom longline closed area off the coast of North Carolina during their time at liberty. Dusky sharks primarily occupied the 20–40 m depth range 26% of the time, and overlapped with bottom and pelagic longline gears 41% and 59% of the time, respectively. Overlap was highest in the winter and spring for both commercial fisheries. Conclusions The use of archival satellite telemetry in this study has provided valuable preliminary information on vertical and horizontal movements of immature dusky sharks in western North Atlantic Ocean. Dusky sharks may be more vulnerable to incidental capture in the pelagic longline fishery due to the high fishing effort, larger areas of horizontal overlap, and greater percentage of vertical overlap. This information will inform mitigation measures of commercial longline fisheries, which can work toward population rebuilding of the species.

IDENTIFIKASI IKAN DI PELABUHAN PERIKANAN TANJUNG LUAR, PULAU LOMBOK, PROVINSI NUSA TENGGARA BARAT

Sumberdaya ikan adalah jenis ikan termasuk biota perairan laut lainnya yang merupakan sumber kekayaan alam yang memiliki daya pulih kembali secara alami, sehingga pemanfaatannya dapat dilakukan secara berkelanjutan. Pelabuhan perikanan merupakan tempat pendaratan hasil tangkapan dan awal pemasarannya. jenis ikan hasil tangkapan nelayan yang didaratkan di Pelabuhan Perikanan Pantai Tanjung Luar, Kabupaten Lombok Timur. Penelitian dilakukan dengan metode survey dan identifikasi. Berdasarkan penelitian diperoleh sebanyak 34 jenis ikan hasil tangkapan nelayan di Pelabuhan Perikanan Tanjung Luar, Pulau Lombok, yaitu: Naso brevisrostris, Chanos chanos, Parastromateus niger, Thallasoma purpureum, Siganus canaliculatus, Plectorhinchus polytaenia, Diodon holocanthus, Upeneus vittatus, Caesio cuning, Dermogenys sp, Epinephelus areolatus, Rastrelliger sp, Pomadasys argenteus, Lepturacanthus savala, Decapterus macrosoma, Sillago sihama, Eusphyra blochii, Carcharhinus brevipinna, Carcharhinus albimarginatus, Carcharhinus limbatus, Carcharhinus obscurus, Galeocerdo cuvier, Prionace glauca, Taeniura lymma, Priancanthus hamrur, Scolopsis sp, Scombermorus guttatus, Euthynnus affinis, Nemipterus japonicas, Sardinella sp, Lutjanus bitaeniatus, Secutor interruptus, Sphyraena jello, dan Caranx ignobilis

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

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.

Dusky sharks (Carcharhinus obscurus) undertake large-scale migrations between tropical and temperate ecosystems

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.

Age and growth of sharks: do vertebral band pairs record age?

Inaccurate age estimates can have severe consequences in the management of elasmobranchs. Numerous studies in shark age validation have demonstrated a disconnect between band pair counts and age, resulting in age underestimation, particularly in older individuals. To investigate the relationship between band pairs, vertebral shape and growth, we quantified intracolumn differences in centrum morphology (size and structure) and band pair counts in seven shark species: Squatina dumeril, Carcharodon carcharias, Lamna nasus, Isurus oxyrinchus, Alopias vulpinus, Prionace glauca and Carcharhinus obscurus. In all species examined, band pair deposition was closely related to body girth and the structural properties of the cartilaginous skeleton, relative to maximum size, and body type. These results have strong implications for accurately assessing age for fisheries management of these species.

The spatial segregation patterns of sharks from Western Australia

The extent to which sharks segregate by size and sex determines the population structure and the scale at which populations should be managed. We summarized 20 years of fisheries-dependent and independent sampling to define the spatial patterns of size and sexual segregation for sharks in Western Australia. Carcharhinus obscurus and C. plumbeus showed a large-scale (more than 1000 km) latitudinal gradient in size. Large individuals occurred predominantly in the northwest and north whereas smaller individuals occurred predominantly in the southwest and south. Mustelus antarcticus and Furgaleus macki showed strong sexual segregation at very large scales. Females occurred predominantly in the west and southwest whereas the proportion of males in catches substantially increased in the southeast. The populations of other shark species did not show sex and size segregation patterns at very large scales; most species, however, showed varying degrees of segregation when data were analysed at a smaller scale. These findings highlight the importance of matching the scale of observation to the scale of the phenomenon observed. As many shark species are highly mobile, if sampling is opportunistic and constrained both temporally and spatially, the observed segregation patterns may not be representative of those at the population level, leading to inaccurate scientific advice.