Dusky Shark

Species: 

Carcharhinus obscurus

IUCN Status: 

Vulnerable
Photo: Malcolm Nobbs www.malcolmnobbs.com

The dusky shark is currently listed as Vulnerable by the IUCN, but populations continue to decline. The dusky shark is one of the slowest-growing, latest-maturing sharks, bearing small litters after a glong gestation period. These characteristics make the shark especially vulnerable to depletion by fisheries. In the Northwest Atlantic, dusky sharks began to decline in the 1970s, when they were targeted by recreational fishers. They continued to decrease due to directed catch in US shark fisheries in the 1980s. Dusky sharks are now protected in US waters, but their population has continued to decline.   Dusky sharks are often caught as bycatch by longlines, nets, and trawls and have a high mortality rate when caught. The dusky shark is most commonly caught in pelagic longline fisheries.  Between 1992 and 2000, dusky sharks were the second most commonly caught elasmobranchs in the US pelagic longline fishery (Beerkircher et al 2002). The dusky shark is also among the most highly sought after for its large fins, which are sold for shark fin soup.  Dusky sharks that are incidentally caught in tuna and swordfish fisheries are often landed for their fins instead of released.

References

Beerkircher, LR, E Cortes, and M Shivji. 2002. Characteristics of shark bycatch observed on pelagic longlines off the southeastern United States, 1992-2000. Marine Fisheries Review 64(4): 40-49. 

Bycatch Threat: 

Longlines, gillnets, hook and line, trawls

Population: 

Decreasing
Patchy, coastal in tropical and warm temperate seas

Type: 

Fish

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Related Field Studies

Location Gear Technique Species Results Catch Effect on Target Catch Type Reference
Seychelles and Reunion Island Hooks-and-Lines Mammals, Sharks N/A Swordfish and tuna Depredation rate was significantly higher for sharks but depredation impact was significantly higher for toothed whales Summary study Toothed whale and shark depredation indicators: a case study from the Reunion Island and Sechelles pelagic longline fisheries
Hooks-and-Lines Sharks Field study in the wild Stress, post-release mortality, and recovery of commonly discarded deep-sea sharks caught on longlines
Seychelles Hooks-and-Lines Bony Fishes, Sharks All marketable species were caught in the 150 m layer during day and night sets. swordfish and tuna Differences in the species composition, quantity and depth of capture for bycatch species Summary study Comparison of bycatch species captured during daytime and nighttime: preliminary results of longline experiments carried out in the Seychelles waters
NE Atlantic Hooks-and-Lines Electromagnetic deterrents Sharks Magnets increased and did not decrease blue shark catch rates. Larger magnets had a stronger effect. Tuna and blue sharks None reported Field study in the wild Hooks equipped with magnets can increase catches of blue shark (Prionace glauca) by longline fishery
Tyrrhenian Sea Trawls Excluder devices Sharks 90 mm excluder grid did not reduce catches of blackmouth catshark (Galeus melastomus) Norway lobster (Nephrops norvegicus) and greater forkbeard (Phycis blennoides) 90 mm excluder grid reduced catches of greater forkbeard and to a lesser extend Norway lobster Field study in the wild Selective characteristics of a shark-excluding grid device in a Mediterranean trawl
Minch and Stanton Banks Trawls Modified ground gear (mobile) Sharks, Skates/Rays Removing the 'tickler' chain resulted in decreased catches of sharks and skates Whitefish Catch rates for flatfish, haddock and whiting did not differ. Larger anglerfish were caught without the 'tickler' in place Field study in the wild A simple technical measure to reduce bycatch and discard of skates and sharks in mixed-species bottom-trawl fisheries
Massachusetts Bay Trawls Excluder devices Sharks There were no observed differences in the behavior of spiny dogfish between gear configurations or grate colors. Silver hake Field study in the wild Development and observations of spiny dogfish Squalus acanthias reduction device in a raised footrope silver hake Merluccius bilinearis trawl
Brazil Hooks-and-Lines Circle hooks Sharks Mortality at haulback was significantly higher for on J-hooks. Tuna (pelagic) sharks (bottom) Field study in the wild Fishing gear modifications to reduce elasmobranch mortality in pelagic and bottom longline fisheries off Northeast Brazil
Macduff Marine Aquarium, UK Non-specific Electromagnetic deterrents Sharks, Skates/Rays, Elasmobranchs Species significantly avoided magnets n/a n/a Study in the lab The effects of neodymium-iron-boron permanent magnets on the behavior of the small spotted catshark (Scyliorhinus canicula) and the thornback skate (Raja clavata)
Central Pacific Hooks-and-Lines Sharks examing post-release of blue shark released from longline gear pelagic sharks and fishes examined in other research Field study in the wild Predicting postrelease survival in large pelagic fish.
Atlantic and Pacific Hooks-and-Lines Sharks examing factors relevant to post-release mortality in sharks released from longline gear pelagic fishes (tuna, swordfish) not examined Summary study Factors influencing mortality estimates in post-release survival studies.
Central Pacific Hooks-and-Lines Sharks examining rates of post-release mortality from longline gear pelagic sharks and fishes examined in other research Field study in the wild Post-release survival, vertical movements and thermal niche partitioning in five species pelagic sharks released from longline fishing gear in the central Pacific Ocean
Lake Maracaibo, Venezuela Gillnets Deep sets, Sub-surface sets (gillnets) Sharks, Skates/Rays, Bony Fishes, Elasmobranchs, Invertebrates None, reduced bycach capture rate shrimp None Field study in the wild Selectivity of three artisanal fishing gears used for the capture of the white shrimp, Litopenaeus schmitti, in the Bay of El Tablazo, Zulia State, Venezuela
Venezuela Trawls Modified ground gear (mobile) Sharks, Skates/Rays, Bony Fishes, Elasmobranchs, Invertebrates Decrease of 26-32% shrimp Maintained or increased Field study in the wild The use of devices to reduce bycatch in Venezuelan shrimp fisheries and the FAO Code of Conduct for Responsible Fishing
Costa Rica Hooks-and-Lines Sharks, Skates/Rays, Bony Fishes, Elasmobranchs, Sea Turtles mahi-mahi, tunas, marlins, sharks Summary study Impact of Costa Rican longline fishery on its bycatch of sharks, stingrays, bony fish and olive ridley turtles (Lepidochelys olivacea)
laboratory Electromagnetic deterrents Sharks Nd did not repel bonnethead or lemon sharks None n/a Summary study Electrochemical properties of lanthanide metals in relation to their application as shark repellents
Scotian Shelf Hooks-and-Lines Electromagnetic deterrents Sharks No effect Swordfish Significantly reduced on hooks treated with control lead (56%) and Nd/Pr (48%) Field study in the wild No effect from rare-earth metal deterrent on shark bycatch in a commercial pelagic longline trial
Seychelles Hooks-and-Lines Night sets Sharks, Bony Fishes 75% of bycatch species were caught during day sets, with lancetfish being the most common. Sharks were most common during night sets. Tunas and swordfish 75% of market species were caught during day sets. Field study in the wild Comparison of bycatch species captured during daytime and nighttime: preliminary results of longline experiments carried out in Seychells waters
Pacific Ocean Hooks-and-Lines Electromagnetic deterrents Sharks Metal affixed to baited hook significantly reduced catches of juvenile hammerhead sharks but not blue, shortfin mako, pelagic thresher, sandbar or tiger sharks None reported None reported Field study in the wild The effects of a lanthanide metal alloy on shark catch rates
Sharks Summary study Trends in shark bycach research:current status and research needs
Western and Central Pacific Surrounding nets Trap-net modification Sharks, Bony Fishes No effect tuna No effect Field study in the wild Development and testing of a release panel for sharks and non-target finfish in purse seine gear
Eastern Pacific Hooks-and-Lines Circle hooks Sharks Small circle hook (C15) had lowest catch of sharks tuna and billfish Small circle hook (C15) had lowest catch of billfish Field study in the wild Comparison of circle hooks and J hooks in the catch rate of target and bycatch species taken in the Korean tuna longline fishery
Eastern Pacific Ocean Hooks-and-Lines Circle hooks Bony Fishes, Sharks Significant differences were found between J4 and C15 and between J4 and C18. Billfish and tuna Significant differences between catch rates were only found between the J-4 and C18 hooks. Large circle hooks (C18) had the lowest catch rates for tunas and small circle hooks (C15) had the lowest catch rates for billfish Field study in the wild Comparison of circle hooks and J hooks in the catch rate of target and bycatch species taken in the Korean tuna longline fishery
Pacific Ocean Hooks-and-Lines Circle hooks Sharks Blue and oceanic whitetip shark bycatch increased. Tuna Tuna catches were significantly higher, but lengths for bigeye and skipjack were smaller; shortbill spearfish and striped marlin catch was lower; no difference in swordfish catch rates, but lengths were greater Field study in the wild Hawaii longline tuna fishery temporal trends in standardized catch rates and length distributions and effects on pelagic seamount ecosystems
Massachusetts Bay Trawls Excluder devices Sharks Over 88% of spiny dogfish were excluded by the grate regardless of color or gear configuration. Silver hake Within typical commercial quantities Field study in the wild Development and observations of spiny dogfish Squalus acanthias reduction device in a raised footrope silver hake Merluccius bilinearis trawl
Northwest Africa Trawls Excluder devices Sharks Allowed between 20-75% of sharks to escape Sardinella None reported Field study in the wild Bycatch and release of pelagic megafauna in industrial trawler fisheries off Northwest Africa
North Carolina Gillnets Tensioning gillnet Sharks Catch rates of Atlantic sharpnose sharks were significantly lower in the 4" modified net Spanish mackerel, menhaden, spot Did not reduce catch rates Field study in the wild Assessment of modified gillnets as a means to reduce bycatch in southeastern North Carolina coastal waters
Brazil Hooks-and-Lines Circle hooks Sharks, Skates/Rays CPUE was significantly higher for night, blue, silky, and oceanic whitetip sharks on circle hooks. tuna (pelagic) and sharks (bottom) None reported for pelagic longlines. On bottom longlines, CPUE was higher for blacknose and nurse sharks and southern stingrays; no CPUE difference between hook type; more tiger and blacknose sharks were alive at haulback with circle hooks Field study in the wild Fishing gear modifications to reduce elasmobranch mortality in pelagic and bottom longline fisheries off Northeast Brazil
Northern Australia Trawls Excluder devices Sharks Reduced shark catches by 36.3% Prawns Reduced the proportion of soft and damaged prawns by 41.6% and reduced catches of tiger prawns by 6.5% Field study in the wild The impact of turtle excluder devices and bycatch reduction devices on diverse tropical marine communities in Australia's northern prawn trawl fishery
North Carolina Gillnets Tensioning gillnet Sharks The selectivity of blacknose sharks varied between the modified and unmodified nets Spanish mackerel, menhaden, spot Did not reduce catch rates Field study in the wild Assessment of modified gillnets as a means to reduce bycatch in southeastern North Carolina coastal waters
North Carolina Gillnets Tensioning gillnet Sharks Significantly less blacktip sharks were wrapped in the 3" modified net Spanish mackerel, menhaden, spot Did not reduce catch rates Field study in the wild Assessment of modified gillnets as a means to reduce bycatch in southeastern North Carolina coastal waters
North Carolina Gillnets Tensioning gillnet Sharks The proportion of hammer-wrapped bonnethead sharks was significantly higher in the 4" unmodified net Spanish mackerel, menhaden, spot Did not reduce catch rates Field study in the wild Assessment of modified gillnets as a means to reduce bycatch in southeastern North Carolina coastal waters
North Carolina Gillnets Tensioning gillnet Sharks Catch rates of bonnethead sharks were significantly lower in the 4" modified net Spanish mackerel, menhaden, spot Did not reduce catch rates Field study in the wild Assessment of modified gillnets as a means to reduce bycatch in southeastern North Carolina coastal waters
Western Australia Trawls Excluder devices Sharks Reduced capture of sharks larger than 100 cm with largest reductions occuring at lengths greater than 180 cm Red emperor, scarlet perch, spangled emperor, Rankin cod, blue spot emperor, rosy threadfin brea, flagfish, frypan snapper, red snapper and goldband snapper None reported Field study in the wild Evaluation of exclusion grids to reduce the bycatch of dolphins, turtles, sharks and rays in the Pilbara trawl fishery
North Pacific, Japan Hooks-and-Lines Circle hooks Sharks Little effect on catch rates, mortality or size composition Swordfish Not reported Field study in the wild Shark catch in pelagic longline fishery: Comparison of circle and tuna hooks
Northeastern Australia Hooks-and-Lines Alternative leader design Sharks Wire leaders had higher catch rates of sharks when compared to nylon monofilament leaders Bigeye and yellowfin tuna Nylon monofilament leaders had higher catch rates of bigeye tunas when compared to wire leaders, but had no effect on yellowfin catch Field study in the wild Large-scale experiment shows that nylon leaders reduce shark bycatch and benefit pelagic longline fisheries
Eastern Tropical Pacific Ocean Surrounding nets Time area closures Sharks Tuna Summary study Trade-offs in the Design of Fishery Closures: Management of Silky Shark Bycatch in the Eastern Pacific Ocean Tuna Fishery
North Carolina, USA Gillnets Tensioning gillnet Sharks Significantly reduced bycatch of some shark species Spanish mackerel and spot No difference in similar mesh size in Spanish mackerel fishery or the spot fishery. The trend was for higher catch rates in modified nets in the spot fishery. Field study in the wild Bycatch mitigation assessment for sharks caught in coastal anchored gillnets
Alaska Hooks-and-Lines Electromagnetic deterrents Sharks Slightly reduced bycatch of spiny dogfish Pacific halibut Did not affect catch of halibut Field study in the wild Field investigation of rare-earth metal as a deterrent to spiny dogfish in the Pacific halibut fishery
Hawaii Hooks-and-Lines Deep sets Sharks, Skates/Rays No differences in catches between experimental and control sets Tuna and swordfish Decreased catch of wahoo, dolphinfish, blue and striped marlin, and shortbill species, increased catch of sickle pomfret and opah, and did not affect catch of sharks and pelagic sting rays Field study in the wild Effects of eliminating shallow hooks from tuna longline sets on target and non-target species in the Hawaii-based pelagic tuna fishery
off Hawaii Hooks-and-Lines Alternative bait Sharks Decreased by 36% Swordfish Swordfish catch increased by 16%, tuna catch decreased by 50%, and mahi-mahi, opah, and wahoo catch decreased by 34% Field study in the wild Reducing sea turtle interactions in the Hawaii-based longline swordfish fishery
Bahamas Hooks-and-Lines Sharks Reduced bycatch shrimp Field study in the wild