Shark conservation: global threats, local concerns
Chip Cotton, PhD Virginia Institute of Marine Science A little taxonomy…
or
“What is a shark anyway?” What is a “shark” anyway? Chondrichthyans = cartilagenous fishes – Sharks, skates, rays, and chimaeras
CHONDRICHTHYAN What is a “shark” anyway? Chondrichthyans = sharks, skates, rays, chimaeras •Holocephalans – fixed upper jaw
What is a “shark” anyway? Chondrichthyans = sharks, skates, rays, chimaeras •Holocephalans = chimaeras •Elasmobranchs – protrusible upper jaw What is a “shark” anyway? Chondrichthyans = sharks, skates, rays, chimaeras •Holocephalans = chimaeras •Elasmobranchs = sharks, skates, rays – Batoids – Gills ventral, eyes dorsal, pectoral fins attached to the head Torpediniformes Pristiformes Myliobatiformes
Rajiformes What is a “shark” anyway? Chondrichthyans = sharks, skates, rays, chimaeras •Holocephalans = chimaeras •Elasmobranchs = sharks, skates, rays – Batoids – skates and rays – Selachii – 5-7 gills on each side of the head, pectoral fins NOT fused to the head
“Local” species
VIMS Long-term Shark Monitoring Survey
• 1973 – present (one of the longest-running, fishery- independent shark monitoring surveys) • 9 standard stations in Chesapeake Bay and Virginia coastal waters • Sampled monthly for 4-6 months per year (summer fauna) • 100 hook longline • Approx. 1.25 nautical miles • Fished for 4 hours • Baited with Atlantic menhaden (Brevoortia tyrannus)
Romine et al. 2010 Sampling Large Coastal Shark Abundance 0.6
0.5
protocols 0.4
0.3
0.2 • Each fish is measured, sexed, tissue sampled, LogCPUE
tagged and released 0.1 – Hallprint nylon dart tag for sharks < 150 cm TL 0 – Hallprint stainless steel head dart tags (SSD) for 74-79 1980 1981 82-85 86-89 1990 1991 1992 1993 1995 1996 1997 1998 1999 2000 2001 2002 sharks > 150 cm Year Group • Habitat utilization studies • Migration • All dead sharks or sharks needed for ongoing research projects are fully sampled – Reproductive biology – Age and growth – Diet • Over 1200 sets – Over 120,000 hooks • Nearly 10,000 fishes – 23 species of sharks – 8 species of batoids – Few teleosts Sharks caught 1973 - 2004
Species Number
Sandbar shark, Carcharhinus plumbeus Atlantic sharpnose shark, Rhizoprionodon terraenovae Smooth dogfish, Mustelus canis Dusky shark, Carcharhinus obscurus Sandtiger shark, Carcharias taurus Blacktip shark, Carcharhinus limbatus Scalloped hammerhead shark, Sphyrna lewini Tiger shark, Galeocerdo cuvier Spiny dogfish, Squalus acanthias Spinner shark, Carcharhinus brevipinna Bignose shark, Carcharhinus altimus Blue shark, Prionace glauca Shortfin mako shark, Isurus oxyrinchus Silky shark, Carcharhinus falciformis Lemon shark, Negaprion brevirostris Common thresher shark, Alopias vulpinus Bull shark, Carcharhinus leucas Night shark, Carcharhinus signatus Smooth hammerhead shark, Sphyrna zygaena Blacknose shark, Carcharhinus acronotus Bigeye thresher shark, Alopias superciliosus White shark, Carcharodon carcharias TOTAL Scalloped hammerhead Sphyrna lewini • Often targeted for its valuable fins • Highly fecund (12-38 pups), age @ maturity = 15 yr • Aggregations are susceptible to overnight longline sets. Ram ventilators with small mouth, post-release survival very low
Endangered Dusky Smoothhound Mustelus canis
• Diet consists mostly of crustaceans • Recent unregulated fishery established in MD, VA, and NC • Females mature @ 4-5 yr, produce 10- 20 pups annually Near Threatened Unique management considerations Most sharks have a very conservative or “K-selected” life history, characterized by:
•Slow growth
•Low fecundity (# offspring)
•Late age @ maturity
Surplus Production Maximum Sustainable Yield
or… “How much can we harvest without crashing the population?”
Shark Fecundity Anchovy
Growth
Age @ maturity
Maximum age Surplus Production Maximum Sustainable Yield
Proportion needed to sustain population
Fecundity
Growth
Age @ maturity
Maximum age Surplus Production Maximum Sustainable Yield
Proportion we can harvest without crashing the population
Fecundity
Growth
Age @ maturity
Maximum age Case study in shark conservation gone wrong Butterfly rays (2 spp.)
Cownose rays (1 sp.)
Coastal Rays of Virginia
Eagle rays (1 sp.)
Stingrays (4 spp.) Cownose ray Abundant throughout Bay Seasonally (Summer/Fall) Forms huge migratory schools Swims at surface Eats mostly shellfish Maximum size 1-m width Slow-growing, long-lived Matures at 8 years, one pup Did NOT wipe out native oysters
Smith and Merriner, 1985 In Chesapeake Bay, cownose rays are purportedly hindering oyster recovery efforts Problems with Myers’ hypothesis
• Diet studies do not support the notion that these sharks significantly prey on cownose rays
Problems with Myers’ hypothesis
• Diet studies do not support the notion that these sharks significantly prey on cownose rays • Diet studies do not support the notion that cownose rays significantly prey on oysters
Rays consume farmed oysters, not wild oysters Smith and Merriner, 1985 Problems with Myers’ hypothesis
• Diet studies do not support the notion that these sharks significantly prey on cownose rays • Diet studies do not support the notion that cownose rays significantly prey on oysters • The life history of the cownose ray precludes an “explosive” population growth
N=2 N=1 Unintended consequences of Myers et al. 2007 • Development of an unregulated fishery on a highly K-selected elasmobranch
Unintended consequences of Myers et al. 2007 • Development of an unregulated fishery on a highly K-selected elasmobranch • Life history of the cownose ray (like many sharks) makes this species particularly susceptible to overharvest, needs regulation, proceed with caution! • There are other, less detrimental approaches to protecting oyster aquafarms that should be investigated (i.e. barriers, deterrents) Thank You!
Photo credits: FishBase www.fishbase.org VIMS Shark Research Program Flickr Jason Romine, Bob Fisher, www.flickr.com Dean Grubbs Feel free to contact with questions: [email protected]