Monday, June 4, 2012

JAWS Returns: Signs of recovery in well-managed shark populations


Humans have done far more damage to great white
shark populations than they have done to us.
(Credit: Tobey Curtis)

     Sharks and their relatives, the skates and rays, have become poster children for the ‘global overfishing crisis’.  Largely due to increasing demand for high-priced shark fin soup in Asia, shark populations worldwide have suffered from excessive exploitation in recent decades.  Tens of millions of sharks are harvested each year, resulting in population declines in excess of 80% in some cases.  Numerous publications over the last decade have lamented these declines, and attempted to describe the ecological consequences of removing these important apex predators from marine ecosystems. 

    
Most Western Atlantic shark populations are well
below historic numbers. (Source)
It may take many decades to rebuild overfished populations due to the low productivity of most sharks (slow growth, late maturity, few offspring, etc.).  For example, it is projected to take 60 years for sandbar shark and nearly 100 years for dusky shark populations in the U.S. to rebuild to pre-fishing levels.  However, rebuilding horizons such as these are dependent on effective fisheries management.  Many countries that contribute to global shark harvests lack the ability to implement or enforce regulations on shark fisheries, and much of the catch and trade goes undocumented.  These limitations paint a dire picture for the future of sharks in our world’s oceans.
  
A basket of shark fins can be worth thousands
of dollars. (Credit: Tobey Curtis)

     There is reason for optimism, however.  Over the last few years, positive signs have sprouted in a few regions that have been able to effectively manage shark resources.  Once the highest volume commercial shark fishery in U.S. waters, the northeastern U.S. spiny dogfish fishery was essentially shut down in 2001 after a near stock collapse.  Following several years of stringent landing limits, the spiny dogfish stock recovered and was declared rebuilt in 2010.  The commercial fishery for large coastal sharks off the southeastern U.S. has also begun to show some positive signs.  Since management of this fishery began in 1993, increasing abundance trends have been detected in tiger, scalloped hammerhead, lemon, spinner, and bull sharks.  Some of these species still have a long road to full recovery, however.  A combination of regulations has contributed to the observed increases, including bans on shark finning, annual catch limits, trip limits, gear restrictions, and time-area closures.

A bull shark (Carcharhinus leucas) being caught by longline in the Gulf of Mexico.  Some large coastal shark populations have declined more than 80 percent over the last few decades.  (Credit: Tobey Curtis)

     Similar management measures implemented in regions of Australia and South Africa have resulted in population increases in several species of commercially-valuable sharks and rays.  Even the iconic great white shark appears to be benefitting from years of protected status.  The white shark is among the most highly-protected sharks in the world, and has been prohibited from harvest in numerous countries since the 1990s.  Relative abundance indices in Australia and the east and west coasts of the U.S. all display upward trajectories over the last decade.  Recovering shark populations are a good sign for ecosystems that historically contained higher abundances of these key predators.  Their presence helps promote ecological stability through “top-down” predatory pressures on prey populations.  


Sharks play critical ecological roles in our oceans, helping to control prey populations and balance ecosystems.  Here a white shark (Carcharodon carcharias) successfully captures a Cape fur seal off South Africa.  (Credit: Tobey Curtis)

     Collectively, these recent observations offer some rays of hope (pun intended) for the recovery of shark populations.  It is possible for shark stocks to rebuild from low levels of abundance.  It is possible to have sustainable shark fisheries when precautionary measures are used.  Without proper management, however, the inevitable outcome is the collapse of shark populations and the commercial fisheries that depend on them.  Not to mention the long-lasting alterations to the ecology of our oceans.  Hopefully we can avoid more negative outcomes with improved management, monitoring, research, and international cooperation.

-Tobey H. Curtis
 PhD Candidate, School of Marine Sciences, University of Massachusetts

Thank you Tobey for this excellent guest post.  We encourage our colleagues to consider writing a guest post...you write the text, we do the rest!

Further reading

Carlson JK, Hale LF, Morgan A, and Burgess GH.  2012.  Relative abundance and size of coastal sharks derived from commercial shark longline catch and effort data.  Journal of Fish Biology 80:1749-1764. 

Lowe CG, Blasius ME, Jarvis ET, Mason TJ, Goodmanlowe, GD, and O’Sullivan JB.  2012.  Historic fishery interactions with white sharks in the Southern California Bight.  Pages 169-185 In: Global Perspectives on the Biology and Life History of the White Shark.  Domeier ML (editor).  CRC Press.  Taylor and Francis Group.  Boca Raton, Florida.

Rago PJ and Sosebee KA.  2009.  The agony of recovery: Scientific challenges of spiny dogfish recovery programs.  Pages 343-372 In: Biology and Management of Dogfish Sharks. Gallucci VF, McFarlane GA, and Bargman GG (editors).  American Fisheries Society, Bethesda, Maryland. 

Skomal GB, Chisholm J, and Correia SJ.  2012.  Implications of increasing pinniped populations on the diet and abundance of white sharks off the coast of Massachusetts.  Pages 405-417 In: Global Perspectives on the Biology and Life History of the White Shark.  Domeier ML (editor).  CRC Press.  Taylor and Francis Group.  Boca Raton, Florida.

Ward-Paige CA, Keith DM, Worm B, and Lotze HK.  2012.  Recovery potential and conservation options for elasmobranchs.  Journal of Fish Biology 80:1844-1869. 

2 comments:

  1. I would like to note that some shark declines, including the ones shown in the graph, have been characterized as exaggerated and overly-pessimistic. Please refer to:

    Burgess, GH, LR Beerkircher, GM Cailliet, JK Carlson, E Cortes, KJ Goldman, RD Grubbs, JA Musick, MK Musyl, and CA Simpfendorfer (2005) Is the collapse of shark populations in the northwest Atlantic Ocean and Gulf of Mexico real? Fisheries 30(10): 19-26.

    ~~Tobey Curtis

    ReplyDelete
  2. nice posting.. thanks for sharing.

    ReplyDelete

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