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Overfishing of sharks is harming coral reefs

September 19, 2013

TORONTO, ON — A team of sci­en­tists from Cana­da and Aus­tralia have dis­cov­ered that the decline in shark pop­u­la­tions is detri­men­tal to coral reefs.

“Where shark num­bers are reduced due to com­mer­cial fish­ing, there is also a decrease in the her­biv­o­rous fish­es which play a key role in pro­mot­ing reef health,” said Jonathan Rup­pert, a recent Uni­ver­si­ty of Toron­to PhD grad­u­ate. Rup­pert was part of a team engaged in long-term mon­i­tor­ing of reefs off Aus­tralians north­west coast.

Team leader Mark Meekan of the Aus­tralian Insti­tute of Marine Sci­ence (AIMS), said that the results might, at first glance, seem strange. “How­ev­er our analy­sis sug­gests that where shark num­bers are reduced, we see a fun­da­men­tal change in the struc­ture of food chains on reefs.”

“We saw increas­ing num­bers of mid-lev­el preda­tors – such as snap­pers – and a reduc­tion in the num­ber of her­bi­vores such as par­rot­fish­es. The par­rot­fish­es are very impor­tant to coral reef health because they eat the algae that would oth­er­wise over­whelm young corals on reefs recov­er­ing from nat­ur­al dis­tur­bances,” said Meekan.

Accord­ing to Rup­pert, the study comes at an oppor­tune time – coral reefs are fac­ing a num­ber of pres­sures both from direct human activ­i­ty, such as over-fish­ing, as well as from cli­mate change.

The reefs stud­ied are about 300 kilo­me­tres off the coast of north­west Aus­tralia where Indone­sian fish­ers tar­get sharks – a prac­tice stretch­ing back sev­er­al cen­turies and which con­tin­ues under an Aus­tralian-Indone­sian mem­o­ran­dum of under­stand­ing.

“The reefs pro­vid­ed us with a unique oppor­tu­ni­ty to iso­late the impact of over-fish­ing of sharks on reef resilience, and assess that impact in the broad­er con­text of cli­mate change pres­sures threat­en­ing coral reefs,” said Rup­pert. “Shark fish­ing appears to have quite dra­mat­ic effects on coral reef ecosys­tems. Giv­en that sharks are in decline on reefs world­wide, large­ly due to the shark fin trade, this infor­ma­tion may prove inte­gral to restora­tion and con­ser­va­tion efforts.”

Track­ing stud­ies show that, in many cas­es, indi­vid­ual reef sharks are close­ly attached to cer­tain coral reefs.  This means that even rel­a­tive­ly small marine-pro­tect­ed areas could be effec­tive in pro­tect­ing the top-lev­el preda­tors and allow­ing coral reefs to bet­ter able to recov­er from coral bleach­ing or large cyclones which are increas­ing in fre­quen­cy due to the warm­ing of the oceans as a result of cli­mate change.

The study will appear in the Sep­tem­ber 28issue of jour­nal PLOS One. Lead author Jonathan Rup­pert com­plet­ed his PhD at the Uni­ver­si­ty of Toron­to and was also based at AIMS for part of his research. Rup­pert is cur­rent­ly a post-doc­tor­al research asso­ciate at York Uni­ver­si­ty. Oth­er team mem­bers includ­ed Marie-Josée Fortin of U of T’s Depart­ment of Ecol­o­gy and Evo­lu­tion­ary Biol­o­gy, Michael Tra­vers of AIM and Luke Smith, the prin­ci­pal envi­ron­men­tal sci­en­tist at Wood­side Ener­gy.

The research was fund­ed by the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da and Wood­side Ener­gy of Perth, Aus­tralia.

Images are at

For more infor­ma­tion, please con­tact:

Jonathan Rup­pert, PhD
Post­doc­tor­al Research Asso­ciate
Depart­ment of Biol­o­gy, York Uni­ver­si­ty

Kim Luke
Com­mu­ni­ca­tions, Fac­ul­ty of Arts & Sci­ence
Uni­ver­si­ty of Toron­to

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