858 words • 4~6 min read

Breaking News! “Most Comprehensive Estimate of Mortality”: Between 63 and 273 Million Sharks Killed Each Year

davesquare

A new study* has estimated that the total number of sharks killed by fisheries each year is between 63 and 273 million, with a average of approximately 100 million.In an interview, lead author Dr. Boris Worm explains the importance of this estimate:

This is by far the most comprehensive estimate of shark mortality yet,” he said,  “because we consider all sources of mortality, from direct fishing, finning, and discard mortality. the estimate was derived by crunching numbers from almost 100 publications on the catches and mortality of sharks.”

Of all the numbers this team crunched, the most important thing to consider is whether the exploitation rate is greater than the rebound rate. In other words, is this level of exploitation more than the populations can recover from? Though many estimates and approximations went into calculating these figures, it seems quite clear that sharks are being harvested at an unsustainable rate.

The average of the rebound rates for 62 sharks that have been studied this far is 4.9%. Three estimates of exploitation rate (for all sharks combined) in this study range from 6.4% to 7.9%.  Additionally, 48% of shark species whose rebound rates are known are currently fished above those rates, and more than 2/3 of all known rebound rates are below the average exploitation rates found in this study. ““This paper shows that a staggering amount of sharks are killed,” Dr. Worm said. “We estimate that one in 15 sharks dies every year from fishing.”

Perhaps the most troubling result of the study is that despite increased public awareness and advocacy, the authors have not detected a significant decrease in fin consumption since the year 2000. They suggest that this may be due to fishermen overfishing a particular area or species, and then moving on to a new area or species, which is clearly not sustainable in the long term.

Figure 2 from Worm et al. 2013. Total mortality = total catch - those that survived being released. Numbers are in 1,000 tons. IUU= Illegal, Unreported and Unregulated

Figure 2 from Worm et al. 2013. Total mortality = total catch – those that survived being released. Numbers are in tons, rounded to the nearest thousand. IUU= Illegal, Unreported and Unregulated

To combat the problem, the authors make several policy recommendations. They reiterate that focusing on finning alone is inadequate because the problem is overfishing, and instead recommend focusing on the most threatened species through policies like CITES, instituting strict science-based fishing quotas (with enforcement), and protecting critical habitats. The authors note that localized protection measures may protect sharks locally, but have little effect on global demand as fishermen can just move their fishing effort elsewhere. Additionally, they suggest an international agreement similar to the International Whaling Commission.

In the press release for this study, Dr. Worm noted “Protective measures must be scaled up significantly in order to avoid further depletion and the possible extinction of many shark species.”

Dr. Worm has particularly high hopes for CITES:

“[Our paper] shows that the problem of unsustainable shark fishing has not been solved by existing regulations, and that a majority of shark species are  threatened by overfishing. CITES is an effective tool in preventing extinction of some of the most vulnerable species. on land, CITES has been 100% effective in preventing extinction of thousands of listed species. I hope this can be applied effectively to ocean creatures as well.”

We’ll have updates on CITES as things develop, but I remain, as always, cautiously optimistic.


Author’s note: Regular readers may recall that “100 million sharks are killed for their fins” ranks as #2 on my list of “13 wrong things about sharks that advocates should stop saying“. This was because the previous best estimate for the scale of the fin trade was between 26 and 73 million sharks a year. That estimate focused on the number of sharks passing through the Hong Kong markets, while this estimate is for total global mortality. They are measuring separate things. As Dr. Worm stated in his interview, It remains true that anyone saying “100 million sharks are killed for their fins” prior to the publication of this study was misrepresenting the state of the science.

*Boris Worm, Brendal Davis, Lisa Kettemer, Christine Ward-Paige, Demian Chapman, Michael Heithaus, Steven Kessel, and Samuel Gruber (2013) Global catches, exploitation rates, and rebuilding options for sharks. Marine Policy 40: 194-204