July 14, 2015

Guest Column: The Science of Shark Bites

By CHARLES 'PETE' PETERSON, JOEL FODRIE, AND STEPHEN R. FEGLEY

Even before “Shark Week” began last week, we all witnessed a recent feeding frenzy of reporters all seeking “sound bites” on what has been causing the high incidence of shark attacks off the North Carolina coast. Unfortunately, sound bites do not lend themselves to proper public communication of science because they are divorced from the process by which scientists reach conclusions.

Documenting the scientific process for reaching conclusions requires an expanded explanation of how we scientists first erect credible hypotheses that may reasonably explain the phenomenon, gather data to test the hypotheses and then present the conclusions that arise from those tests. The public interest is better served with press publication of details of this more elaborate scientific process than with sound bites because the publication of the entire process exposes to scrutiny the basis on which conclusions are reached and can enrich and enliven the public debate by showing how the data are transformed into conclusions.

Here is what we know. In 2007, UNC-IMS faculty joined with shark experts from Dalhousie University in Canada to publish in the journal Science a study of temporal changes in the populations of great and small sharks along the East Coast. A core data set analyzed in this paper was a 32-year fishery-independent sampling of sharks on the coastal shelf off Shackleford Banks in Carteret County. 

These analyses revealed that all 11 species of great sharks sufficiently abundant to be analyzed exhibited dramatic declines in abundance from 1972 through 2003. In contrast, of the 14 species of smaller sharks and rays, eaten solely by these great sharks, 12 exhibited dramatic increases up to 2003. This paper we wrote interpreted the expansion in abundance of smaller sharks and rays to be a consequence of reduced predation pressure from the overfished great sharks.

So one might reasonably ask — what have the shark counts been doing since 2003? We have indeed continued this scientifically valuable shark survey biweekly over all the warm months of each year. Over the 11 subsequent years, 2003-2014, there is no evidence of increasing numbers of any of the 11 species of great sharks that exhibited dramatic declines. Recovery of great shark populations in the coastal ocean off North Carolina, then, cannot serve as an explanation for the unexpectedly high numbers of recent shark attacks off N.C. beaches.

Some suggest that the unusually warm weather in eastern North Carolina during June may, along with lower gasoline prices, have led to increased numbers of beach visitors. The hot weather may, in turn, have motivated a higher percentage of those visitors to spend more time in the ocean to cool off. More humans in the ocean increase chances of shark attacks because of the increased probability of human-shark encounters. 

While this explanation has merit, it fails to explain fully why the numbers of shark attacks have increased in North Carolina during June as compared to previous years. The eight shark attacks during the last three weeks contrast dramatically with full-year totals for North Carolina of five in 2010, four in 2011, two in 2012, one in 2013 and four in 2014. The number of humans in the water this year has not increased that much over the preceding years to account for this much change in the number of shark bites alone.

One could hypothesize that the warmer weather in June and warmer waters may have driven great sharks further north from southern waters and into North Carolina this year, bringing the increased risk of shark attacks along with them. Seasonal migrations of coastal sharks are evident along the East Coast, regularly increasing numbers of sharks off North Carolina in early summer. This northward movement of migrating sharks could explain the general geographic pattern of the shark attacks in June, beginning at North Carolina’s southern beaches and then subsequently extending to the northern Outer Banks of Dare and Hyde counties. 

Our shark survey data for June show no indication of this suggested temporal increase in great shark abundances, but such comparisons over a single month (two shark sampling trips) have little power to detect significant short-term changes in abundances. When we examine patterns in our full 43-year survey records, shark numbers regularly peak in the month of June.

Accounts offered by victims and witnesses suggest that the sharks in the recent attacks may have differed in length from 5 to 8 feet, a difference large enough to imply that more than one individual shark, and perhaps multiple species, were involved. Even if several sharks have been involved in biting bathers, a single great shark is capable of inflicting injuries to all the victims.

Satellite tagging of several great white sharks on the East Coast over the past few years and subsequent tracking by the Ocearch program have revealed that these animals swim far and fast enough to have moved sequentially from the first June attack through each subsequent one and finally ending up off Surf City for the last attack. This is not to imply that an individual great white shark is responsible for the all the attacks or even for one of them. Based upon their aggressiveness towards humans, their coastal habitats and their known propensity to chase people at shallow wading depths, bull sharks may be the most likely candidate for some or all attacks.

To the degree that the northward seasonal migration of sharks explains why June produced high numbers of shark attacks, one might suggest that after the month of greatest risk in North Carolina has passed that we could reasonably anticipate that shark attacks off North Carolina will revert to very low frequencies and fade from the news headlines.

(About the authors: Charles H. “Pete” Peterson, Stephen R. Fegley and F. Joel Fodrie are professors and researchers at the University of North Carolina's Institute of Marine Sciences in Morehead City.)

comments powered by Disqus