To avoid dangerous shark encounters, information trumps culling

Risk of great white shark attack in California waters down 91 percent since 1950, researchers report

FOR IMMEDIATE RELEASE: Thursday, July 9, 2015
Contact: Liza Lester, 202-833-8773 ext. 211,


A great white shark (Carcharodon carcharias) approaches a kayaker in Mossel Bay, South Africa. Credit: C & M Fallows/; used by permission.

A great white shark (Carcharodon carcharias) approaches a kayaker in Mossel Bay, South Africa. Credit, C & M Fallows/; used by permission.

The great white shark (Carcharodon carcharias) has a terrifying reputation. Shark attacks, though very rare, loom large in our imaginations, drawing intense media attention when they occur. Recent injuries in North Carolina are putting sharks in the limelight again. But going after sharks à la Jaws is not the best way to protect people in the water, said shark researchers.

California scientists found that the risk of white shark attack for individual ocean users in California has fallen strikingly, by over 91 percent, since 1950, in a study to be published online ahead of print in the Ecological Society of America’s journal Frontiers in Ecology and the Environment later this month.

Information that empowers ocean users to avoid the large predators is far more effective for public safety than culling sharks, the authors said.

“Just like we check the weather before going boating or the surf forecast before surfing, information about the risk of encountering large predators can become a normal precaution we take before going into the ocean,” said first author Francesco Ferretti of Stanford University.

Ferretti and his colleagues at Stanford and the Monterey Bay Aquarium looked at the number of reported white shark attacks that caused injuries on the California coast from 1950 to 2013, as recorded by the Global Shark Attack File—86 injurious attacks, of which 13 were fatal. They weighted the numbers with information on coastal population growth and seasonal and weekly beach going, surfing, scuba diving, abalone diving, and swimming.

Many more people are enjoying the ocean, and so, although the number of shark bites per year has increased over the last six decades, these numbers actually hide a much reduced risk to individuals. Three times as many people live in coastal California now as did in 1950. But the popularity of ocean sports has expanded far more dramatically. The 7,000 surfers hitting the waves in 1950 became 872,000 by 2013. Certified scuba divers grew from about 2,000 at the beginning of the 1960s to about 408,000 in 2013.

“Doing this kind of analyses can inform us on hot spots and cold spots for shark activity in time and space that we can use to make informed decisions and give people a way to stay safe while they are enjoying the ocean,” said Ferretti. For example, in the fall there is a higher chance to find big white sharks on the California coast than in spring, when they migrate to Hawaii. The risk of encountering a shark is higher in the evening. The authors found that in Mendocino County, it is 24 times safer to surf in March than in October and November – and if surfers choose the coast between Los Angeles and San Diego in March, they can be 1,566 times safer than they would be during the fall months in Mendocino.

Co-author Fiorenza Micheli, Ferretti’s postdoctoral advisor at Stanford, sees a win-win for healthier oceans and safer coasts.

“We don’t necessarily have to see conservation and public safety as at odds with each other. This is also true of coastal economies. People can coexist with predators,” said Micheli.

There is no evidence that culling sharks improves the safety of ocean users, said the authors. Culling efforts often end up killing shark species that are not dangerous to people. Culling sharks is not under discussion in California, but has been a contentious issue elsewhere.

Culling is also expensive. Western Australia spent A$22 million on a 2014 cull and did not catch a single white shark. The authors argue that resources could be better spent getting good information to the public and collecting more data on the numbers, patterns, and behaviors of both sharks and people. Even in California, where monitoring is relatively good, estimates of the white shark population vary by an order of magnitude, from 500 to 5,000. Additional data on ocean users is needed.

“The more we know about the behavior of users and predator population, the more the analysis can be refined. There is a lot that can be done to really improve the information that we have for conservation and public safety,” said Ferretti.

Sharks species are among the most endangered animals worldwide, due to habitat degradation, declining prey, fishing bycatch, and growing consumer demand for shark fins. In recent years, fisheries management and international agreements have included more protections for sharks. But every year, 100 million sharks are killed. Slow growing, with late maturity and few progeny, sharks are sensitive animals that do not have the biological characteristics to sustain this level of exploitation.

Removing top predators like white sharks from ecosystems causes a cascade of bad consequences for the health of the ecosystems and local economies. Prey populations can suddenly boom, putting heavy demand on species further down the food chain. The collapse of the bay scallops (Argopecten irradians) fishery in North Carolina is linked to the rise in numbers of scallop eating cownose rays (Rhinoptera bonasus), due to overfishing of the predatory sharks which once kept ray numbers in check. Crowding resulting from a boom in prey numbers can also lead to disease outbreaks.

“Loss of predators can result in species that have negative impacts on economic activities becoming unchecked,” said Micheli. “You don’t know what you have until you lose it.”

The trend in increased number of shark attacks may indicate an increasing population of white sharks off California’s coast, but it is difficult to know without baseline historical data, the authors said. Conditions have improved for the sharks. Commercial and recreational fishing of white sharks is not permitted in California waters. Restrictions on use of drift gill nets, which have a high rate of bycatch, have improved the survival of juvenile sharks. But perhaps the best boost for the white shark population is the remarkable recovery of a favored prey species, the northern elephant seal (Mirounga angustirostris), from fewer than 100 at the end of the nineteenth century to over 100,000 today.

Ferretti and colleagues suspect that the return of the elephant seal may be the key to the greatly reduced risk of white sharks for humans, as sharks are drawn to seal rookeries and away from areas heavily used by people.

“Earlier, sharks were wandering around California waters, searching for food. Now they know where their preferred prey is,” said Ferretti. Knowing the sharks congregate near the seals is useful to ocean users who wish to avoid encountering the predators.

“This is an important result,” said Micheli. “In this case, the recovery of prey has not meant an increase in risk for people, as has been proposed for predators both on land an in the ocean.”

Shark encounters are extremely rare, on the order of 10 per year worldwide, Micheli stressed. Swimmers are 1,817 times more likely to die from unintentional drowning in California than shark attack, according to statistics available from the Centers for Disease Control and Prevention. California scuba divers are 6,897 times more likely to be hospitalized for decompression sickness.

“You have a higher chance to win the lottery, a much higher chance to drown in the ocean, than to be attacked by a shark. At the same time, people need to approach the ocean with precaution and respect. We are entering the realm of predators and they are fulfilling their ecological role,” said Ferretti

“Even a single attack is a tragedy,” said Micheli. “More information can help reduce the risk of tragedy.”


Francesco Ferretti, Salvador Jorgensen, Taylor K Chapple, Giulio De Leo, and Fiorenza Micheli. (In press). Reconciling predator conservation with public safety. Frontiers in Ecology and the Environment.

This research was funded by the Lenfest Ocean Program.


Also of interest:

Neil Hammerschlag, Annette C. Broderick, John W. Coker, Michael S. Coyne, Mark Dodd, Michael G. Frick, Matthew H. Godfrey, Brendan John Godley, Du Bose B. Griffin, Kyra Hartog, Sally R. Murphy, Thomas M. Murphy, Emily Rose Nelson, Kristina L. Williams, Matthew J. Witt, and Lucy A. Hawkes. (2015) Evaluating the landscape of fear between apex predatory sharks and mobile sea turtles across a large dynamic seascape. Ecology (preprint; version-of-recorded scheduled for August 2015)


The Ecological Society of America (ESA), founded in 1915, is the world’s largest community of professional ecologists and a trusted source of ecological knowledge, committed to advancing the understanding of life on Earth. The 10,000 member Society publishes six journals and a membership bulletin and broadly shares ecological information through policy, media outreach, and education initiatives. The Society’s Annual Meeting attracts 4,000 attendees and features the most recent advances in ecological science. Visit the ESA website at

Climate change, predators, and the trickle down effects on ecosystems

Ecologists are just beginning to understand how the impacts of climate change are affecting predatory keystone species and their ecosystems. Ecologists will report on this and other climate-ecosystem research news at the Ecological Society of America’s 2014 Annual Meeting in Sacramento, Cal., August 10–15.

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99th Annual Meeting
The Ecological Society of America

FOR IMMEDIATE RELEASE: Monday, August 4, 2014
Contact: Alison Mize (703) -625-3628;
Liza Lester (202) 833-8773 x 211;


Predators play important roles in maintaining diverse and stable ecosystems. Climate change can push species to move in order to stay in their climatic comfort zones, potentially altering where species live and how they interact, which could fundamentally transform current ecosystems.

A symposium focusing on climate’s effects on predators—causing cascading effects on whole ecosystems — will take place on Tuesday, August 12th during the Ecological Society of America’s 99th Annual Meeting, held this year in Sacramento, California.

There will be “winners” and “losers” as species adapt to a changing climate. Ecologists are just beginning to understand why different competitors may be favored by climate change and how consumer-resource interactions are modified. Impacts on one species can affect many organisms in an ecosystem. Because predator species are animals that survive by preying on other organisms, they send ripples throughout the food web, regulating the effects other animals have on that ecosystem. This cause and effect process is called a “trophic cascade,” or the progression of direct and indirect effects predators have across lower levels in a food chain.

Sea otters consume sea urchins and help keep the undersea kelp forest healthy. Credit, Vancouver Aquarium

Sea otters consume sea urchins and help keep the undersea kelp forest healthy. Credit, Vancouver Aquarium.

Sea otter populations provide a historical example of this phenomenon. The fur trade spanning the late 1700s to early 1900s decimated their numbers across their range, from Alaska to Baja California, Mexico. Populations went from an estimated several hundred-thousand to more than a million down to 1,000–2,000. Today, there are estimated to be just over 106,000 worldwide, with just under 3,000 in California. Now sea otters and other important predator species face the challenges of a changing climate.

“The near extinction of sea otters is one of the most dramatic examples of human-induced impacts to the structure and functioning of temperate nearshore marine ecosystems,” said Rebecca G. Martone, of the Center for Ocean Solutions at Stanford University.

In the U.S., there are two distinct sea otter subspecies, the Northern sea otter (Enhydra lutris kenyoni) and the Southern sea otter (Enhydra lutris nereis). Northern sea otters are found in the Aleutian Islands, Southern Alaska, British Columbia, and Washington. Southern sea otters, also known as California sea otters, live in the waters along the California coastline and range from San Mateo County in the north to Santa Barbara County in the south.

Sea otters live offshore in forests of kelp—huge, yellow-brown, rubbery seaweed reaching from the sea floor to the surface, like tall trees. In coastal North America, sea otters help maintain healthy kelp forests, which benefits other marine species dependent on this habitat.

Sea otters must eat about 25% of their body weight daily to maintain their body temperature since unlike other marine mammals they rely solely on their fur rather than an extra layer of blubber to stay warm—it’s like a 120-pound human eating 30 pounds of food per day. Some of otters’ favorites are abalone, clams, crabs, mussels, shrimp, and sea urchins. Few predators can crack the globe-shaped spiny urchins, which in unchecked hordes will chew through the holdfasts of the kelp, leaving vast barrens in place of the vibrant forests. The otter is a “keystone predator” whose presence has an outsized effect on its kelp forest habitat.

Without sea otters, the undersea sea urchins they prey on would devour the kelp forests, resulting in dense areas called sea urchin barrens that have lower biodiversity due to the loss of kelp that provide 3-dimensional habitat and a food source for many species. Researchers found that when sea otters arrive in an area from which they have been absent, they begin feasting on urchins. As a result, the kelp forest begins to grow back, changing the structure of kelp forest communities.

Many fish, marine mammals and birds are also found in kelp forest communities, including rockfish, seals, sea lions, whales, gulls, terns, snowy egrets as well as some shore birds. Otters might also offer a defense against climate change because healthy kelp forests can grow rapidly and store large amounts of carbon.

Dr. Martone’s analyses of the effects of sea otters on kelp forest ecosystems can help shape predictions of how climate change and trophic cascades, in concert with other drivers, affect coastal ecosystems. The ecological impacts of a changing climate are evident, from terrestrial polar regions to tropical marine environments. Ecologists’ research into the tropic cascading effects of predators will assist decision makers by providing important scientific findings to prepare for the impacts of climate change occurring now and into the future. Speakers for the symposia include marine, freshwater and terrestrial experimental ecologists who will present their research and offer insights from different approaches used to studying consumer-resource interactions.


Ecological Society of America’s 99th Annual Meeting, August 10–15th, 2014, in Sacramento, Cal.

Main * Program * Press Information * App

Symposium 9:  From Oceans to Mountains:  Using Abiotic Gradients to Investigate the Effects of Climate on the Cascading Effects of Predators
Tuesday, August 12, 2014; 1:30 PM–5:00 PM; Magnolia, Sheraton Hotel
Organizer: William L. Harrower
Co-organizer: Mary I. O’Connor

1:30 PM SYMP 9-1 Ecological stoichiometry: A chemical approach to understanding trophic interactions across spatial gradients
Angélica L. González, University of British Columbia; Rana W. El-Sabaawi, University of Victoria

2:00 PM SYMP 9-2:  Spatial and temporal patterns of trophic control across marine ecosystems
Daniel Boyce, Queen’s University and The Bedford Institute of Oceanography; William Leggett, Queens University; Brian Petrie, Bedford Institute of Oceanography; Boris Worm, Dalhousie University; Kenneth T. Frank, Department of Fisheries and Oceans

2:30 PM SYMP 9-3:  Ecological responses to predators and temperature in California mountain lakes
Celia C. Symons, University of California- San Diego; Jonathan B. Shurin, University of California- San Diego

3:10 PM SYMP 9-4:  Space use patterns and tropic interactions among woodland caribou, wolves, and moose across an anthropogenic disturbance gradient
John M. Fryxell, University of Guelph; Tal Avgar, University of Alberta; Anna Mosser, University of Minnesota; Andrew Kittle, University of Guelph; Garrett Street, University of Guelph; Madeleine Mcgreer, University of Guelph; Erin Mallon, University of Guelph; Ian D. Thompson, Canadian Forest Service; Arthur R. Rodgers, Ontario Ministry of Natural Resources; Brent Patterson, Ontario Ministry of Natural Resources; Glen S. Brown, Ontario Ministry of Natural Resources; Doug Reid, Ontario Ministry of Natural Resources; Merritt R. Turetsky, University of Guelph

3:40 PM SYMP 9-5:  Indirect effects of sea otter-driven trophic cascades vary across environmental and anthropogenic gradients
Rebecca G. Martone, Stanford University; Russell W. Markel, University of British Columbia; Gerald Singh, University of British Columbia

4:10 PM SYMP 9-6:  Trophic cascades and detrital subsidies in montane temperate grasslands
William L. Harrower, University of British Columbia; Lauchlan H. Fraser, Thompson Rivers University; Roy Turkington, University of British Columbia

4:40 PM SYMP 9- Panel Discussion


The Ecological Society of America is the world’s largest community of professional ecologists and a trusted source of ecological knowledge. ESA is committed to advancing the understanding of life on Earth. The 10,000 member Society publishes five journals, convenes an annual scientific conference, and broadly shares ecological information through policy and media outreach and education initiatives. Visit the ESA website at

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