Every year during the winter and spring, a population of great white sharks that normally live off the North American West Coast travel to a 160-mile wide stretch of seemingly barren ocean in the Pacific known as "White Shark Café."
This unusual migration has puzzled scientists for decades. But now, research carried out by an interdisciplinary team from Stanford University, the Monterey Bay Aquarium and other institutions, has begun to unravel the mystery of why the region attracts these apex predators.
The scientists have recently returned from a month-long expedition aboard the Schmidt Ocean Institute's research ship Falkor during which they used cutting-edge and traditional oceanography techniques to study the Café ecosystem in unprecedented detail.
Newsweek spoke to one of the researchers involved in the project—Aaron Carlisle, a marine biologist at the University of Delaware—to find out more.
What is the White Shark Café, and why has it puzzled scientists for so long?
The White Shark Café is a region approximately the size of the state of Colorado that is midway between Hawaii and Baja California, Mexico. We only learned about it when we started tagging white sharks in California in the late 1990s and early 2000s with the first generation of pop-off satellite archival tags.
These tags record light, depth and temperature every few seconds before popping off the animal, floating to the surface and transmitting those data to orbiting satellites. We can use the light data to estimate the position of the shark using light-based geolocation algorithms. After the tags were deployed, they started transmitting from the middle of the Pacific Ocean, in an area we never thought white sharks went to.
Prior to this, the general paradigm was that white sharks were coastal species, moving up and down the West Coast of the U.S. for their entire life span. This was pretty shocking when you consider it's the largest predatory fish in the ocean, had been studied for many decades, and yet we didn't have a clue about this aspect of their life history until about 20 years ago.
So, this finding completely upended our understanding of the biology, ecology, and life history of white sharks. I find it to be tremendously exciting when you realize how little we still understand about many aspects of the oceans and its inhabitants, and the story about the White Shark Café exemplifies this. We only know about the Café because the sharks led us there.
What are we learning about the behavior of the sharks and the Café itself?
The more tagging that scientists have done on this population of white sharks the more we've come to realize that they actually spend most of their adult life out in the central Pacific. Males spend on average six to eight months offshore, or longer, and females may spend as long as 18 to 20 months out in the high seas of the Central Pacific before returning to coastal regions.
In addition, there is a fascinating difference in how males and females use this area. Males go out to the Café itself—it's really a male hotspot. Females go there but move more broadly and do not spend as much time in there as males.
So, males go out there every year like clockwork and then return to coastal feeding aggregations in California and Mexico. Females have a much more irregular pattern. They may stay in coastal regions for a year or two before moving offshore, and when they move offshore they generally stay out there for much longer. One hypothesis is that females may move offshore to gestate their young.
Males and females also exhibit very different behaviors when offshore, with males exhibiting an amazing energetically costly diving behavior where they go from the surface to 400 to 500 meters (1,312 to 1,640 feet) 100 to 200 times a day. This is a unique behavior that has a tremendous energetic cost to the sharks, and we only see it in males when they are in the Café.
Females exhibit a behavior we commonly see across open ocean, or "pelagic" species, called diel vertical migration. Diel vertical migration is where the sharks are primarily in shallow waters during the night and dive to deeper depths (400 to 600 meters or deeper) during the day. This behavior is one that has been attributed to foraging in other pelagic species, with predators following the vertical migrations of creatures in the deep, scattering layer, or DSL.
The DSL is a remarkable biological soup of small mobile, organisms such as fishes, squids and crustaceans, that move from deep midwater habitats to surface waters at dusk to feed and then back to depths at dawn in what is the world's largest daily migration.
These animals come to the surface at night to access shallow water prey and resources when they are hidden by the dark from visual surface predators. The DSL forms the backbone of pelagic food webs, and is an important forage base for many potential prey items of white sharks, such as tunas, squid, billfishes.
The marine ecosystem off the West Coast of the U.S. and Baja California—known as the the California Current—is one of the most productive in the world, filled with white sharks' favorite prey, marine mammals, and in particular pinnipeds such as elephant seals and sea lions.
The Café, on the other hand, is generally thought of as being an oceanic desert. There's nothing there that is immediately obvious. While there are fisheries for pelagic fishes out there, there has not been anything that historically has stood out as being a hotspot of biological activity. We know there are things for sharks to eat out there—because fishing boats target the region—but nothing close to the abundance and richness of prey that are present in productive coastal waters.
So, why leave the one of the most productive marine ecosystems in the world and head offshore for most of the year to what is to all appearances an oceanic desert? It is an amazing puzzle. There is just no apparent reason for them to go out there and spend so much time in that region. Clearly we are missing something.
Ultimately, there have been two general hypotheses about why these sharks are going to the Café. One is that they are going out there to feed, the other is that it is related to reproduction. To date, we've never had a full oceanographic expedition to this part of the ocean. So we've never been able to get direct observations in order to get an idea what they may be feeding upon in the region, what the actual oceanography of the Café is like, or how the pelagic food web in that area is structured and functions.
We've been limited to using the data from the sharks' tags, so we've only had half the story, an important half that allowed us to generate hypotheses, but not enough to answer these basic questions. We're hoping that the data we collected during this cruise will help shed light on this.
Does the data collected during the expedition provide an answer to the question of why the sharks are migrating to the Café?
Science takes time, so it is a bit premature to talk about what we found or to speculate further about what the sharks might be doing in the Café.
However, based on our preliminary work and observations, what we found in the Café was an ecosystem that was richer and more biologically and oceanographically dynamic than we expected. This doesn't mean that white sharks are feeding on the small creatures of the DSL. But they could be eating the larger predators that directly feed upon the DSL, such as bigeye tuna, opah, swordfish, or flying squid.
We found evidence of a robust food web that could support the potential prey of white sharks. This doesn't answer the question about why the sharks are there, but it may provide insight into how they could support themselves while in those habitats.
What are the implications of these findings?
Our hope is that we'll be able to answer that fundamental question—why do white sharks go to the Café? This question is fascinating from a scientific point of view, and is fundamental to understanding the biology, ecology, and life history of these sharks. Importantly, if we understand why they are there, we can better understand what role this region plays in the life history of this important apex predator.
White sharks are of conservation concern. They are listed as globally "Vulnerable" by the International Union for Conservation of Nature Red List. So, if you want to effectively protect this species, you need to understand why it is spending most of its adult life in these unprotected and generally unmonitored oceanic habitats.
If you don't know why an animal is in a place or what it is doing there, it is impossible to develop effective management or conservation strategies. Is the Café a critical habitat for white sharks? If it is, we need to manage this region, and white sharks, accordingly. But we can't do this until we know what they are doing out there.
Uncommon Knowledge
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Newsweek is committed to challenging conventional wisdom and finding connections in the search for common ground.
About the writer
Aristos is a Newsweek science reporter with the London, U.K., bureau. He reports on science and health topics, including; animal, ... Read more
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