A DOLPHIN - SHARK MYSTERY

We Talk with Shark Expert Andrew Nosal about Dolphins, Humans, and How Not to Get Bitten

Dr. Andrew Nosal of the Scripps Institution of Oceanography and Pt. Loma Nazarene University cradles a leopard shark.

The other day, my daughter and a friend were walking down the beach and saw this:

The San Diego media immediately assumed the dolphin had been killed by sharks, probably great whites, since the area is a great white nursery.  

Here’s the video:

You be the judge. 

Now, it’s usually the other way around in mythology and anecdotal accounts.  Dolphins protect their calves.  From stories by the ancient Greeks through the present, dolphins get the credit for saving shipwrecked sailors, often head-butting sharks away, or by circling swimmers to protect them.

So what's going on here? I asked my colleague Dr. Andrew Nosal, a shark expert at Point Loma Nazarene University.

ANDREW NOSAL:

Having looked at the picture of the dolphin that washed ashore, certainly it shows signs of various shark bites. That's not to be denied and watching the video taken just offshore, it clearly shows at least one white shark taking bites out of a dolphin. The first question that came to mind when I heard about this was could the sharks have actually killed the dolphin, or might the dolphin have already been dead or perhaps very sick or dying when the sharks came upon it?

The reason I think that's an important question is because Southern California is known to be a white shark nursery area and so, of course, we occasionally have the larger adult passing through, but for the most part we have just juvenile and subadult white sharks in the range of 4-8 feet. These immature white sharks are typically eating fish. They don't transition to eating marine mammals until they reach maturity at about 8-10 feet, so I think it's quite unlikely that these white sharks would have taken down a healthy dolphin, possibly a very sick or dying dolphin, but watching the video--and I'm watching it again right now as I'm speaking to you, Steve--I don't see any signs that the dolphin was moving or struggling or alive. In fact, by the time the video starts, there are already some very large bites taken out of it so I suspect these white sharks were scavenging on a dead dolphin. We've seen this before with even whales that show up along the coastline, dead. They're floating at the surface and various white sharks have been seen chomping on them as well.

HOT GLOBE:

So in the normal course of events a single dolphin would be too fast and a pod of dolphins could actually attack sharks that were menacing them or their offspring. Is that the case?

NOSAL: I would imagine that if there was a pod of dolphins and there was a threat from the shark, that the dolphins could certainly intervene or block the shark from actually biting, and I would say even for an adult white shark it'd be pretty difficult to take down a healthy dolphin. They’re pretty fast compared to really large size sharks, which are certainly fast, too, but most of the marine mammals that adult white sharks are praying on, namely seals and sea lions, though very agile and fast as well--the sharks are typically hunting them from below and trying to attack them quickly before they even realize what has happened to them. I don't think they can do that in the same way with dolphins.

HG: This coast is called a nursing area, but sharks don’t nurse the way mammals like us or dolphins do, right, by giving milk?

NOSAL: Unlike marine mammals that show very high levels of parental care and actual nursing because they're producing milk, sharks for the most part exhibit no parental care. As soon as those baby white sharks are born, they have all the instincts they need to find food and avoid predators themselves. What we mean by a nursery area in the case of these sharks is a place where the sharks may be born or even if they're born somewhere else they quickly migrate to this particular area. where there's likely favorable conditions like water temperature and food availability and they use this area for multiple years.

White sharks give live birth without a placenta. When they give birth the baby white sharks will emerge from the mother only a few feet long and they are basically ready to go. They start swimming immediately.

HG: What would cause a dolphin to die of natural causes?

NOSAL: Just like any other mammal they age and so once you get to be of old age certain organs may stop working or they just become a little bit slower. Maybe it's harder for them to catch fish or catch their normal prey because they're not as agile and so they may slowly starve or certainly they could get sick. They could get cancer. I mean there there's all kinds of reasons why older dolphins like any other older mammal would die, and honestly most times when these animals would die we probably wouldn't know about it because they would either sink or they would float away and be quickly eaten and scavenged by all the other creatures in the ocean. So the fact that we see just one wash ashore, that doesn't mean that there's not many more that probably die every day from natural causes.

Worldwide it's estimated that somewhere between 1/4 and 1/3 of shark and ray species are threatened with extinction, but we also have a number of species that are doing quite well. Leopard sharks, for instance, are considered “Least Concern” and their population seems to be quite stable.

HG: Why is that?

NOSAL: Well, there have been a number of steps that have been taken in the US and specifically in California to protect various species of sharks and rays. Let's say one of the most important in helping leopard sharks, soup fin sharks and even white sharks is the banning of nearshore bottom set gill nets, but if you go south of the border in Baja nearshore bottom set gill nets are still widely used by small scale fisheries and so it's unclear how the populations of these species are doing once they leave the US territorial waters.

HG: For people who are a little afraid of swimming out here, can you tell us why sharks bite in the first place? Are they testing?