Dolphin Stampede: 1,000 Dolphins Seen Swimming Off the California Coast

While some people dream of swimming with dolphins, a group of people off the coast of southern California got to boat with a massive group of 1,000 or so of the ocean mammals. The animals — spotted off Dana Point in Orange County — swam in the waters near a whale-watching boat for four hours.

"They are so graceful even in the frenzied behavior and we are so amazed to see them right of our coast,” wrote the Dana Point Whale Watching group, which posted a video of the event. The group takes credit for coining the term “dolphin stampede” to describe the frenzied activity.

Dolphins are very social animals that normally travel in groups called pods. Most pods are much smaller than this though, typically from a handful to a few dozen individuals. Very large pods of hundreds or sometimes thousands of dolphins, however, do get together often, especially to look for food or mates.

Nick Kellar, a marine mammalogist with the National Oceanic and Atmospheric Administration (NOAA) Southwest Fisheries Science Center in La Jolla, California, is not far from where these dolphins were spotted. We checked in with him to see what it’s like to observe a group of this size, how often it happens, and what they’re likely doing.

Treehugger: Are you familiar with this group of dolphins in the video?

Nick Kellar: I don't know if I'm familiar with the individuals who comprise this group of dolphins but I'm very familiar with the species. They are long-beaked common dolphins and currently designated as the Delphinus capensis (or sometimes Delphinus delphis bairdii). 

You can see in the near ground of the video that in this group there are “young-of-the-year” calves that I would guess are six to nine months old because of their well-developed pigmentation and size. 

The reason I say that it's unlikely that I'm familiar with these particular individuals is that group composition in this species is often very fluid, coming together and breaking apart over the course of hours to tens of hours to multiple days. In fact, it is often noticed that big aggregations can start out the day as small pockets or subgroups or sub-schools and then at a certain point start aggregating together as larger cohesive units but usually not all at once. And then they might break apart again or slowly peel off as smaller groups and sometimes reform if necessary or desired. 

On the other hand, these aggregations can remain cohesive for many hours, and perhaps certain core elements can remain together for days. But schools of common dolphins off California are not like pods of killer whales, for instance, which are founded on matrilineal lines such that they have close associations that remain stable through years and even decades.

How common is it for dolphins to be in a group of this size? 

It's not totally uncommon for these dolphins to be in groups this large but more common are groups sizes between 50 to 400 individuals. Depending on the time of year and the location, I would say somewhere between 1/30 and 1/100 schools that we observe are sized greater than 1,000. 

What is a group this large called?

There's no official term for schools this size but we often refer to them as “mega schools” — it probably should be “kilo-school” but that just sounds wrong. We don't typically use the term superpod or megapod, I think because most scientists reserve the term pod as a plural noun for cetaceans when they are species that are closely connected via family lines like killer whales.

However, the term “pod” is occasionally used to describe aggregations of these schooling dolphins. It wasn’t too long ago that the preferred term for these large aggregations of dolphins was “herd” perhaps a nod to the fact that they are closely related to ungulates. Common dolphins from the same school off California aren't really any more related to one another than to dolphins from other schools; with some exceptions.

When the animals separate into smaller groups we sometimes notice that they appear to be of similar life-history states. For instance, we see groups that are composed of a lot of moms with calves that we call nursery schools or we see small schools that have a high number of adult males that we call bachelor schools.  

Why are they together when they are in groups this large? 

Though we don't know with great certainty we suspect that dolphins school together in large aggregations for some of the same reasons that other mammals create herds or other large aggregations. The two most common reasons are to mitigate predator risk and increase foraging success. 

The dilution effect is one hypothesis, in which the risk of being preyed upon for any particular individual is lessened within a larger group. The idea being that although greater aggregations result in potentially higher detection rates, that the relationship isn't a one-to-one such that at some point the risk of being attacked is less even when you factor in potentially higher detection rates. 

And this is particularly true for small dolphins in which their predators don't typically use sight for detection but instead rely on hearing. You can imagine how much more difficult it is to detect, for instance, 20 additional animals in a group of 1,000 than it is to detect 20 additional animals in for instance a group of 40.

Another benefit of schooling for predator avoidance is having a kind of collective vigilance. The idea is that there are always some alert animals within these aggregations to alert the rest when a predator is detected. This may be particularly important for dolphins as they engage in resting/sleeping behaviors such that they're not fully attentive. We know that dolphins sleep one brain hemisphere at a time and during those times they probably aren’t as aware as when both hemispheres are active.

And being in a large group somehow helps with foraging?

You might think that finding enough food might be more difficult when they're in such large aggregations because there wouldn't be enough to go around for all the individuals. And this likely would be problematic for some cetaceans given the differences in preferred prey but I think that this isn't such a problem for common dolphins because they're often hunting prey that also engages in schooling behavior but with much greater abundance (e.g., anchovies, sardines, and squid). 

And it's been shown that dolphins likely have an advantage when they forage in groups with schooling fish as they're able to herd their prey together and often toward the surface of the water and keep them in a tight ball for efficient prey capture.

What is it like to observe dolphins when they are in a group like this?

It is interesting that the whale watching group was calling this school a dolphin stampede; that’s a good analogy in that the animals are moving rapidly in a tight formation and yet are in a large aggregation. 

Now we do see this fairly often but the two times that I've witnessed this most distinctly were in the presence of hunting killer whales. During one of those encounters, the first dolphin was captured before any others were alerted to their presence. As soon as the killer whale landed its quarry there was an instantaneous chaotic spree of porpoising animals. This quickly turned into a more organized but very rapidly moving line or flattened ark of individuals fleeing all together in a very tightly associated group. What was impressive was that even young calves seem to be able to keep up with the group at least over the first few minutes of escape.

I was just looking at the images from that day and you can clearly see that all the surfacing dolphins were fully out of the water so likely moving at a much faster rate than these dolphins. So I don't think it's as likely that they were fleeing killer whales on this day. At least not fleeing from an imminent attack.

So my guess for the reason that these animals are together forming a large tight school is that they were either foraging and were close enough to their target prey that they were no longer in search mode (which the animals in the group are more spread out) but not yet close enough to start encircling their prey. But there are many other possibilities including that there was something that spooked them and they were fleeing but that something didn't seem as threatening as an imminent attack from a pod of killer whales.