It's easy to get pessimistic about the state of the oceans. Coral reefs are dying. Various fish species are getting wiped out by overfishing. Jellyfish are taking over.

The blue whale recovery is a sign that ocean conservation measures really can work

But not all news is bad news. A recent study published in Marine Mammal Science found that blue whale populations off the coast of California have almost entirely rebounded after being nearly hunted to extinction in the 20th century. (Many thanks to Andrew Revkin for pointing out this study.)

Unfortunately, the same can't yet be said for blue whale populations down near Antarctica — which have yet to fully recover from the massive whaling frenzy that only ended in the early 1970s. (Previous studies have suggested that Antarctic blue whales, which used to be the most numerous of all, are still at just 1 percent of historic levels.) Still, the whale rebound off the coast of California is a reminder that protections for threatened species really can make a difference.

How we nearly hunted blue whales to extinction

Blue whales are the largest, heaviest animals ever known to exist on this planet. Growing to nearly 100 feet and weighing more than 160 tons, they're more than twice as large as even the biggest land dinosaurs that have been discovered to date.

So it's stunning to think that humans nearly eradicated these whales entirely in the 20th century. The animation below, made by Cole Monnahan, a PhD student at the University of Washington, tells the tale:

Prior to the late 19th century, blue whales were simply too big and powerful to pursue. But the advent of steamboats and advanced harpoon guns made it easier to go after much larger whales for oil and meat — and catches soon began surging, first in Iceland and Norway, then around the world.

By the time the International Whaling Commission banned blue-whale hunting in 1966 — and after illicit Soviet whaling finally tapered off in the 1970s — most of the damage was done. Roughly 380,000 blue whales had been killed in all, and the species was at 0.2 percent of its initial numbers.

But now there's recovery — at least off the California coast

Now there are signs that the whales are starting to recover. The latest paper, by Cole Monnahan, Trevor Branch and André Punt of the University of Washington, estimates that there are currently some 2,200 California blue whales in the eastern North Pacific — more or less the number that existed in the region before the advent of whaling.

Figuring out the numbers involves some detective work

What's more, they conclude that the blue whale population has now plateaued because it's nearly reached the maximum size that this region of the ocean will support — and not because too many whales are being killed by ship strikes, as some researchers had previously thought.

How did they figure this out? Calculating the precise numbers of whales (both now and historically) involves a lot of detective work, as this breakdown from the University of Washington details. The historical data on blue whales caught in the region wasn't even known until recently — because Soviet whaling figures were kept secret for so long. In a June paper for PLOS One, the researchers combined those numbers with data on whale acoustic calls to estimate that roughly 3,400 blue whales were caught in the eastern North Pacific between 1905 and 1971.

And in their newest paper, the researchers put all that data together to model a more precise population estimate over time. Among other things, they calculated that there are currently about 2,200 California blue whales today — and that that's about 97 percent of the carrying capacity for the eastern North Pacific. It's a full recovery.

Now, note that this is only a single region of the world. Down in Antarctica, the situation is still quite bleak. An earlier paper by Branch, using a similar model, estimated that the Antarctic blue-whale population went from 239,000 before the advent of whaling down to just 360 in the 1970s, and was still only back up to around 1,700 whales by the late 1990s. Recovery there has been very sluggish.

Ship strikes are a problem — but they're not thwarting the California blue whale recovery

Another big concern about California blue whales is that too many are being hit and killed by large ocean vessels. In their paper, the authors agree this is a problem, but they find that this doesn't appear to be thwarting the overall recovery of whales in the eastern North Pacific.

Some groups have estimated that a dozen blue whales a year are killed by ship strikes

Estimating the precise number of whales rammed by ships and killed is also difficult — since, as Monnahan discusses in a blog post, ship impacts aren't always recorded and carcasses aren't always recovered. Some groups have estimated that roughly a dozen California blue whales per year are killed, but there's a range.

In the Marine Mammal Science paper, the authors consider even higher-end estimates (35 per year) and found that ship-strike deaths still wouldn't prevent the California blue whale population from rebounding to historic levels.

This is in contrast to earlier research, which had suggested that blue whale populations in the area might be stagnating lately because of ship strikes. The authors of the new paper also calculate that it would take an 11-fold growth in ship strikes for there to be a 50 percent chance of long-term population depletion.

That said, the authors certainly don't condone ship strikes. The paper does note that current ship-strike numbers for California blue whales "are likely above legal limits set by the U.S." and add that "ship strikes are currently a major concern for other endangered cetacean populations." (In the north Atlantic, for instance, ship strikes are the leading cause of death for right whales.)

For that reason, many researchers have argued for policies to reduce these strikes — say, by moving shipping lanes away from known feeding areas.

Further reading:

Monnahan has set up an excellent blog, Blue Whale News, to discuss this and other blue-whale research.

This Nature piece takes an in-depth look at the debate over limiting ship strikes off the coast of California — with some discussion of how this latest research fits in.