Before I get to the oceans, I’ll describe something that Boston builds every year on the Boston Common: a fishweir. A fishweir is a fence used to catch fish. Believe it or not, it used to work:

During the excavation and construction of Back Bay, a fishweir, thought to be about 3,700-5,200 years old, was discovered at what would today be Clarendon and Boylston streets–which then would have been a shallow marine intertidal zone. I bring up fishweirs because it’s important to realize what makes the large investment in building them useful: lots and lots of fish. You’re basically trapping fish against a fence and either spearing them or grabbing them.

For anyone who is familiar with New England salt marshes and seashores, this strategy seems ridiculous. When is the last time, in those habitats, you have seen enough fish (and minnow-sized fish don’t count) to justify this strategy? What many people don’t realize is that these habitats, until a couple of centuries ago were teeming with wildlife; in terms of productivity, these habitats are marine equivalent of Iowa farm soil. In Maine, lobster, in shallow habitats, were so plentiful, they were used to fertilize soil–and feeding lobster to servants, slaves or orphans in the 1600s was illegal (it was considered feeding people cockroaches). Ditto the fish.

A couple of weeks ago, there was an article which led to some debate about whether human activity is actually leading to a Sixth Extinction. It might not be, but, at the same time, the devastation we wreak on ocean communities pales to anything we do on land (boldface mine):

The oceans have endured a similar transformation in only the past few decades as the industrial might developed during World War II has been trained on the seas. Each year fishing trawlers plow an area of seafloor twice the size of the continental United States, obliterating the benthos. Gardens of corals and sponges hosting colorful sea life are reduced to furrowed, lifeless plains. What these trawlers have to show for all this destruction is the removal of up to 90 percent of all large ocean predators since 1950, including familiar staples of the dinner plate like cod, halibut, grouper, tuna, swordfish, marlin, and sharks. As just one slice of that devastation, 270,000 sharks are killed every single day, mostly for their tasteless fins, which end up as status symbol garnishes in the bowls of Chinese corporate power lunches. And today, even as fishing pressure is escalating, even as the number of fishing boats increases, even as industrial trawlers abandon their exhausted traditional fishing grounds to chase down ever more remote fish stocks with ever more sophisticated fish-finding technology, global fish catch is flatlining.

Closer to shore, coral reefs, the wellspring of the ocean’s biodiversity, have declined in extent by as much as a third since only the 1980s. These paradises are plagued by overfishing, pollution, and invaders, but 500 million people, many of them poor and living in the developing world, rely on them for food, storm protection, and jobs. As in the handful of reef collapses of the geological past, modern reefs are expected to collapse from warming and ocean acidification by the end of the century, and possibly much sooner. During record-breaking temperatures in 1997–1998, 15 percent of the world’s reefs died. In the past few months a similar wave of death struck the Great Barrier Reef. It won’t be the last.

These organisms might not become extinct, but the oceans have been radically altered, perhaps irrevocably so. Assuming human civilization doesn’t collapse, we’re going to look back on the last century and wonder why we were stupid stupid about the ocean.