In the 19th century, visitors to European coastal resorts were often greeted with the sight of huge skates hanging from wooden stakes along the shore, some the size of dining tables. The meat of these fish was considered too tough and rank to be eaten fresh, but a few days in the open air softened them up nicely.

Although their name is “common” skate, such giants are unfamiliar to modern shoppers because there are virtually none left. Even by the mid-19th century they had begun to decline as bottom trawlers dragged their nets across ever larger areas of seabed and hook-studded longlines stretched farther.

The common skate is but one of dozens of fish that have come and gone from markets in the past century, victims of their popularity and of our inability to restrain overfishing. Fisheries the world over made the revolutionary transition to engine power in about 1900, allowing boats to deploy bigger nets, fish deeper and farther offshore. Add fast-freezing to the package — an invention of the American entrepreneur Clarence Birdseye in the 1920s — and fishermen were freed to move into the haunts of exotic new species, one of many reasons why organizations like the Blue Marine Foundation, which works to establish marine reserves, are so important.

Elsewhere, the ongoing collapse of the iconic bluefin tuna — the world’s most expensive fish — could see them disappear from the table within a few years. Anticipating this loss, the Japanese Mitsubishi Corporation has reportedly been stockpiling frozen bluefin.

Some kinds of seafood have boomed, busted and made a comeback as fish farms have spread. But unless overfishing is brought under control, we will see further losses of common seafood species, and many scientists predict a future in which we must eat jellyfish or plankton for want of anything else.

How we create toxic algae, killing animals

Watch where you throw out that six-pack of soda! Up to 1 million seabirds and 100,000 mammals die each year from trash-related problems, says the IOI. Bags routinely choke turtles and seagulls and can act as magnets for other types of contaminants.

The bad stuff travels far, as well. Steve Gittings of the National Oceanic and Atmospheric Administration says his group sees more problems from trash among wildlife in offshore areas than from the coastal pollution problem, as contaminants from urban businesses, factories and agriculture flow into the ocean through our waterways.

Another issue: Hypoxia eutrophication — which has been attributed to the increase of agricultural chemicals, industrial byproducts and waste from population growth in our oceans — depletes oxygen and increases harmful algae.

Our desire for oil has us tearing up the sea bottom

Levin, of the Scripps Center, points out that our need for more fossil fuels is driving more companies to drill for oil in areas like off the coast of Africa and the Arctic. In the Gulf of Mexico, there are 4,000 oil rigs in waters of 3,000 meters in depth or more. Hydrothermal vents will soon be mined for precious minerals. Besides the noise pollution that affects animals that depend on vocalization to communicate, drilling brings in other contaminants -not to mention the extra boat traffic — and creates too much change for the habitat.

“Deep sea oil and gas and energy extraction already has the potential for major accidents,” Levin says. “Now, mining companies are set to mine precious metals in the deep ocean. They target countries with limited regulations when it comes to the deep sea. As a biologist, I’m not happy to have the bottom torn up.”

Watch “The End of the Line,” Blue Marine’s documentary on ocean health, for free here