In 2000, I caught a ferry across the North Sea from Esbjerg in Denmark to Harwich in Essex. As we neared the Essex coast, I peered over the side of the vessel and was surprised to see swirls of orangey-brown... er, stuff, near the surface. I didn't expect such a well-used waterway to be clean or teeming with life, but neither did I think it would be opaque. I never got to the bottom of what it was – an algal bloom of some sort, or petroleum sludge, or sewage – but it was not pleasant.

At least a decade later, and half a world away, I witnessed some of the clearest sea water I've ever seen – off the east coasts of Tasmania and New Zealand's South Island, in particular. Near Kaikoura I saw what looked like a wall of fish (top), which was extraordinary… and increasingly rare, I fear.

Last week I read something that made me depressed, which, since I read a lot of environmental stuff, happens quite often. But this article was worse than most. It described a yachtsman who repeated a 37,000-km journey he'd made ten years before, from Australia to Japan and then across the Pacific. Previously, he'd caught fish every day but this time noticed a 'severe lack of marine life'. There were no dolphins playing alongside his boat, or turtles, or whales passing by, or even birds accompanying him. He also encountered huge amounts of rubbish, some of it large and a danger to his boat. He would only use the motor during the day when someone could keep watch for large pieces of debris in his path. He chooses words such as 'dead' and 'barren' to describe thousands of the kilometres he covered. You, too, can get depressed here. 

I don't know if the yachtie sailed through the Great Pacific garbage patch, also known as the Pacific Trash Vortex. Estimates of the size of this dreadful phenomenon vary widely depending on the size of the plastic particles you use to define it, but it could be as much as 15 million square kilometres, or twice the size of the North American continent (24.7 million square kilometres x 2). It consists of plastic, chemical sludge and other debris trapped in the circulatory systems of the North Pacific. The effects on wildlife are dire, of course: you may have seen photographs of the plastic contents of the stomachs of sea birds that have starved to death. Albatrosses on Midways Atoll eat a lot of plastic and as many as a third of their chicks die as a result.

Overfishing is obviously a major reason why there were few fish for our boatie friend to catch. We have been surprised here how unaware most people seem to be of dwindling supplies of their favourite fish. They blithely go out most weekends in their boats with their mates, oblivious of the fact that it's not just commercial fishing that is depleting stocks. There are millions of recreational fishers in Australia. And bycatch is not only a problem when you're dragging an enormous net across the ocean floor.

For some years marine scientists have noticed reduced numbers of ocean predators. The reasons are many and various, from overfishing to habitat change, pollution to increased demand for shark's fin soup. The loss of top-end predators has serious consequences all the way down the food chain. The decline in shark numbers off the North Carolina coast, for example, led to a dramatic proliferation of their prey, cownose rays, which expanded into areas where previously sharks had lurked. The rays decimated the local scallops, not to mention the century-old scallop fishing industry. Such cascade effects on the food chain can even impact on the regulation of carbon emissions. Everything joins up in the end.

I'm sure few people underestimate the importance of the world's oceans in regulating climate. We've learned recently from climate scientists, however, that the sea has been absorbing much more carbon from the atmosphere than was previously thought. This is not the time to abandon marine park maintenance or fishing quotas, or increase the chances of environmental damage to an already challenged Great Barrier Reef, or turn a deaf ear to inconvenient truths.