But useful as they are, marine reserves – often around points like coral reefs or rock islands – are only effective if governments have the resources to patrol and protect them. Also, many marine creatures, from whale sharks to whales, are migratory – they don’t stay in the protected areas, making them easy prey for fishermen. What’s needed, many argue, are mobile reserves that follow migratory animals, and those that shift habitat due to currents or climate phenomena like El Nino.
The zones need to be well-targeted and needn’t impact on fishermen’s livelihoods. For example, one study found that designating just 20 sites – 4% of the world’s oceans – as conservation zones could protect 108 species (84%) of the world’s marine mammals.
The rivers in many European cities were so overfished, polluted and dammed up by the mid 20th century that they emptied of fish, and many species went locally extinct. But thanks to clean-ups, riverbank restoration and fishing restrictions, fish are returning to waterways, even in inner cities. A decade ago, few people would have imagined that salmon would return to my local river, the Thames. If it is possible to bring back fish to 'dead' rivers, there is surely hope for the world's oceans.