Ghosts of oceans past and future

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Marine ecosystems are facing a litany of threats, ranging from overfishing to climate change – but the Census of Marine Life is key to mitigating them, say Ian Poiner and Poul Holm.

In this week’s BBC’s Green Room, they argue that everything from long-ago tax accounts to eyewitness whale encounters are crucial in understanding the future of ocean life.

Joni Mitchell once famously sang that “you don’t know what you’ve got ’til it’s gone.” But when it comes to marine life, in many cases we’re only just starting to realize what the planet once had.

Imagine a nearshore off Cornwall, England, teeming with orcas, blue whales, shredder sharks, dolphins and harbour porpoises. Such were conditions in the 17th Century before humanity removed the top predators. It is now estimated that inshore regions of the seas historically held 10 times the volume of marine life seen today.

Establishing environmental history in mainstream marine science will be one of the great enduring legacies of the Census of Marine Life, which has united thousands of world researchers to assess and explain the diversity, distribution and abundance of marine life, past, present and future.

The historical research involves many disciplines, including palaeontology, archaeology, history, fisheries and ecology, and such diverse sources as old ships’ logs, literary texts, tax accounts, newly translated legal documents and even mounted trophies.

Some 400 of these marine historians will gather from around the world this week at the University of British Columbia, Vancouver, Canada. And the images they are piecing together reveal fish of such sizes, abundance and distribution in ages past that they stagger modern imaginations.

They are also documenting the timelines over which those giant marine life populations declined. Sadly, due largely to fishing and habitat destruction by humans, the scale of decline is much larger than generally thought a few years ago.

‘Cause for optimism’

The work has very practical value today, enlightening the management of fisheries worldwide, which typically use reference information spanning no more than 20 to 40 years.

Consequently, policies, management strategies and conservation targets are set to a standard much below the oceans’ potential productivity and can now be reviewed with a larger historical perspective.

Some of the research also provides cause for optimism, showing the ocean to be much more resilient to human pressures than the land. History shows that a moratorium on fisheries works. This is well illustrated by the rich harvests of fishermen after World War II and the rebuilding of North Sea herring after the decline of the 1970s.

In the last two or three decades citizens and politicians in rich and poor countries alike have come to recognise that our planet is small and vulnerable: a historic turn of the public mind.

With the triumph of social engineering and science in the second half of the 20th Century, history was widely considered irrelevant to the practical concerns of modern society. Today, however, the need for historical insight is pressing.

While few marine species have gone extinct, there is concern that some marine ecosystems have been depleted beyond recovery and generally humans would benefit economically by fishing less and fishing smarter.

Understanding historical patterns of resource exploitation is a key to identifying what has actually been lost in the habitat – essential to developing and implementing recovery plans.