Winding back the ocean clock

Looking back into oceans past, Poul Holm tells us about novel ways of understanding how the marine realm has changed.
07 October 2010

Interview with 

Poul Holm, Trinity College Dublin




As well as exploring the oceans for present-day marine life, the Census of Marine Life has also been using innovative ways of winding back the clock to build a picture of what the oceans used to be like and the impact humans have had on them. We talk to Poul Holm about how the Census has been recreating past oceans and how this can help us plan for the future.

Fisherman clothes

Find out more:

History of Marine Animal Populations (HMAP)

James Barrett et al (2004) The origins of intensive marine fishing in medieval Europe: the English evidence. Proc. Roy. Soc. Lond. B (PDF)

Poul - Ten years ago my colleagues said to me writing the history of the oceans - you can't do that because we don't have the sources. Historians don't do fish. What we now know is that we have actually go plenty of evidence for now much we've taken out of the oceans, what kind of footprint that has left.

We also know a bit about how that impacted the development of human society. So it's learning more about our relationships with the oceans. I think that's exciting because so little work has been done in that field.

Helen - And you're looking into the past. How do you do that, and how far are you able to look back into the past now, when it comes to oceans and fisheries and so on?

Poul - well you need to look into all sorts of sources. It can be, well obviously, written records. As a historian I would go to port books, I would go to skippers' log books, I would go to monastic records. You can go to archaeological evidence, you can look at trash heaps as the Americans would say, or the kitchen middens and look at old fish bones. We can look at sediment layers in the ocean bed. 

Some of the most imaginative uses of records was the use of restaurant menus. The New York city library had one, possibly slightly insane collector, who had been collecting restaurant menus and had build up this fantastic series of a hundred years of restaurant menus, which provides a fantastic record of what people used to eat and how we've been eating down the food web. A hundred years ago abalone would be on the menu whereas today that would be an endangered species.

One of the exciting things about this is actually when you use that kind of evidence you will also use evidence that people will be able to relate to very easily. When I talk to fishing communities very often they will flatly deny that they've had any footprint in the oceans. They say, well the fishes are doing alright.

But if I tell them, actually if I look into your grandparents log books they were catching fish that would be three times as long as the cod that you're landing today. That seems to be an indication of some change and they will actually have to say, yeah, that's pretty bad news.

Helen - so it sounds like that during the Census you unleashed this new way of looking into the past. You said that people thought, well how can you do that, and now there are all these creative and imaginative approaches. That's really quite new isn't it, that now you've almost unleashed this whole new field of research?

Poul - Yeah, and the exciting thing is that because so few people have been looking at this in the past there's lots to discover. I think one of the most amazing things is the discovery by James Barret and his group in Cambridge of the so-called fish horizon. We now know that around 1050 - 1100 fishermen all around the north sea, in England, Scotland, Holland, Belgium, Denmark, were going much further out into the open sea and catching fish at depths of two-three hundred metres, bringing back gadoids of all sorts so cod, ling, that sort of species and marketing the fish fairly deep inland. That's new. It really reveals that there was a tremendous change in the diet of ordinary people.]

It explains a lot about the impact that we humans have had on the open ocean because everywhere we look the first thing you do is take out the big fish, the big marine mammals or the big fish. And of course, when we look at the cod bone way back then, these were humungous creatures that they were landing, because no one had been fishing them before so they lived their full natural age of 20 years. They could grow well in excess of 1.5m. 

Helen - Presumably we use this looking into the past to think about the present, and even the future? How does your research help us think about things like conservation and what we really need to be doing to the oceans now? It's all very well to say in the 12th century we changed our diets. How does that now help us to look into the future?

Poul - It's kind of sad to know we took out all of that, but it's also something we can turn to really inform us for future ocean policy because if it used to be out there it would actually be rebuilt. Because the oceans are very resilient. Although we have wiped out many of the commercial fish stocks , it's not that the fish have gone in a biological sense. They're still out there. They're waiting for their chance. The oceans are deep and there are many places to hide. So there's a huge potential for us to rebuild fish stocks in the future. 

Remember, all the fisheries biology science really rests of knowledge that's been accumulated in the past fifty years. Whereas what we're looking at is how did the oceans look 100, 200, 300 years ago. They were very different, they could be rebuilt to that stage if we want to. Of course that's a political question, will we want to do it? But the economic gains would be sizeable. It would mean there would be plenty more fish and it would be fish of a much more valuable nature.

Because of course if you have a fish which is allowed to grow to longer scales you actually have much more attractive fisheries. And you will also have much more resilient fisheries because it makes no sense now  that we killing off cod at the age of less than three years of age that's just before they spawn for the first time.

It would make more sense to wait until they are at least 8 or 10 years, have spawned a few times, and actually we know that the eggs of cod that have spawned a few times is actually much more likely to survive all the hazards that confront a little fish egg. So, lots of gains to be made, and we can learn that from history that the oceans will actually provide that future for us, if we want to. It's not doom and gloom it's actually tremendous potential.


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