Photo: Mikal Schlosser

Fisheries physicist boils theory down to five words

Monday 04 Sep 17


Ken Haste Andersen
Professor, Head of Section
DTU Aqua
+45 35 88 33 99

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The defence of the doctoral dissertation will take place on
18 September from 2 - 4 p.m. at DTU Lyngby Campus.
Everyone is welcome.

More information on the doctoral defense.

More on Doctor Technices—the highest academic distinction within engineering and technological science in Denmark.

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This month Professor Ken H. Andersen will defend his doctoral dissertation written on the basis of 10 years’ research and which resulted in his development of new fishery management models.

It is not often that a doctoral dissertation can be boiled down to five words, but Ken H. Andersen’s case in an exception. The five words are: ‘Big fish eat small fish’—which has an intuitive ring to it. However, the five words actually reflect a break with the way in which fish stocks are traditionally investigated.

“The earliest attempts to design an optimal fishing management system date back 70 years when UK researchers showed how the fishing industry affected plaice and cod of different ages in the North Sea. Their research showed that it was most advantageous to fish the oldest fish and let the younger ones live. Similar models have subsequently been devised for other species of fish. Generally speaking, this is the same approach employed to this day,” says Ken Haste Andersen, Professor at DTU Aqua:

“A given species is typically described using a range of parameters—e.g. size, growth rate, maturity, and its risk of being eaten. But if all these parameters have to be in place, it becomes extremely difficult to assess the consequences of various types of fishing,” he says.

Hidden reality
Add to this real-life conditions at sea—e.g. that fish also eat each other—and you end up with a highly complex picture of an entire ecosystem. So complex, in fact, that it can be to hard to focus on the area you are truly interested in knowing more about.

For the past ten years, Ken Haste Andersen—who has a background as a theoretical physicist—has therefore worked on creating models that can describe what is actually going on below the sea surface. It is all about obtaining data about fish stock numbers, permissible fishing quotas, and ultimately how much money society can earn from the fishing industry.

"What ultimately defines what you eat and who you are eaten by is your own size."
Professor Ken Haste Andersen, DTU Aqua

“Fishing for cod, for example, will affect sand eel numbers, which will increase as there are fewer cod to eat them. The whole ecosystem is inextricably linked—an action in one area affects the entire ecosystem. Of course, researchers are aware of this and have tried to improve the old models by grouping populations, but these efforts are little more than stop-gap solutions,” says Ken Haste Andersen.
In his doctoral dissertation, he therefore proposes new models which, among other things, means talking about the size of the fish rather than different species such as cod and eel.

“What ultimately defines what you eat and who you are eaten by is your own size,” says Ken Haste Andersen.

Information overkill
‘Big fish eat small fish’ reflects a break with the current models’ focus on species and their many characteristics.

“It means throwing away some information. Conversely, I can say something about a less complex part of the ecosystem—e.g. size—with greater certainty. It’s enough for me to know that I’m dealing with a fish that weighs two pounds, for example. Based on this, I can say a lot about the other parameters, and then make an impact assessment of different types of fishing activity and their effect on the other fish stocks.”

According to Ken Haste Andersen, the approach will pave the way for improved fishing management in places where—unlike Denmark—there are insufficient fish stock data.

“We need methods for managing stocks we don’t know much about. But if we have to examine each individual species and the age of the fish, it will take a very long time and be very expensive. All things considered, it is much easier to say something about the size of the individual species. The nature of the species itself is less important. For no matter where we look, we know that big fish eat small fish.