Closer to solving the cod mystery

Avoiding starvation and predators are the two basic survival challenges for the billions of small fry in coastal waters. In marine environments, the rule is either eat and survive or be eaten.

In the project Effects of North Atlantic Climate Variability on the Barents Sea Ecosystem (ECOBE) , scientists from a number of Norwegian and international research groups are collaborating and using new methods to find answers to a longstanding mystery: What happens to the billions of fish eggs and fry that drift out to the Barents Sea from their spawning grounds along the Norwegian coastline? Those that survive comprise the new year-class of cod and other important fish for consumption.

The project has received funding under the research programme on Climate Change and Impacts in Norway (NORKLIMA) at the Research Council of Norway.

New models expand possibilities

Over the years, scientists have proposed a number of hypotheses to explain the annual fluctuations in fish stocks. But these have been untestable hypotheses, mainly because marine physics has not been an integrated component of this research, and because various processes have not been quantified in relation to each other to determine their significance.

"But with models developed in the ECOBE project, we can finally do just that," says project manager Svein Sundby of the Institute of Marine Research (IMR).

There are complex factors involved in recruitment (the number of new fish entering a population in a given year). The primary objective of the ECOBE study is to develop physical-biological modelling for running simulations. The researchers have succeeded in creating an intricate model that quantifies all the major processes within the marine environment. Using powerful computers, they can now test various fish-stock scenarios.

Subject to dramatic impacts

The model explains what happens when a new year-class is establishing itself, from the hatching of fish eggs in the spawning grounds along the Norwegian coastline until making its way to the Barents Sea as fry, five months later. The fish must survive a variety of environmental challenges, including being transported by currents, changes in light and temperature, and turbulence. In addition, the supply of zooplankton, their food source, may vary.

Turbulence is one of the most critical challenges for fish larvae, the researchers on the ECOBE project have discovered. The rapid variation of pressure and velocity of marine turbulence is a constant threat to larvae survival. Turbulence varies so greatly in time and space that it is impossible to measure in practice -- it has to be modelled.

For as long as records have been kept, the abundance of fish stocks in the seas off Norway has fluctuated tremendously from year to year. Great Norwegian marine researchers such as G.O. Sars (1837-1927) and Johan Hjort (1869-1948) believed that a particularly crucial factor must be the availability of plankton, which comprises the food supply of the fish larvae.

Today's researchers agree. But food supply involves more than biology. There are several critical processes, and each process in turn has an impact on the others. A factor such as turbulence, for example, must be considered in the context of temperature, light conditions and nutrients, explains Professor Sundby.

As a professor at IMR in Bergen, he has long been researching recruitment of Norwegian fish stocks. For years he has lamented that hypotheses about recruitment of cod and other fish have been untestable.

"A good hypothesis has to be testable," he insists. "Otherwise it remains nothing more than speculation."