By Carolyn Fry
in Vigo, Spain
A scientific model of the North Sea's ecosystem suggests the total stock of fish has dropped from 26 million tonnes to 10 million in just over a century.
The model should aid future management of the North Sea
Some fish, such as the bluefin tuna, have disappeared completely following intensive fishing in the 1960s.
Others, including cod, haddock and mackerel, have declined considerably, while seals appear to be on the rise.
The model's details were revealed at the annual meeting of the International Council for the Exploration of the Sea.
The researchers hope their work will help in forecasting the future impact of fishing and climate change on species within the North Sea.
"We're trying to find out who eats whom and how much, and how the ecosystem has changed since 1880," explained ecologist Steven Mackinson, at the Ices conference here in Vigo, Spain.
"Fisheries and the Ices are calling for ecosystem-wide management and this is a response to that," he told BBC News Online.
Mackinson and Georgi Daskalov, both senior scientific officers at the Centre for Environment, Fisheries and Aquaculture Science (Cefas) in Lowestoft, UK, began by gathering information on the biomasses of organisms within the ecosystem for the year 1991.
They chose this date primarily because a lot of detailed marine information was gathered that year.
Sources included the International Bottom Trawl Survey, which has gathered data on fish for over 30 years; and the Year of the Stomach, a regular Europe-wide survey of fish stomach contents.
Cod stocks in the North Sea are said to be in crisis
"There have been other models of the North Sea made in the past, but we've not been entirely happy with them," said Mackinson.
"We now have better quality data and skills, so we're trying to build a model which is highly detailed and can be defended scientifically."
The next step was to define variables for the model. Major fish species such as haddock, mackerel and lemon sole are listed individually, while others are grouped into types exhibiting similar behaviour, such as predators living close to the seabed.
As some juveniles have distinctly different eating habits from the adults of their species, they are considered as separate, but linked, groups.
Non-fish groups include whales, dolphins, seabirds, squid, echinoderms, benthic crustaceans and small worms. "We're trying to capture what happens from the very bottom of the food chain to the very top and look at the dynamics throughout the ecosystem," said Mackinson.
The scientists also added figures for the amount of dead material such as discarded fish and other detritus that routinely sinks to the seabed. Including a "dead" variable is important as it is a vital link in the food chain.
"Lots of organisms, particularly tiny bacteria, eat and take nutrients from detritus and in doing so they make it available to the higher organisms who eat them," said Mackinson.
"There's a recycling of nutrients through the system. Traditionally people have believed that a lot of energy within the ecosystem is dependent on primary production of phytoplankton but we believe recycling has a strong input."
Comparing the 1991 data with information gathered for the year 1800, prior to the age of steam and mechanisation in the fishing industry, showed up clear reductions in the numbers of cod, haddock, saithe, cetaceans and seabirds.
Seal numbers appeared to have risen but this could simply be because there is more data available now.
The scientists aim to use information on North Sea stocks in 2004 to test the model's ability to accurately forecast change under different conditions.
Then they will ask various "what if?" questions to predict how the ecosystem may react to changes in fishing and the climate in the future.
"There used to be two stocks of Atlantic bluefin tuna in the North Sea but during the 1960s, fisheries caught 70,000 fish a year," explained Mackinson.
"They seem not to be present now but one was recently caught off Scarborough, which may indicate we'll see bluefin tuna in the North Sea in future."
Cefas scientists will continue to refine the model
Fellow ecosystem modeller Henrik Sparholt, a fishing assistant at the Ices Secretariat in Copenhagen, believes such work will yield valuable results, provided people are aware of the limitations.
"This kind of modelling is a bit controversial in scientific spheres because a lot of the elements and links between them are not well understood," he said.
"But it is an important step forward in our progress towards understanding the ecosystem. Ultimately, working out how the North Sea functions will be a job for a whole group of scientists over a long period of time."
Images are Crown copyright and reproduced with the permission of Cefas, Lowestoft