How to count all the fish in the sea

Scientists in Denmark are developing a new technique to work out how many fish are in a certain area of the sea, without having to throw down a net.

The new method, described in the "PLOS ONE" science journal, involves testing water samples for the genetic material of different types of fish.

"Currently, monitoring of marine fish is largely based on invasive procedures, such as trawler fishing, and on official reports of global hauls, which can be unreliable," says Philip Francis Thomsen of the University of Copenhagen.

Improving upon these monitoring methods could be important for scientists assessing the impact of climate change, as well as for fishing fleets that need to know where to look for good stocks of fish.

Fish DNA in the water comes from their scales and excrement. Researchers say that the number of these traces found in the water gives a reliable indicator of not only which kinds of fish are present, but also how many.

Putting it to the test

The first round of tests involved taking 21 water samples at depths of around 190 meters (623 feet), just off the west coast of Greenland. The DNA analysis was compared with the haul from fishing trawlers that were active in the same area directly after the samples were taken.

Sea creatures from 26 biological groups, such as ray and halibut, were identified in both the DNA analysis and the fishing nets.

There were, however, some discrepancies in the two sets of results. The researchers say this could be because fish lose more or fewer scales depending on their age and species. Alternatively, the genetic material could have been moved to a different area by ocean currents.

Certain species of fish, such as the Greenland shark, are also able to escape fishing nets.

But the researchers remain confident that such problems can be overcome by improving their current methods.

rs (dpa)