DNA tests could be used to check up on fish populations in the sea Denmark scientists think.

DNA samples taken from underwater can help researchers monitor how many fish are there and what types are present without having to physically catch them. The technique involves taking water from 1km below the surface of the water and then sequencing the fragments of DNA.

Dr Philip Francis Thomsen, based at the University of Copenhagen, said the environmental DNA (eDNA) method could give information about a large number of species including rays, sharks and flatfish.

He told BBC News: “We are basically doing equivalent to CSI [crime scene investigation] work for a biologist. Investigating the biodiversity of the ocean by using environmental DNA as a proxy for what is actually living there.”

Samples from water can identify species

The reason the technique is effective is that fish leave tiny bits of their DNA in the water in which they live. This cannot be seen but can be extracted from samples of water and then sequenced to gain valuable details about fish populations.

Scientists took samples of seawater off Greenland as part of a study, published in Plos One, and found that 26 out of the 28 species of fish caught there had left traces of their DNA in the water. And another three types of fish, including the rare angler fish, were identified simply from their DNA.

The researchers were then able to use the information gathered from the genetic material to create maps of where certain fish species were found.

Dr Thomsen said: “If we can fine tune this method, we might be able to use this as a supplement to estimating stock sizes and potentially to more sustainable fishing.”

DNA testing is more commonly associated with identifying biological relationships between people. Paternity tests can establish the biological father of a child while there are also tests to identify other family relationships.