The number of arrests for the smuggling of eels in Europe has increased by 50% after a concerted effort by enforcement agencies to tackle the problem.

Eels are in demand in China and other east Asian countries and about 350m are trafficked out of the European Union each year, in a trade worth about €3bn (£2.7bn) annually. It is the world’s biggest wildlife crime in terms of the number of creatures trafficked. About 15m eels were seized last year and 153 arrests were made, compared with 98 arrests the year before.

Eels are increasingly threatened by overfishing and illegal fishing, as well as from pollution and other water contaminants, including illegal drugs.

The pan-European police agency, Europol, said it was unable to provide further details such as how many arrests resulted in convictions. The majority of arrests were in Spain, France and Portugal.

Eels, once so common they were a staple food for poor people, have declined by about 95% to 99% in recent decades and are classed as endangered. The fish has an unusual lifecycle, needing to travel from rivers in Europe to the Sargasso Sea in the Gulf of Mexico to spawn, and taking as long as two decades to reach sexual maturity.

In the intermediate stages it becomes a “glass eel”. These are often smuggled across borders, finding markets in China in particular, where they are put in farms and then sent to other countries.

Europol has developed technology to identify and track the DNA of such trafficked eels. In Europe, the export and import of European eels has been suspended. However, Europol believes about 350m eels are illegally trafficked each year from the continent, which is about a quarter of the total of juvenile eels calculated to enter European waters each year.