Illegal fishing is depleting the seas and robbing poor nations in Africa and Asia of resources, but a lack of global cooperation is undermining efforts to track rogue vessels, according to an environmental group.

The Pew Environment Group, a Washington-based think-tank, has found that a UN scheme to oblige ports to crack down on illegal fishing boats is handicapped by a lack of accurate information, implementation and participation.

In the five years from 2004, of 176 vessels blacklisted by regional fishing authorities, only 55 turned up on port records, Pew said in a report it presented to the UN Food and Agriculture Organisation in Rome.

In some cases, ports were not checking ships' identity, using the unique vessel number on their hulls.

In others, ships had found ways of avoiding detection, such as changing their names, sometimes doing so mid-voyage before entering a region where enforcement was stricter.

Blacklisted vessels are, in theory, banned from landing fish at ports in the regions signed up to the scheme.