A new study has found that 42 countries or territories around the world permit the harvest of marine turtles - and estimates that more than 42,000 turtles are caught each year by these fisheries.

The research, carried out by Blue Ventures Conservation and staff at the University of Exeter's Centre for Ecology and Conservation, is the first to comprehensively review the number of turtles currently taken within the law and assess how this compares to other global threats to the creatures.

All seven marine turtle species are currently listed on the IUCN Red List of Threatened Species.

Frances Humber of Blue Ventures and a PhD student at the University of Exeter, who led the research, said: "This is the first study to comprehensively review the legal take of turtles in recent years, and allows us to assess the relative fisheries threats to this group of species. Despite increased national and international protection of marine turtles, direct legal take remains a major source of mortality. However, it is likely that a fraction of current marine turtle mortality take is legal, with greater threats from illegal fisheries and bycatch."

The first marine turtle harvest legislation was instigated in Bermuda in 1620 to protect "so excellent a fishe" and prohibited taking any turtle "under eighteen inches in the breadth or diameter".

But large scale commercial taking of turtles continued all over the world for centuries, with global capture peaking at over 17,000 tonnes in the late 1960s. For example, during the peak of Mexico's sea turtle exploitation in 1968 it is estimated that the national take was over 380,000 turtles.

Increased conservation awareness at an international scale has led to greater protection of marine turtles, with 178 countries now signed up to the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) restricting the international trade of turtle products.

The direct take of turtles has continued legally in many regions and countries, often for traditional coastal communities to support themselves or small-scale fisheries supplying local markets with meat, and sometimes shell. The fisheries are an important source of finance, protein and cultural identity, but information can be scarce on their status - despite often being listed as one of the major threats to turtle populations.

The researchers collated data for all seven species of marine turtles from over 500 publications and 150 in-country experts.

They estimate that currently more than 42,000 marine turtles are caught each year legally, of which over 80% are green turtles. Legal fisheries are concentrated in the wider Caribbean region, including several of the UKs Overseas Territories, and the Indo-Pacific region, with Papua New Guinea, Nicaragua and Australia together accounting for almost three quarters of the total.

The data indicates that since the 1980s more than 2 million turtles have been caught, although current levels are less than 60% of those in the 1980s.

Bycatch - the unwanted fish and other marine creatures trapped by commercial fishing nets during fishing for a different species - is thought to be a far higher cause of death for marine turtles, likely running into hundreds of thousands each year.

Illegal fishing also continues to be a major cause of mortality, with the researchers estimating a minimum of 65,000 turtles taken from Mexico alone since the year 2000. The scale of global illegal capture is likely to be severely underreported due to the difficulties collecting information on such an activity.

Dr Annette Broderick, of the Centre for Ecology and Conservation at the University of Exeter's Penryn Campus in Cornwall, added: "We were surprised to find that there are 42 countries with no legislation in place that prohibits the harvest of marine turtles, although for many of these countries these harvests provide important sources of protein or income. It is however important to ensure that these fisheries are operating at a sustainable level."

The article, 'So Excellent a Fishe: A global overview of legal marine turtle fisheries' is published in the latest edition of the journal Diversity and Distributions.