For the environmentalist who has done it all, from greening their home to decarbonising their travel, there's a new frontier: greening their pet.

Later this year the UK's 8m cat owners will, for the first time, be able to feed their kitties their favourite fish with a clear conscience.

In a move announced today, Whiskas and Sheba pet foods are to become the first to sell products using Marine Stewardship Council-certified fish, which is caught sustainably and without threatening further dwindling stocks.

While eight out of 10 cats are likely to have no opinion on the provenance of their fish, Mark Johnson of manufacturer Mars Petcare said people were increasingly aware of the importance of sustainability.

"The End of the Line film [a documentary about overfishing] has had a big impact," said Johnson, the company's UK general manager. "We are now the first pet company to make a commitment to sustainable fish, and we hope that will act as a catalyst for the whole industry."

Supermarkets have been quick to respond to rising human demand for sustainable fish, with the Co-operative eliminating threatened species from its own-brand products and Marks & Spencer recently becoming the first high-street name to sign up to WWF's new seafood charter.

But pet lovers have so far been limited to giving their animals human food such as sustainably caught tinned tuna: an expensive way to limit their pets' ecological impact.

While cats and dogs may seem unlikely environmental villains, UK pet owners buy 1.5m tonnes of food a year and globally there are an estimated 750m pets who consume 20m tonnes annually.

The authors of a recent book, Time to Eat the Dog?, warned that the energy required to feed a cat is the same as that required to build and drive a Volkswagen Golf for 6,000 miles a year.

Robert Vale, one of the authors, has said that poultry and rabbits have a lower impact than red meat and fish when used as pet food. "When feeding a pet… the advice is to think feathers and long ears, not horns and fins," he said.

Under the Mars Petcare plan, MSC-certified fish will be available in Whiskas and Sheba brands to all European consumers by Christmas, with certification eventually coming to all the company's fish products in Europe. It pledged to source all its fish globally from sustainable sources by 2020.

The MSC badge is awarded through a voluntary process whereby fisheries approach the council to be assessed under 23 different criteria, followed by an annual audit.

The process can cost $150,000 (£100,000); Mars has said it will absorb the cost of buying the more expensive fish.

Conservation groups WWF and the Marine Conservation Society welcomed the news. Jason Clay of WWF US said: "There is no quick fix to this problem but when companies as influential as Mars take a leadership role, it is great news for the world's oceans."

However, experts also argue that pet food companies need to reduce the prime fish in their products, which is included as whole fillets in some of Mars Petcare's products but will be phased out under the new initiative.

Callum Roberts, professor of marine conservation at University of York and the author of Unnatural History of the Sea, said: "Clearly it's more sustainable using certified products than uncertified ones, but what makes me uncomfortable is we are feeding so much fish protein to pets when there isn't enough fish in the world to give everyone a healthy amount of fish in their diet.

"I say this as a cat owner: Pets are definitely second rate when it comes to eating fish, and should be largely fed by the trimmings market."