The "hacktivist" group Anonymous recently took down many Icelandic government websites, in protest at the country's practice of whaling.

"Whales do not have a voice. We will be a voice for them. It's time to speak out about this impending extinction of a species. It's time to let Iceland know we will not stand by and watch as they drive this animal to extinction," Anonymous said.

Iceland is not the only country that still practices whaling: Norway and Japan also do so, as do a few smaller populations.

This often baffles and horrifies people from elsewhere. If so many people are opposed to it, why are countries still whaling?

In 1986, in response to declining populations and widespread outrage at perceived inhumane practices, the International Whaling Commission (IWC) issued a global moratorium on commercial whaling.

Iceland signed up to the moratorium, but it did so "under reservation". This means it is not actually illegal for Iceland to continue whaling, so long as it abides by certain rules.

Iceland hunts two species: minke whales and fin whales.

Whales do not eat all the fish in the seas around Iceland

In 2015, Iceland's whalers were allowed to hunt 154 fin whales and 229 minke whales, a quota set by the Ministry of Fisheries and Agriculture.

A report published 30 September on the Icelandic National Broadcasting Service says that they hunted their full quota of fin whales this year, but only 29 minke whales.

Whaling is very visible if you visit the country. In early 2015 I went filming in Iceland, and in a restaurant my colleague and I noticed that whale meat was readily available. We remarked on it to our Icelandic contact.

He offered two justifications. First, there are too many whales in the ocean and that they eat all the fish. Second, Icelanders do not even tend to eat whale meat: it is the tourists that do that.

That first claim is arguable. "Contrary to popular belief, whales do not eat all the fish in the seas around Iceland," says Whale and Dolphin Conservation (WDC), citing a 2004 report. In fact minke and fin whales both eat varied diets, including plankton and krill as well as small fish, according to NOAA's National Marine Fisheries Service in the USA.

However, our contact's second point is largely true. According to the WDC, only 1.7% of Icelanders eat whale meat. The population of Iceland is small, so that is only about 5,600 people. In contrast, 35-40% of visitors to Iceland eat whale meat, although even this is now falling according to the International Fund for Animal Welfare.

That still leaves a lot of leftover whale meat. Oddly, hardly any of the fin whales caught in Iceland are actually eaten there. Instead their meat is exported to Japan where the demand for whale meat is somewhat larger.

Fin whales are an endangered species

Iceland's Ministry of Fisheries says that its whaling practices are "sustainable and legal and in accordance with the rules of the IWC". They claim that fin whales are abundant, and that Iceland is an "advocate of international cooperation in ensuring sustainable use of living marine resources".

The best available data on whale populations suggests that they are largely correct.

Minke whales are found in all of the world's oceans. The International Union for Conservation of Nature (IUCN) lists them as "least concern", meaning they are not in any imminent danger of extinction. The IUCN estimates that there are over 100,000 minke whales in the wild, so it should be possible to continue hunting them without endangering the species.

However, fin whales are an endangered species, and the IUCN says commercial whaling in the 19th and 20th centuries was the main cause. But even in this case, it is not clear that Iceland's whaling practices are a problem.

While the IUCN assesses whether a species is threatened, it assesses its entire global population. But within each species, there may be some local populations that are critically endangered and others that are very healthy.

"When you talk about whaling, you talk about whaling a particular population, not a species," says Kate Wilson of the IWC.

The North Atlantic fin whale population is considered healthy. It is populations elsewhere that are low and that have led to the species being classed as endangered.

For that reason, hunting fin whales around Iceland is not a conservation threat, says Geneviève Desportes of the North Atlantic Marine Mammal Commission. "It has no consequences, it's sustainable in the longer term."

The only reason not to allow it is for emotional reasons or cultural reasons

Whaling is a long tradition in Iceland and brings small communities income, says Desportes. "Our organisation thinks that you can use marine resources, as long as you do it in a sustainable and responsible way."

A 2008 study questioned whether Iceland's practices are quite sustainable. A team of scientists reported "a significant decrease" in the number of minke whales near Iceland's shores since their last survey.

However, accurate population estimates are difficult to obtain. "These creatures spend 80% of their time under the water, often in remote, inaccessible parts of the world, so the science to get accurate abundance figures is time-consuming and expensive," says Wilson.

Regardless, Iceland takes the view – along with the other whaling nations – that the practice can be sustainable, at least for certain populations.

"Therefore, reasons not to allow it include welfare, emotional and cultural reasons," says Wilson. That might include a reluctance to cause the whales to suffer - it is not clear whether it is possible to humanely kill a large animal in open water - or distaste for the killing of such intelligent animals.

Not all cultures share these concerns. "Some don't agree there are reasons they shouldn't be whaling," she says.

Its aim is to hunt 333 minke whales

The Japanese government has a similarly pro-whaling attitude. The country has long continued whaling, on the grounds that it does so to further scientific research – which the IWC allows.

This week, a Japanese vessel set off for Antarctica. Its stated aim is to hunt 333 minke whales, to find out how many are now living around Antarctica.

However, Japan's scientific whaling program has been widely criticised on the grounds that it produces hardly any research. Most of the whale meat ends up in restaurants.

What's more, a 2013 report concluded that the industry was not even profitable, and had to be subsidised by the Japanese government.

Melissa Hogenboom is BBC Earth's feature writer, she is @melissasuzanneh on twitter.

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