The issue has attracted increasing attention with the rise of the vegan movement.

But those raising ethical concerns about eating fish face arguments about the nutritional benefits, tradition, culture and economics of the ancient practice.

University of Newcastle marine ecologist Vincent Raoult said the ethics of eating fish were complicated.

The question of whether fish feel pain, for example, was the subject of disagreement among researchers.

"That is the focus of a lot of current studies. Most researchers would agree that fish have pain receptors and can feel pain," Dr Raoult said

"The difficult question is whether they experience pain in the same way we do."

Questions on this subject include whether fish pain was "just an autonomous reaction".

"Or does it leave lasting effects on the way the animal feels, even once the pain has gone?" he said.

"This is much harder to show and what leads to disagreements among scientists.

"I think given the complexity of fish intelligence, fish do feel and experience pain."

Dr Raoult believes more ethical standards will emerge around fish consumption, as research into fish feeling and learning "trickles up to management and authorities".

"Given what we now know about fish thinking and pain, this does have implications for industries that work with them," he said.

"If we compare fisheries and aquaculture to livestock, then certainly the death of livestock is much more ethical than fish, which generally are left to asphyxiate."

Dr Raoult said aquaculture conditions were usually better, with "fish euthanised in ice baths or anaesthetic".

"For wild fisheries this is something that's not feasible," he said.

He said there were trials for "electro-trawling in Europe that stuns the fish before they are captured".

"This would potentially be more ethical.

"Obviously the issue here is aligning the needs of cultures and nations to capture fish as food, and the complexity of doing this in an ethical manner."

From a nutritional perspective, the Australian Dietary Guidelines recommend one or two fish meals a week.

"Fish, especially oily fish such as salmon and tuna, can be a valuable source of essential omega-3 fatty acids," the guidelines state.

"Regular consumption of fish may help reduce risk of heart disease, stroke, dementia in older adults and macular degeneration."

The spotlight on fish consumption is increasing with wider concern about animal cruelty.

For many years, some people concerned about animal welfare kept fish in their diet, following the so-called pescetarian diet. Some had believed that fish were less sentient and less able to suffer than other animals.

Laura Weyman-Jones, of People for the Ethical Treatment of Animals, said "scientists agree that fish feel pain and suffer just as we and other animals do".

"People are rightfully questioning the ethics of who they eat, as we become increasingly aware of speciesism".

She said speciesism was the "assumption of human superiority leading to the exploitation of animals".

She said this was not surprising to "anyone who has seen a fish struggling and fighting for their life when pulled from the water".

"Fishers toss the animals they've caught into piles of ice on boat decks, where they slowly asphyxiate, freeze or get crushed to death."

She said these were "horribly cruel and drawn-out ways to die".

"Those who are pulled up from deep waters often suffer from rapid decompression.

"Farmed fish spend their lives in cramped filthy enclosures and commonly suffer from parasite infections, diseases and injuries."

She said there was a "grave environmental cost associated with humans' appetite for fish and seafood".

"Fishing vessels are decimating the world's oceans, leaving them empty, lifeless and on the brink of ecological collapse.

"Massive trawlers with nets the size of football fields destroy coral and marine plants on the seabed and scoop up all life in their path - including dolphins, turtles and seals.

"In addition, there's a huge 80,000-tonne floating patch of garbage in the Pacific Ocean - and 46 per cent of the large items of debris is made up of fishing gear, including nets."

She believes humans don't have to eat fish for health.

"Why support an industry that is not only cruel to animals but also terrible for the environment?"

Fish stocks have been a key issue for environmental groups for years.

Dr Raoult said these concerns were "not always well placed".

"Most fish stocks that are managed with a scientific approach are actually improving on a global scale.

"Australia typically has some of the better managed fisheries."

He said there were reasons to eat fish instead of other animal protein.

"For example, a study recently came out showing that the CO2 [carbon dioxide] produced for wild-caught fish is significantly lower than red meat and even less than chocolate and cheese."

As such, pescetarianism was a "viable option" for those looking to reduce climate change impacts.

When it comes to intelligence, though, Dr Raoult said it had been known for decades that fish "have the ability to learn tasks and solve problems".

Anyone with a fish tank knows that fish "associate the presence of people near their tank with food".

"Captive fish typically move towards the glass when people are present," he said.

"Some fish have shown behaviours that we would consider to only occur in 'smart' animals like our close relatives, the chimpanzees."

For example, there are records of fish using rocks to break shellfish, which is effectively using tools.

"There are also many species of fish that have learned to work with other species for their benefit. Some gropers work in tandem with eels to capture fish, making sure they both get meals."

Dr Raoult observed differences in shark personality when working closely with sharks in an aquarium.

"Some were very docile, some cheeky, but each one was clearly different."

Humans and all other vertebrates "evolved from fish millions of years ago".

"We share a lot of our physiology and DNA with them.

"This means a lot of our hormones like cortisol and sex hormones are shared."

When the byproducts of medicines, like the pill, end up in human urine and wastewater, they can affect wild fish "sometimes causing feminisation of male fish".