In many ways, last week’s scandal was unsurprising. The Age reported that Melbourne fish-and-chip chain Hunky Dory was substituting Vietnamese catfish, or basa, for locally caught John Dory. Hunky Dory’s owner Greg Robotis denies the allegations, but did suggest the chain store sometimes labelled basa as “H-Dory”.

But the real scandal isn’t Hunky Dory's alleged conduct; it’s that those actions would be perfectly legal.

Next month, new rules come into effect that require mandatory country-of-origin labelling for all packaged and fresh goods sold as retail – and those rules are already in place for fish. Even with these laws in existence, it’s still hard to identify where your fish really comes from, and labels such as “line caught” can be deceptive.

But if you’re selling that same fish at a restaurant, cafe or fish-and-chip shop, you’re under absolutely no legal obligation to tell your customers the origin of that fish or even identify it by its correct name. To be sure you’re getting what you paid for, Food Standards Australia and New Zealand suggests that you should just “find a reputable fishmonger/restaurant".

Frankly, it’s weird. Last year, Senator Nick Xenophon introduced a simple amendment to the legislation that would require restaurants and cafes to correctly label their fish in exactly the same way that applies to retail. Despite support from the Greens and the crossbench for this eminently sensible measure, both Labor and the Coalition voted it down. As usual, there was talk of red tape and added costs to business – despite the fact that most reputable venues have already adopted the practice of labelling.

Obviously, it’s an issue because diners have a right to get what they order. But there’s a bigger issue at stake: having any seafood left to eat in the long to medium term.

Fish is in a terrible state. We’ve gone from catching around 17 million tonnes in 1950 to around 79 million tons in 2005, quadrupling what we take from the oceans. We’re catching fish faster than they can breed, and because of that, stocks are declining by around 450,000 tonnes a year. More than 85 per cent of fish species have been exploited, over-exploited, depleted or are recovering from depletion. Often, fish farming isn’t much better, with deleterious run-off polluting adjoining waterways, and the high yields offered by intensive farming practices can create monocultures highly susceptible to disease, which is reflexively treated by antibiotics (which then adds to the terrifying new problems caused by the growth of antibiotic-resistant bugs).

The horrible irony is that seafood is appearing on more menus than ever – and, for the most part, we expect to eat it cheaply. It’s little wonder businesses may be tempted to substitute imported varieties such as catfish for $5 a kilo when local John Dory rings in at around $50. We’re straight-up demanding it.

With increasingly strict regulations around Australian food production, some operators are, naturally, looking for a cheaper option. Imported fish such as basa provide just that. The Seafood Importers Association of Australasia says basa are fast-growing fish from the fresh waters of the Mekong River, fed by Himalayan snowmelt, and are imported into Australia under strict quarantine conditions (it also mentions that basa should not be sold under the name “Dory”). And, certainly, the Vietnamese aquaculture industry is rapidly improving, working with organisations such as the World Wide Fund for Nature, which is accrediting farmers to certify they’re being ecologically responsible.

But compared with the Australian fishing industry – which is by no means perfect – consumers buying imported seafood have little to no information about the conditions in which their dinner was raised, slaughtered, packed, stored or transported. It certainly could be the case that the catfish served at Hunky Dory was, in fact, raised in WWF-approved, ecologically responsible fish farms in a vibrant new industry for Vietnam.

But the problem is that it’s so difficult for us, as diners, to figure out where our fish really comes from, how it’s been caught, and whether the workers who caught and produced it were paid an adequate wage. Plenty of reports suggest there’s little regulation of the Vietnamese industry, and sometimes the fish are literally swimming in shit. A report published in 2015 found little enforcement of antibiotic use in domestic aquaculture, and a general lack of knowledge about antibiotic use by Vietnamese farmers. Mercury levels in fish such as basa are so high that Food Standards ANZ guidelines still state that pregnant women, those planning pregnancy, and kids under six shouldn’t eat it more than once a week.

The marginally good news is that Australia’s fisheries are some of the best managed in the world, and there’s evidence that criminally over-caught species such as southern bluefin tuna might be starting to recover. But when operators bring in poor-quality, cheaply available products from overseas, they set an expectation that fish should be cheap. That makes it much harder for restaurateurs committed to buying sustainable (and safe) seafood to justify to customers why they should pay what it’s really worth. That anything should be available for five bucks a kilo should immediately ring alarm bells – to get things down to that price, corners have been cut, whether environmentally or socially.

As Dan Barber forcefully points out in his brilliant book The Third Plate, our chefs and restaurateurs are on the frontline of ethical eating. They have a direct line to producers, and a growing power to change the way we eat. It’s their obligation to educate their diners on what’s truly the best in terms of quality and sustainability. Barber believes that chefs, in collaboration with farmers, will be the people who solve the problem of feeding a growing population – and serve it to their customers.

“Just as chefs will tell you a chicken gizzard has more flavour than a breast, a very fresh sardine is more flavourful than a slab of tuna. So why do sardines sell about as well as chicken gizzards on most menus?” Barber asks. “Blame chefs, because after all, [they] are the supposed arbiters of taste. Identifying the best ingredients is an essential part of the cooking craft. Of all the complicated problems confronting the oceans, overfishing is not only the most immediate threat, but also perhaps the easiest one to remedy. Inasmuch as the industry story of the depleted oceans is about greed, the chef’s story of the sea is about a lack of imagination.”

With that in mind, restaurateurs might not be legally obligated to feed diners sustainable, ethically sourced produce, or even to tell people when they’re not doing so. And certainly, laws could be changed to make everyone accountable for what they’re serving. But whether it’s a legal obligation or a moral obligation, our restaurants have a great responsibility.

For more information on sustainable seafood, visit the Australian Marine Conservation Society’s website or download its free Sustainable Seafood Guide via the app store.