When consumers buy butterfish or white tuna at a grocery store they may instead receive a fish dubbed "the laxative of the sea," according to an investigation into seafood fraud that found nearly half of seafood samples it tested at Canadian grocery stores and restaurants were wrongly labelled.

"The results show widespread mislabelling," said Julia Levin, seafood fraud campaigner for advocacy group Oceana Canada, which conducted the study.

It collected 382 samples of snapper, sea bass, sole and other fish that other studies indicate are often substituted. They chose samples from 177 retailers and restaurants in five Canadian cities.

Scientists at Tru-ID, a Guelph, Ont.-based lab, used DNA barcoding to determine the species of fish. That was compared to the Canadian Food Inspection Agency's Fish List, which contains acceptable market names for various fish species.

They found 44 per cent of the fish were not what the label claimed -- 52 per cent of the time in restaurants and 22 per cent of the time at retailers, including grocery stores and markets.

Snapper, yellowtail and butterfish were mislabelled 100 per cent of the time, according to the study. Half of the sea bass samples were wrongly identified, while more than 30 per cent of cod, halibut, tuna and sole samples were mislabelled.

Such practices can harm consumer health and wallets, as well as hurt the environment, the report claims.

Most often the fish turned out to be escolar, tilapia or Japanese amberjack.

The CFIA, which is responsible for mitigating food safety risks and monitors food fraud in the country, says it is in the process of modernizing the way food is labelled in Canada.

"The Safe Food for Canadian Regulations will improve traceability requirements throughout the supply chain, including for seafood products," spokesman Brian Naud wrote in an email.

He said food fraud occurs around the world and is mostly driven by economic gain. Canadian food laws make it illegal to misrepresent a food.

The CFIA found that 13 per cent of the 304 fish samples tested for species authenticity since April, 2014, were unsatisfactory.

A recent publication by the University of Guelph's Biodiversity Institute of Ontario and the CFIA showed mislabelling occurred in 14.8 per cent of samples tested between 2013 and 2016 using DNA barcoding, Naud added.

Previous studies based on retail sampling found mislabelling was greater than CFIA results but less than identified by Oceana.

Seafood is susceptible to food fraud because of a complicated global supply chain that has opportunities for mislabelling at many stages from the fishing boat to the restaurant or store.

While some mislabelling happens accidentally, Levin said, the majority appears to be deliberate. She stressed the restaurants or stores where the samples were collected are not necessarily the ones responsible for misguiding consumers and may instead be victims themselves.

"Economic profit is the primary driver," she said, noting the pattern is for more expensive fish, like red snapper, to be replaced with a cheaper alternative, like tilapia.

Industry insiders often try to convince Robert Hanner, an associate professor at the University of Guelph whose lab tested the samples, that the problem amounts to no more than random mix ups: a confused employee laying out fish under an incorrect label.

"If it were purely random, you would expect that once in a while you'd get the good stuff when your'e paying for the cheap stuff," said Hanner, whose lab demonstrated the first use of DNA barcoding to show seafood fraud in Canada about a decade ago.

"There's no evidence that that ever happens."

This means shoppers pay higher prices for lower value fish, and may unknowingly consume harmful products, like escolar that can cause diarrhea, vomiting and other stomach problems. People living with allergies are especially vulnerable.

People may also mislabel seafood to mask illegally caught fish, Levin said. When this happens, it hampers efforts to curb overfishing and protect at-risk areas, among other things, according to the report, which adds illegal fishing is often linked to troubling practices like modern slavery and child labour.

Restaurants Canada's James Rilett said he was surprised to see Oceana Canada found 52 per cent mislabelling in its restaurant samples when the CFIA's figures are so much lower.

Still, "any level of mislabelling is concerning," said the not-for-profit industry association's vice-president for central Canada.

Restaurants Canada works with partners like Ocean Wise -- a program that lends its symbol to what it deems sustainable seafood choices -- to educate its members on how to avoid mislabelling, he said.

Illegal, unreported and unregulated fishing is a global problem that hurts everyone, said Paul Lansbergen, president of the Fisheries Council of Canada, a non-profit trade association that calls itself "the voice of Canada's fish and seafood industry."

However, this type of report into seafood fraud is not new, he wrote in an email.

"I find it unfortunate that Oceana Canada continues to exaggerate what is a rare occurrence in the overall market," Lansbergen said, adding the study and others like it "are designed to arrive at a pre-determined outcome."

Oceana Canada wants the federal government to increase labelling requirements to match those in the European Union. In the EU, labels must show the fish's scientific species name, catch method, and origin -- among other information. It also requires catch documentation. Studies show that seafood fraud rates appear to have fallen since the union implemented the more stringent labelling practices.

"We need Canada to implement a traceability system to keep everyone accountable."