For the residents of Grassy Narrows who have fished Clay Lake and the river downstream for generations, walleye is a dietary staple.

Now a comprehensive analysis of provincial data conducted for the Star confirms what has long been suspected: the walleye they are eating are the most mercury-contaminated in the province.

“It’s overwhelming for me,” said Ryan Kokokopenace, 36, when told of the Star’s finding. Kokokopenace and his family fish for walleye in the Wabigoon River, which is connected to Clay Lake. “It’s been our way of life for so long. I’ve been doing it since I was 3.”

The mercury in an average meal of walleye from Clay Lake is 15 times the daily tolerable intake limit for adults, and about 40 times the limit for women of child-bearing age, pregnant women and children.

Using the stricter guidelines recommended by the U.S. Environmental Protection Agency, that same meal would be about 90 times the daily limit for women of child-bearing age, pregnant women and children.

As another point of comparison, the average walleye mercury concentration in Clay Lake is 13 times that found at one point in Lake Ontario, where walleye meet the acceptable daily mercury consumption guideline.

Clay Lake and two other sites along the Wabigoon River with top mercury contamination readings are within 80 kilometres downstream from the site of the former paper mill in Dryden, Ont., that in the 1960s dumped 10 tonnes of mercury into the river.

At one of those spots along the Wabigoon called Ed Wilson Landing, 10 kilometres from Grassy Narrows, walleye had the second highest mercury concentration for that species in Ontario — about 40 per cent greater than the third highest concentration and also well over recommended daily consumption levels.

“We have a contamination problem in this site that’s unequivocal,” said Western University environmental science professor Brian Branfireun, a mercury expert, who reviewed the results of the fish data analysis performed by his colleague, Heidi Swanson of the University of Waterloo.

“It’s clear that there’s something different happening (on the Wabigoon River) than elsewhere in the province,” added Branfireun, Canada Research Chair in environment and sustainability.

An ongoing Star investigation — which began in June with a report about a retired labourer who said he was part of a small crew that “haphazardly” buried drums filled with salt and mercury in the early 1970s — has renewed concerns from residents and scientists that nothing has been done to clean up the still-contaminated area.

Physical symptoms of mercury poisoning include loss of muscle co-ordination, slurred speech and tunnel vision. Recent scientific research on the effects of the neurotoxin show the poisoning occurs at low levels previously thought harmless, and that fetuses are particularly vulnerable to cognitive damage.

The provincial data set includes 25,000 fish samples taken from hundreds of lakes and rivers between 2011 and 2016. Collected for the Guide to Eating Ontario Fish, the data is used to advise the public about consuming fish from the lakes and rivers of Ontario.

Current advisories issued by the province generally say walleye from Clay Lake should not be eaten. But the underlying data — which shows just how contaminated the fish are — has not been public. It took the Star two months and multiple requests to obtain the data from the province.

The Star asked Swanson, who specializes in mercury levels in fish, to analyze the data set. Swanson, who holds Waterloo’s University Research Chair in biology, sorted the data so that lakes across the province could be compared to other lakes, rivers to rivers and fish species to the same fish species of the same size.

This “size standardization” prevents possible individual high values (such as very large, old fish with off-the-charts mercury readings) from skewing the data. Swanson only considered sites where there was enough data available. The end result is an “apples to apples” comparison, said Branfireun.

People from Grassy Narrows have told the Star they still eat walleye from the lake and particularly from the river farther downstream, in part because they do not have enough money to buy food off store shelves or for boats to fish farther away, and also because living off the land is a key part of their culture. Before the 1970s, many from Grassy Narrows worked as commercial fishers and as fishing guides for popular tourist camps that later shut down because of the mercury contamination.

Environment Ministry spokesperson Gary Wheeler told the Star that government officials are working with the people of Grassy Narrows to “further evaluate the current situation” in the river system and to provide information to the community “on safe consumption of fish caught from lakes on their traditional lands.” This includes putting up signs and posters, Wheeler said.

Fish from other lakes and rivers in Ontario were found to have elevated mercury levels, but nowhere near as high, and the risk to people near many of those other sites is low because they do not regularly eat the fish. Many factors can explain the elevated levels, such as the impact of coal-fired generating plants and other industries around the globe that emit mercury, which is then deposited in rain and snow. Climate, geology, water chemistry and other factors can also make some water bodies more sensitive to mercury than others.

What makes points in the Wabigoon River system “a standout,” explained Branfireun, is that its rivers and lakes have clay-rich bottoms that can help trap mercury. Other rivers and lakes adjacent to Grassy Narrows have far lower concentrations.

“It’s not chance, and it’s not some natural phenomenon, because otherwise the lakes around it would be equally high,” said Branfireun.

An unexpected finding from Swanson’s data analysis is that there were high mercury contamination levels at points in the Wabigoon system across a range of fish species.

Some highlights of the research:

Clay Lake has some of the highest size-standardized mercury concentrations for lake whitefish, northern pike and white sucker in the province. (Whitefish, however, were below consumption guidelines in other parts of the Wabigoon system, including nearby Grassy Narrows Lake and Ball South Lake).

• Ed Wilson Landing has the highest mercury concentration in the province for cisco (lake herring), which is above consumption guidelines.

• Wabigoon River Steel Bridge — which is 13 kilometres away from Grassy Narrows — has the second highest concentration for white sucker, which is also above consumption guidelines.

The fact that white sucker, a fish near the bottom of the food chain, has such high levels of mercury is indicative of a source of mercury in the sediment, said Branfireun.

For mercury to really build up in fish, the fish need to eat other fish. That’s why walleye and pike (species at the top of the food chain) are most likely to have higher mercury levels, he explained.

“You would not expect a white sucker to have that concentration of mercury in its tissue if it were not exposed to quite a high source in its diet,” said Branfireun of the bottom-feeding fish that eats plants, snails and insects in the sediment.

Mercury levels in fish increase with both size and age. The data shows that fish caught in Clay Lake and along the Wabigoon River are, however, of average size. For example, the walleye in Clay Lake were in the 50th to 75th percentile for length and in the 99th percentile for mercury concentration.

As recently as May, the province said there was no evidence that the Wabigoon River system near Grassy Narrows needed to be cleaned up.

Since then:

• A group of environmental scientists released a report that says high mercury levels in fish and sediment near Grassy Narrows suggest a source of mercury is still leaking into the system. The scientists also put forward a plan to clean the river system.

• A retired mill worker came forward out of “guilt” to say that more than 40 years ago, he was part of a small crew that “haphazardly” dumped drums of mercury and salt into a pit behind the old mill in Dryden.

• Another report by leading mercury expert Donna Mergler found that the level of mercury in the umbilical cords of babies tested in Grassy Narrows and nearby Whitedog First Nation between 1978 and 1992 was high enough to affect brain development. (Health Canada won’t give leaders of the affected communities the names of at least 300 residents at risk.)

• Japanese experts found that 90 per cent of Grassy Narrows and Whitedog First Nation residents tested in 2014 had a symptom of mercury poisoning, including the younger generations.

During a June trip to Grassy Narrows, Ontario Environment Minister Glen Murray and Indigenous Relations Minister David Zimmer promised funding for further testing of the river system. In the legislature last week, NDP environment critic Peter Tabuns blasted the government for taking too long to release the funds. As a result, he said, scientists have lost time in the field.

Asked Tabuns: “How much longer will this government stall and dissemble while Grassy Narrows families suffer?”