N.S. wild brook trout hold firm against millions of hatchery releases: study

Canadian scientists are "pleasantly surprised" by the results of a new study that found wild Nova Scotia brook trout have not interbred with hatchery-raised trout — even though millions have been released into the wild over decades.

The first genetic analysis of native and hatchery brook trout in Nova Scotia sampled 1,500 wild brook trout in 12 river systems along with 200 hatchery-raised trout between 2016 and 2018.

The study revealed the wild and hatchery fish are genetically distinct and there has been little to no mixing in the province.

The study is part of research to determine the impact of environmental factors like climate change on brook trout at a genetic level throughout their eastern North America range.

Why the findings matter

The findings suggest that stocking — underway in Nova Scotia since the early 1900s — has fulfilled its main purpose to supply fish for angling without damaging the native population.

"The genes that make a good hatchery fish in a hatchery are genes that make you good at living in a tank but not as good at living in a river," said lead author Sarah Lehnert, a research scientist with the Department of Fisheries and Oceans in St. John's.

"So, if those genes are introduced into the wild they could be bad for the wild population and make them less able to survive, potentially, in that river environment."

Wild brook trout in Nova Scotia are dwindling because of pressures that include warming water temperatures, increasing acidity and invasive species.

The study reported less than five per cent of mixing, or transfer of genetic information, from the fish from the hatchery to the wild stock.

"It's good for the genetics of the wild populations because the integrity of those populations isn't being impacted, potentially, by these bad hatchery genes," Lehnert said.

2 million trout released annually

Provincial records show 23 million brook trout raised in hatcheries have been released into the wild since 1976.

It's believed the hatchery stock originally came from New Brunswick and Rhode Island.

The inland fisheries division of the Nova Scotia Department of Fisheries and Aquaculture now releases two million brook trout a year into lakes and rivers from hatcheries located in Margaree, Cape Breton, McGowan Lake, Queens County and Frasers Mills, Antigonish County.

The program costs about $1 million a year.

Large fish are released in the spring for anglers. In the fall, fingerlings are released that are meant to survive.

Why hatchery fish don't make it in the wild

The study indicated fish raised in hatcheries are less averse to risk than the wild population and the hatchery strains may be not as well adapted to Nova Scotia conditions.

In any case, if they're not caught right away they are not surviving, or if they do survive they're not producing offspring to the next generation, said Lehnert.

The results came as a relief to John MacMillan, a provincial government biologist with the Inland Fisheries division, which helped fund the study.

"When you're undertaking research on the strategies that we use to try to improve fisheries across the province you never know what the results are going to tell you," he said. "So we're quite pleased with the findings.

"It showed that our stocking, although it really works well to provide fish to anglers and satisfy that need, it hasn't resulted in transferring genetic material into the wild populations that we have."

Findings raise questions

The findings raise questions for the provincial government.

They confirm the main hatchery is producing trout that do not do fare well in the wild.

If brook trout get to the point where they need rescuing, wild fish would likely have to be used.

The one place scientists found intermixing was in Lake O'Law Brook in Cape Breton.

It is fed from the hatchery at Margaree, where eggs from native wild trout are used to raise fish they release.

Shauna Baillie, a science advisor with a Department of Fisheries and Oceans biotechnology and genomics group in Ottawa, is also a co-author of the study.

For her, the study established that Nova Scotia has a genetically healthy native population which can be used to study impacts like acidification and temperature on genetic makeup.

Brook trout samples from other Canadian provinces and the United States will be sent to Dalhousie University in Halifax for analysis by geneticists as part of this research.

"So that was one of the first steps — how are brook trout influenced by our hatchery-stocking practices," said Baillie. "It's been going on for a long time and it looks like genetically our stocking practices have not altered the population in a detrimental way.

"So that's a very positive thing and it means we do have a nice healthy genetic baseline to look at future changes."