Round goby fish, the invasive species that has monopolized the Great Lakes for almost 40 years, is now thriving in Hamilton Harbour because "they're able to live in these highly polluted environments," new research suggests.

A team of McMaster University researchers discovered this trend from studying the round goby population in this ecosystem for the the last 15 years.

Erin McCallum, the lead researcher at McMaster's Aquatic Behavioural Ecology Lab recently published her findings about the population size of this aggressive fish, famous for stealing bait from fishermen, and how its growth is affected by industrial pollution, such as steel making, coal gasification and petroleum refining.

Erin McCallum casts a minnow trap used to lure round goby fish in Hamilton Harbour. She has been studying the invasive species for the last 15 years. (Erin McCallum)

The round goby is native to Eurasia, particularly the Black Sea and Caspian Sea. It was introduced to the Great Lakes through the ballast water of ships. The first confirmed sighting in Lake Ontario was in 1998.

The goby has been destructive because it is more aggressive than native fish and competes for the same food source. It also spawns more often than fish local to the Great Lakes — with more than 100 per square metre on the lake or river bottom in some areas.

When it was introduced, it had no natural predators. But that has changed, McCallum says.

"When invasive species first arrive in new environments the predators there won't recognize them as a food source," she said. "It allows the invasive species to get really big and then suddenly those predators will start to recognize them as a source of food."

Hamilton Harbour is still home to some shipping businesses, including the city's steel plants. (John Rieti/CBC)

But tracking how these fish are impacted by pollution in the industrial sites of Hamilton Harbour hasn't been easy.

"There are some challenges with trying to study animal behaviour near polluted sites," she told CBC News.

"A lot of fish monitoring in Hamilton Harbour doesn't go into the industrial area. There are some limitations with some of the collection methods in those areas, but also it's hard to get boats in there and permits for that."

McCallum claims that this might be why a lot of other research isn't investigating this in one of the country's most contaminated sites in the Great Lakes.

"Our study, I think is one of the only studies that has been tracking fish in close proximity to the industry areas of Hamilton Harbour," added McCallum.

The Randle Reef area of Hamilton Harbour is considered the most polluted site on the the Canadian side of the Great Lakes. (Kevin Gamble/CBC)

The study monitored round goby at six sites throughout the harbour — two contaminated sites near Randle Reef, an area of the harbour that's considered a toxic hot spot because of its legacy of industrial contaminants — such as coal gasification, petroleum refining, steel making, municipal waste, sewage and overland drainage, and four "clean" sites, areas that are contaminate free.

Since round goby fish invaded Hamilton Harbour, their population has declined at clean sites, while their numbers remained consistent in the polluted areas, McCallum says.

'When invasive species first arrive in new environments the predators there won't recognize them as a food source. It allows the invasive species to get really big and then suddenly those predators will start to recognize them as a source of food.' - Erin McCallum, McMaster researcher

"At our two contaminated sites, we found that the population was staying at equal numbers to the clean sites," she said. "They're able to live in these highly polluted environments at equal numbers."

She also discovered that round goby at the contaminated sites were smaller overall, a trend researchers can only explain by an account of varying possibilities.

"We're not sure why we see smaller fish at contaminated sites," she stated. "It could mean that there are only younger fish at that site, so they only live there when they're younger and maybe they decide to switch environments after.

"Or, they could have lower growth rates, and maybe they're actually the same age as fish on the other side of the harbour and they're just smaller overall because pollutants have stunted their growth."

But so much can still be learned about the round goby.

McCallum hopes that her annual monitoring of this aquatic species during the summer months will be able to look and track their behaviour, and answer questions about migration and relationships with native fish in this area.