The round goby spread from Lake Ontario to the St. Lawrence River in the 2000s. Since then, the total number of round gobies has increased over time, with some fluctuations. The species is now an integral part of the food web in the river.

Today, round gobies can be found throughout the Upper St. Lawrence River. There are much higher densities of round gobies in the upstream portion of the river. There are several reasons for this pattern. Two important reasons have to do with round gobies’ habitat preferences.

First, there are fewer round gobies in wetlands compared to non-wetland areas in the river. This is likely because round gobies need hard surfaces to lay their eggs, so they tend to prefer rocks over mud (like those found in wetlands).

Second, round gobies prefer areas of the river with mineral-rich waters like those in their native range. When mineral levels are too low, it negatively impacts round gobies’ diet, hunting ability, and survival rates.

Round gobies live and feed along the bottom of waterbodies. Their diet can include small fish, fish eggs, and macroinvertebrates, like snails, worms, and mussels.

When round gobies arrived in the St. Lawrence River, they found familiar food sources, including the invasive zebra and quagga mussels. Like the round goby, zebra and quagga mussels are native to the Black and Caspian Seas. The mussels were introduced to the St. Lawrence River in the 1980s and spread rapidly. When round gobies arrived in the river, they were one of the few fish species eating these invasive mussels.

All species in an ecosystem are connected through the food web. When invasive species are introduced to a new environment, they become a part of the food web. Their introduction also changes relationships in this food web. When round gobies were introduced to the St. Lawrence River, they changed the food web by becoming a predator of native species.

For example, round gobies eat fish eggs, which can impact native fish populations. Species like smallmouth bass are particularly vulnerable because they build their nests in the same rocky habitats where round gobies live. Male smallmouth bass guard their nests to protect their eggs. On occasions when a male is caught by an angler, leaving the nest unguarded, a round goby can eat thousands of eggs in a short period of time. In 2021, the fishing season in the river was changed to eliminate fishing pressure during smallmouth bass spawning season to reduce nest predation.

Round gobies’ appetite for native snails is another example of a food web impact. Native snails play an important role in the food web by eating algae. When lots of round gobies are eating snails, it can lead to declines in snail populations. A decline in snail populations can, in turn, lead to higher amounts of algae, which impacts ecosystem health.

Round gobies have also changed the food web by becoming prey for other species. Native fish species, including yellow perch, northern pike, smallmouth bass, and walleye, all eat round gobies. So do many birds, like the double-crested cormorant and the ring-billed gull.

What does this mean for the river’s health? One impact is that predators may be eating fewer native fish, which could be good for these native prey species. In fact, research has found that round gobies are now the main food source for cormorants in the region.

Other impacts are more concerning. For example, zebra and quagga mussels can trap contaminants (like mercury) in their bodies. When round gobies eat the mussels, and then get eaten by other animals, it can introduce these contaminants into the food web. As contaminants travel up the food chain, they can cause harm to fish, wildlife, and local communities.