The round goby uncovered: Science behind a freshwater invasion

Among the many invasive species that have found their way to Ontario’s waterways, few are better known to anglers than the round goby. These small fish from the Black and Caspian seas of Eurasia can be found in all five Great Lakes, many Great Lake tributaries, and, more recently, in some inland lakes and rivers of Ontario.

Within the angling community, gobies can be a surprisingly contentious topic, owing to their ability to change the prey base of sport fisheries. These little creatures become so abundant in the waters they invade that sport fish like bass and whitefish begin to target them specifically — including in shallower water — creating a niche that anglers can exploit. The introduction of gobies into Lake Simcoe, for example, has given way to dozens of goby imitation baits, none more popular than the now classic Meegs jig.

“After gobies invaded Lake Ontario, their abun­dance significantly increased and so did their presence in the stomachs of lake trout,” Ontario Federation of Anglers and Hunters Fisheries Biologist Adam Weir said. “Although alewife is the preferred preyfish in this Great Lake, gobies take second place, with some lake trout even becom­ing specialists.”

As a result of this interesting phenomenon, many anglers attest that the sheer abundance of the round goby makes a fishery better. But, is this true?

Strength in numbers

Round gobies, like many invasive species, have high fecundity. That’s science-speak for “They’re good at making babies.” Females reproduce multiple times each season, with generally high rates of egg fertil­ization and hatching success, and the result is a spe­cies that becomes abundant very quickly. In some cases, round gobies can reach densities of 100 fish per square metre of nearshore lake bottom.

That’s a lot. As you might imagine, all this com­petition for resources can make life challenging for the critters that would normally occupy that part of the water column. Gobies are notoriously aggressive, outcompeting native bottom-feeding fish like sculpin, logperch, and darters. Although the long-term impacts are not yet fully understood, goby introductions have been directly connected to reduced abundance of other bottom dwell­ers, as well as significant changes in their diets.

Regardless of which species is swimming around bottom, though, it’s undeniable that the presence of round gobies produces an abun­dant food source for sportfish. In fact, research has found that a goby introduction can lead to an increase in the average size of some species, notably smallmouth bass and yellow perch. Good news, right?

Hundreds of gobies

Not exactly. While there’s no doubt that pull­ing in a five-pound smally is one of the most pure forms of entertainment around, this highly abun­dant food source comes at a cost to the very fish it appears to be helping. As an aggressive, bottom-oriented predator, round gobies have become a significant nest predator for our favourite sport fish species.

Where a nesting muskie or bass may have once had to worry about a few mischievous sculpins and perch around its nest, it now has to contend with literally hundreds of gobies, all waiting for their first chance at a meal. This is one of the primary reasons that the introduction of round gobies has been connected to the decline in the overall population of many sport fish spe­cies across North America.

Big picture stuff

So, what we’re left with is potentially bigger fish, but less of them. While this may not seem like a problem to some, from an ecological perspective, less is rarely more. Typically, healthy fisheries have a variety of size and age classes, rather than an abundance of large, mature fish but little else.

Beyond their more direct impacts, round gobies have also proven the unlikely spreaders of disease. Researchers believe these little invad­ers are linked to outbreaks of botulism type-E in Great Lakes’ fish and fish-eating birds, which have led to substantial die-offs of both.

Conversations around invasive species can be tough, especially when our personally inter­ests come into play. The trouble is, the long-term implications of any non-native introduction can be almost impossible to predict. Even with the serious consequences of species like emerald ash borer, for example, we won’t have the full picture until many, many years from now. As it stands, the round goby simply does more harm than good and their expansion promises to be an added and unwelcome pressure to any Ontario sport fishery. While it’s unlikely they’re going to disappear any time soon, we can all do our part of prevent further spread in Ontario’s inland lakes and rivers by paying attention to what’s in our bait buckets and spreading the word.

To report an invasive species, contact the Invading Species Hotline at 1-800-563-7711 or by emailing info@invadingspecies.com

Originally published in the Jan.-Feb. 2025 issue of Ontario OUT of DOORS

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