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