hatch rates of up to 95%. Invasive species The species was accidentally introduced into the North American Great Lakes by way of ballast water transfer in cargo ships. First discovered in North America in the St. Clair River in 1990, the round goby is considered an invasive species with significant ecological and economic impact; the consequences of introduction are quite complex as the fish both competes with native species and provides an abundant source of food for them while consuming other invasive species. In other words, the round goby behaves much like most biological invasive controls. An aggressive fish, the round goby outcompetes native species such as the sculpin and logperch for food (such as snails and mussels), shelter, and nesting sites, substantially reducing their numbers. Round gobies are also voracious predators of eggs of native fish, many of them important to the angling industry. The goby's robust ability to survive in degraded environmental conditions has helped to increase its competitive advantage compared to native species. Many native predatory fish such as smallmouth bass, largemouth bass, walleye, salmon, and trout have begun to prey on round gobies. The incorporation of the round goby into native foodwebs, coupled with the goby's ability to consume large numbers of invasive mussels (zebra and quagga), may result in greater bioaccumulation of toxins such as PCBs higher in the food chain, since these mussels filter-feed and are known to accumulate persistent contaminants. However, this is partly beneficial because even though they do not reduce the population of zebra mussels, they do control their population. Hence, it prevents a large-scale spread of the zebra mussel, which is also an invasive species in the Great Lakes. Another unintended benefit of the round goby's introduction is that the Lake Erie watersnake, once listed as a threatened species, has found the goby to be a tasty addition to its diet. A recent study found the introduced fish now accounts for up to 90% of the snake's diet. The new food supply means that the water snake is now staging a comeback. Round gobies also serve as food for a variety of predatory fishes in the Great Lakes, including bass, lake trout, lake whitefish, burbot, lake sturgeon, and walleye. The round goby is also considered invasive in parts of Europe. This process was started by its introduction to the Gulf of Gdańsk (southern Baltic Sea) in 1990. Locations recently invaded