The Round Goby - This bony, bug-eyed, sardine-sized fish won't win any piscine beauty contests, but it is poised to compete aggressively for food supplies and spawning territory throughout the Great Lakes. Having only recently arrived from the Black Sea, probably in the ballast water of ocean-going ships, it has already proven itself to be a tough contender in the battle for aquatic dominance.

In spite of its diminutive size, the round goby has the survival capability of a super-fish. Its well-developed lateral line provides keen sensory abilities, allowing it to eat around the clock. It has a wide and varied diet, including the fry and eggs of native fish. It can live in the murkiest and most polluted of waters, and it inhabits both shallows and depths. It can spawn up to 6 times a year (laying thousands of eggs), and it does not hesitate to chase larger fish away from food sources and spawning grounds. (The goby is a particular threat to the bottom-feeding Lake Ontario sculpin.) The fearless goby is becoming the bane of sports fishers, who report that it has the audacity to steal their bait. If it succeeds in reducing native fish populations, it will also threaten a sports fishing industry that is worth almost $150 million a year.

The round goby has also become part of the chain of toxic bio-accumulation. It feeds on zebra mussels, and is in turn eaten by sport fish such as smallmouth bass, walleye, sturgeon, yellow perch and brown trout.

The Spiny Water Flea - Like a burr hitching a ride on a passing animal, the spiny water flea, a crustacean about 1 centimetre in length, uses the thorn-like barbs of its long tail spine to attach itself to fishing lines and downrigger cables. The water flea's sharp tail - about 70% of its body - makes the organism hard to swallow, protecting it from preying fish and allowing it to survive, multiply and consume a disproportionate amount of plankton. The flea's hearty appetite places it in direct competition with plankton-eating juvenile fish.

The water flea also benefits from the ability to reproduce both asexually and sexually. In the warm water conditions of late spring and summer, females can produce genetically identical female offspring without fertilization, while in lower temperatures, both males and females are produced. Eggs are fertilized, and protected by a coating that allows the species to survive the harsh conditions of Great Lakes winters.

Like the zebra mussel and the goby, the water flea probably arrived in the discharged ballast water of seagoing freighters. (The most likely source is St. Petersburg, Russia, where the spring runoff temporarily transforms the port into a freshwater harbour.) Although water flea populations in the Lakes are currently increasing, the long-term effects of water flea proliferation are not yet known. Successful predation by some fish species, such as perch, may keep the flea under control and even have a beneficial impact on overall fish populations.