Invasion of the habitat snatchers....what lurks below....

The Great Lakes, and other Canadian waterways, are threatened by more than just climate change. Invasive species, imported either accidentally by cargo ships or purposefully to deal with a natural problem, are starving out or eating up indigenous fish.

Some invasive species, like razor-sharp zebra mussels and round goby fish, are already established in all the Great Lakes. Others, such as grass carp, are causing problems in US waters and could invade the Lakes if we don't take measures to stop them. Our current barriers are outdated, and there are various proposals being considered, but budgets and the difficulty of making cross-border decisions have hampered progress.

There are three main types of defence against invasive species: physical barriers, which are the most expensive and take the longest to build; electric barriers, which shock invading fish and stop them; and filtration systems, which replace incoming water in locks with treated, pest-free water.

Our current measures are outdated, and we know this because grass carp DNA has already been detected in Lake Michigan. These fish, imported from Asia in the 1960s to deal with acquatic weed control, can grow up to 1.3 metres long and weigh up to 50 kg. They can eat up to 20 per cent of their body weight in plankton every day, making it difficult for other species to co-exist with them.

Zebra mussels, native to the Caspian and Black Seas, also eat massive amounts of plankton. As well as starving out local species, they also reproduce fast, clogging intake pipes, weighing down buoys, and coating docks and boats. Their painfully sharp shells litter beaches and shores, posing a hazard to bathers and leaving smelly decaying matter strewn on the sand.

The round goby, first introduced into the St Clair River from Eastern Europe in the 1980s, eats the eggs of trout and bass, reducing the numbers of those species. There are more than 100 goby per cubic metre of water in some areas now.

Found on the southern shore of Lake Superior and in Lakes Huron and Michigan, the Eurasian Ruffe resembles a young bass or perch, but has a spiny dorsal fin that makes it less vulnerable to predators. Used as bait fish, it now threatens local populations of sport fish.

There are measures you can take to fight invasive species: visit the Ontario Ministry of Natural Resources for more information.

Finally, cormorants--once near extinct--are a native species whose resurrection is causing environmental concern. Capable of eating a pound of fish per day per bird, their nesting colonies threaten trees and other birds: they strip bark to build their nests and the ph levels of their droppings can kill vegetation. Some conservation areas cull the birds, but Hamilton came up with a really creative solution: large, inflatable Santa figures placed in desirable nesting areas made the cormorants think twice about settling.