Beware of Alien Invaders!

alien invaders
Posted: May 19, 2017
Category: Newsletters
Tags: alien, carp, fish, invaders, lakes, water

BEWARE of Alien Invaders!

In dealing with any type of alien invasion there are realistically only 4 options:

  • Prevent the invasion
  • Eradicate the invaders
  • Manage alien populations to mitigate their impact
  • Sit back and observe

Obviously prevention is the best option but in the case of fish invading our native waterways, it is not an easy task.  Alien species can be introduced in a variety of ways and preventing the invasion requires complex measures and strategies.

People are the main cause of this problem.  Fish have been deliberately moved and waterways have been altered without consideration of the ecological effects.  Flooding in some areas has allowed species from one ecosystem to be transported into another where it is non-native.  When a non-native species is introduced to a new environment, although it may seem harmless, with no natural enemies it can crowd out the native population and create other unforeseen issues.

Once established, a species is extremely difficult to eradicate!

The Common Carp

When overfishing in the great lakes caused the decline of the local fish population in the late 1800’s it was decided that the introduction of Carp as a food fish would be a good idea.  Within the first 20 years the so called “evil results” of this introduction could be seen.  The common carp can have severe impacts in shallow lakes and wetlands.  Their feeding disrupts shallowly rooted plants and muddies the water.  They release phosphorus that increases algae abundance and induce declines in water quality harming the native flora and fauna.  This is a problem we are still dealing with today and have not yet been able to resolve.


The Royal Botanical Gardens in Hamilton is making great strides locally in decreasing the number of these invaders in the surrounding waters.  They have installed a fishway in the Desjardin Canal to intercept migrating carp and deny them access to their preferred spawning grounds.  This is helping to eliminate carp in the marshlands, an important step in the rehabilitation of this wetland as well as causing the population in the overall area to decline.

Brown Trout

Brown trout were imported from Germany and Scotland in the mid-1800’s as an alternate food fish.  These brown trout are bigger than brook or golden trout and can thrive in water too warm for other trout species.  Brown trout outcompete native species and displace them in the local ecology.

Rainbow Smelt

Rainbow smelt were introduced into Crystal Lake, Michigan as a forage species for salmon but managed to migrate into the great lakes where they have contributed to the extinction of local lake trout and blue pike populations.  These rainbow smelt have now found a way to infiltrate the Lake Winnipeg basin and have been caught as far north as the Nelson River estuary on Hudson Bay.  These aliens are expected to cause declines in populations of other native species and severely impact the commercial fishery in Lake Winnipeg which generates 15 million dollars annually.  Since rainbow smelt have such a high fat content, the fish feeding on them develop a soft greasy bland quality and an increase in the concentration of pollutants such as mercury, make them a much lower quality and less desirable food source when caught, further impacting the fisheries.


The alewife, which invaded the great lakes when the Erie Canal was constructed, along with the rainbow smelt had created an ecological disaster by the 1960’s with no more viable commercial fisheries left standing.  Since then, government efforts have been attempting to re-introduce many of the native species with limited success and are still spending billions of dollars to rectify this problem.

Asian Grass Carp

Asian grass carp have recently been discovered in three of the great lakes, and beginning this spring government agencies are focusing on eradicating the population before they can become established and cause irreversible damage to the great lakes fisheries.asian carp alien invader

Prevention of outcomes such as these is something we must all take very seriously and something we can all do something about.

  • When fishing, be extremely careful with the disposal of excess bait.  Alien species and even fertilized eggs have been introduced to waterways by careless disposal methods.
  • When travelling between river systems make sure your bilge is clean and empty to avoid introducing any alien species to a new system.
  • Create aquaculture facilities that are escape proof even during flooding.
  • Discourage stocking of exotic species for sport or commercial fishing purposes.
  • Don’t release any fish which are not native into a watershed.
  • Guarantee that when building canals we do not unknowingly create pathways for alien species.

It is vitally important that we all do our part to safeguard our local waterways and fish habitats so we can continue to enjoy recreational water activities for years to come.

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