The shortsightedness of using grass carp as a lake management tool

4 September 2014

Pondweeds are a vital part of healthy aquatic environments (Photo: Bill Harding)

Pondweeds are a vital part of healthy aquatic environments (Photo: Bill Harding)

In nearly thirty years of working in the field of aquatic ecosystems, I have yet to see a single example of using grass carp as a means to ‘control’ aquatic plants actually being successful.  I was asked about this again this week and thought it best to lay out a few facts here:

For some reason or other, people who live around lakes and ornamental ponds don’t want to see any plants in the water.  Now, I am not talking about noxious floating species such as Kariba weed or water hyacinth or red water fern, rather the rooted, filamentous pond weeds that are characteristic of many local waters.  Pondweeds, belonging to the genus Potamogeton, together with many smaller sub-canopy types such as Chara and Nitella, provide essential functions and ecosystem stabilisation which, once lost through irresponsible management, are almost impossible to replace.

Rooted, submerged aquatic plants provide a range of natural ecosystem services, including:

  • the production of oxygen to aerate the water (photosynthesis);
  • provision of shelter for fish and aquatic invertebrates;
  • consolidation of the bed and banks of the waterbody in which they occur (sediment stabilisation);
  • provision of food for aquatic organisms;
  • provision of essential surface area for attachment of the organisms that process much of the unwanted nutrients;
  • a spawning medium for fish;
  • nesting material and food for waterfowl such as coots;
  • aesthetic appeal.

So, what do grass carp do?  Firstly, using an alien species (sterilised or not) to control a ‘problem’ that is caused by something else entirely (“too much pondweed” may be due to a variety of factors), is simply idiotic (even more so when these fish are used to control a pest species – i.e. using an alien to control an alien!).  Let’s assume the grass carp actually eat the target plant(s): their feeding simply amounts to ingestion of plant nutrients neatly encased in the pondweed fronds, and then excreted as bioavailable nutrients out the other end.  Many guidelines and manuals state “the excretions of grass carp will lead to continual enrichment of the aquatic environment with nutrients.  This may lead to the production of algal blooms“.  Elsewhere is warned: “[the use of grass carp] has been ecologically disruptive through reducing aquatic vegetation generally and displacing native fish species.  These fish are no longer considered suitable for biological control“.

This then sets in motion triggers that force the stabilised, clearwater lake towards one dominated by algae (which feed happily on the nutrients kindly made available by the digestive system of the grass carp).  Eventually, the system may swing to completely algal dominated, all the other aquatic plants will die, toxins produced by the algae will render the lake dangerous and the ecosystem generally decays to a soupy mess.  Examples of this abound throughout the world.   It is quite mind-boggling that organisations such as CapeNature continue to issue permits for this practice.

This is what using grass carp may lead to… (Photo: Bill Harding)

This is what using grass carp may lead to… (Photo: Bill Harding)

In the Western Cape of South Africa, attempts to control pondweed has racked up a list of spectacular failures:  Zeekoevlei was the first – attempts to clear the water for yachtsmen led directly to the algal dominated mess this beautiful lake has now been in for decades; Paardevlei in Somerset West was similarly abused until we rehabilitated it a few years ago at great cost.  Zandvlei nearly went the same way as over-harvesting (a much more desirable management method) threatened to eradicate the pondweed entirely.  Many many other smaller systems have suffered  the same fate.

None of the above information is in any way new.  Using grass carp for aquatic plant control is a short-sighted, single-focus approach to a complex problem – the outcome of which is likely to be an even bigger and very nasty problem!

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