Got the following email from Don Garling professor at MSU specializing in aquaculture. Thought you would find this of interest. Thank you Don!

Cecil:

Their (Grass Carp) digestive efficiency is even lower than the DNR statement you
cite. It is because they are herbivores and are similar to many other
herbivorous fishes. FAO reports grass carp digestive efficiencies of 40%
or less. Juvenile grass carp consume as much as 30 to over 70% of their
body weight per day and at the higher rates retain much less because feed
remains in their intestinal track for as little as 30 minutes. It is
basically sped through the tract by newly consumed material. So, it is
retention time that is much less than other herbivores.

There is a published report that grass carp can absorb nearly 74% of the
phosphorus that they consume. Unfortunately, the authors checked excreted
fecal phosphorus collected from tanks for phosphorus. It has been shown
that about 90% of fecal phosphorus can be lost from feces in water in 60
minutes. As a result, they probably have phosphorus uptake nearer to
10-15% at best.

Combine those factors with an inability to eat algae:
high intake of grassy plants + rapid passage rates + low retention of P = algae

It is usually bluegreen algae because it can fix its own nitrogen which
also becomes limiting under the above scenario.

Don Garling

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.