Grass carp and reducing the tops of milfoil beds has no affect on the herring and bass populations. In Ball Pond milfoil bio-mass was reduced 90% by the grass carp with 50% of that reduction replaced by native plants and the herring, bass and trout populations all did great.
On the other hand, zebra mussels compete with herring for zoo plankton so any population of zebra mussels will decrease the available zoo plankton that herring have to eat and impact herring recruitment. That is what happened at East Twin. The cycle was: 1. Lots of kokanee salmon and zoo plankton 2. Herring arrived, kokanee populations fell. 3. Zebra mussels arrived, herring populations fell. 4. Brown Trout suffered, were skinny, never got fat like footballs anymore. 5. Zoo Plankton crashed, herring died off, zebra mussels suffered. 6. Kokanee started to make a comeback.
There are only so many zoo plankton to go around. As zebra mussels increase, herring populations will decrease and that will negatively impact bass, trout, perch, etc.