According to a document by the SRAC titled 'Using Grass Carp in Aquaculture and Private Impoundments,' "Generally, grass carp prefer the soft, tender tips of young, growing plants and submerged vegetation. It is known that grass carp consume literally hundreds of aquatic plant species. It is probably safe to say that if they are hungry and deprived of preferred vegetation, they will consume any plant material they can find, including terrestrial plants overhanging the water."
Another benefit to using grass carp as a natural control is that they cannot reproduce or spawn in ponds. So, there is no worry of the species overtaking the pond. They naturally spawn in rivers where there is constant water flow. This allows their eggs to stay suspended in the water. When in a pond setting, the eggs will settle to the bottom of the pond and most likely be covered, preventing adequate oxygen needed to hatch out.
It is important though to keep in mind that before stocking grass carp, check with state game and fish/natural resource agencies or Extension fisheries or aquaculture specialists for the legal requirements. Some states require permits to stock grass carp, depending on the size of the pond, and others have stipulations as to whether diploid grass carp or triploid grass carp can be stocked.
The basic difference between the two is that diploids are fertile and triploids are not. Therefore if a diploid escapes from a pond into a stream or river, it will be able to reproduce and affect that water system. In Texas, pond owners must obtain a legal permit to stock grass carp and they may only stock triploid grass carp. In Oklahoma, there is no permit required and both diploids and triploids can be stocked.
According to Will Moseley, wildlife and fisheries consultant for the Noble Foundation, grass carp are a long-living species, so while they are not able to reproduce, they will be an investment that may last 20-plus years.
Moseley said though they are useful for vegetation control, they are not necessarily going to be something that fishermen can expect to reel in. Since they are vegetarians for the most part, it is hard to remove them through basic methods.
"They are really difficult to remove if you need to remove them," Moseley said. "So, when we stock, we stock at a really low number. Like five per acre, and if you don't get the control that you are looking for, maybe in two years time, put two more per acre. If you are still not getting what you are looking for, then maybe stock a few more.
"If we stock at 12 per acre and they remove all your vegetation, what we are going to start doing then is just start mudding up your water," he continued. "So you can have some other issues if you stock too high, so you should stock yourself low and work yourself up."
According to the SRAC, grass carp are easiest to remove when ponds are drained. They can occasionally be caught using heavy-duty fishing tackle and a dough-type bait or, if necessary, they have a habit of staying near the surface of the water making them a fairly easy target for a bow or firearm.
Because grass carp are naturally from rivers, they will readily and instinctively escape ponds that overflow. According to Moseley it doesn't take but a couple of inches of water overflowing from a pond for grass carp to escape. Many states like Texas mandate, that barriers be placed on spillways before grass carp can be stocked into ponds.
"You will know when they either die or they leave because we have ponds that are free of vegetation and in one year you can walk across it the vegetation is so thick,"Moseley said. "Just watch your pond -- it will tell you what is going on with your grass carp even though you can't see them in there."
Moseley said that in Texas. pond owners who want to stock grass carp have to have a parallel bar barrier over their spillway or over the over flow pipe to be able to stock grass carp.
"What this does, it protects your investment," Moseley said. "These things are $10 to $12, $15 a piece and we are talking five to 10 per acre in large ponds, that is a pretty significant investment. So. it protects your investment and it also protects public water. This protects your investment because once you stock them. you know they are going to be there. So. like I said when you have that plush vegetation. you don't sit there and wonder, did they die or did they get out -- you know they were there."