[bekko]

In the time I spent as a gov'ment fish guy, I learned one imprtant fact. Its easier to obtain forgiveness than permission.

The contractors were probably using a laser level - one transmitter/ receiver on the bank and one on the dozer. They may have gone 4 feet below grade instead of 4 feet overall depth. They probably know something about your soil and water that we do not.

They do make net pen floats out of PVC. Remember, you need the net to be a foot or more above the water to keep them from jumping out. Here's one way to do it - but this example is on a very small scale:
http://www.raingarden.us/spawningcage.jpg

You will also need something to keep the net from billowing up off the bottom. Maybe tie a weight to each corner at the bottom.

I would start with something like 20' x 20'. Any larger and it might be too cumbersome to deal with by yourself. A small net pen will cost more per gallon of water contained therein. You should call Nylon Net and Memphis Net and ask them for a quote on the net material and a ready-made pen (cage). They sew a piece of 1/8 inch nylon line into the seams and around the top edge.

For 2 inch fish at the first culling, you will need 1/4 inch knotless netting like Ace Knotless
http://gmocart.gmotion.com/store/merchant.mv?Screen=CTGY&Store_Code=memnet_fishing& Category_Code=netting_knotless_ace
or Delta Knotless
http://gmocart.gmotion.com/store/merchant.mv?Screen=CTGY&Store_Code=memnet_fishing& Category_Code=netting_knotless_delta
For fish larger than about 8 inches, you could use 252 Green Knotless
http://gmocart.gmotion.com/store/merchant.mv?Screen=CTGY&Store_Code=memnet_fishing& Category_Code=netting_knotless_252
Note that the prices on the linked web site are per pound of netting, not per square foot or square yard. You have to call them to figure out the price per square yard.

Some other things to consider:
1) small mesh costs more per square yard of material than large mesh.
2) small mesh is more versitile in that it can be used with both small and large koi.
3) small mesh will restrict water flow through the net pen more than large mesh.
4) small mesh will clog with algae and other crud faster than large mesh.
5) small mesh with crud all over it is a LOT heavier than large mesh with crud all over it.
6) small mesh will keep all but the smallest bream out of the pen.
7) bream will grow up inside the pen with the koi.
8) bream eat koi food faster than koi eat koi food.

I am sure nice ponds like yours were stocked with bass and bream and may have the optional catfish. I do know how bad the bream problem will be or whether it will take a lot of effort to keep them from growing large enough to eat koi food in the pens.

-steve hopkins