1. Lots, probably a lot more than you think. Some waters I fish definitely have crucians including big ones but you simply cannot get past the bl**dy skimmers, at least Ican't - on one of the waters I haven't had a crucian for nearly 20 years yet they are still there and even the most ardent of specialist has given up the unequal struggle, on the other I know they are there as I've witnessed them but have yet to get one. That's the difference between managed and unmanaged. It can still be difficult to catch fish on managed waters especially where there is lots of pressure; that's where skilful fishing comes in plus a bit of luck in the fish being in a feeding mood. The crucians on the waters where I'd love to catch them are totally unpressured simply for the reason you can't get near them. It's the same with big roach on the unmanaged waters. Nearly all the roach are very small with odd big ones but getting to the big ones is extremely difficult. Three local stillwaters have produced 2lb plus roach in the last few months; I am surprised by this because I didn't think any of them capable of it but the facts are there - these are genuine two pounders (in one case nearer 3lbs), not hybrids. All I need to do is figure out how!
2. No. Crucians, likeall species, breed true most of the time. One water I fish, a gravel pit of about 8 acres, is not managedbut has brown goldfish and true crucians (DNA confirmed). The goldfish do not breed at all, just get older and bigger (about 4lbs now) yet the crucians continue to breed true. With the crucians you either get an old big one of 2lbs plus (an example in the pole book p. 126)or one of the newly minted ones of various sizes, 4oz, 8oz or 1lb which suggests three year classes.It is helping generally that the EA are promoting stocking true crucians; I just wish they'd bar bl**dy F1s and brown goldfish. I don't thinkcrucians get caught because not enough people fish for them in many waters and hence their presence is unsuspected.
If I had a water like MF that close to my home I'd fish it too; pole fishing like that is the very best. I'll have to put up with my local challenges, none of which I seem able to manage - carp to 50lbs, tench to 12lbs, perch to 4lbs, roach to 3lbs+, crucians to 3lbs, barbel to 18lbs, chub to 8lbs, grayling to 3lbs, bream to 15lbs... all on my local club books!