I do yet believe, that a sort of fish, bred in great numbers in bad waters, over-stocked, and almost starved, may in process of time degenerate, and both lose a good shape, and be less apt to grow up to a due greatness, than others that have been better descended of a cultivated stock: and on the other side, it is no less possible, that by coming into good quarters, fish may improve and mend; so that a gentleman is to expect the goodness of his fish from the cleanness of his waters, and the plenty of their feed, and not from any choice of his stock or breed; and let him get them where he may, if well ordered, he may assure himself they shall answer his expectations.
It is a common observation, that some waters will, and others will not breed. It is my experience, that most waters, the first year after having lain dry a summer, do breed, and that numerously, especially carps, which I have known increase to such an incredible fry, that I have been troubled how to dispose them, so as to have them again after three or four years, when they became good stock for great waters. Eels and perch are of very good use to keep down the breed of fish; for they prey much upon the spawn and fry of bred fish, and will probably destroy the superfluity of them.
The quality of breeding is scarce to be found out by any certain symptom; for some very promising ponds do not prove useful that way. The best indication I know of a breeding pond, is, when there is good store of rush and grazing about it, and gravelly shoals, such as horse-ponds usually have. When a water takes thus to breeding, with a few milters and spawners, two or three of each, you may stock a country.
As for pike, perch, tench, roach, &c. they are observed to breed in almost any waters, and very numerously; only eels never breed in perfect standing waters, and without springs; and in such are neither found, nor increase, but by putting in; but where springs are, they are never wanting, though not put in: and which is most strange of all, no person ever saw in an eel the least token of propagation, either by milt or spawn in them; so that whether they breed at all, and how they are produced, are questions equally mysterious.