Such small creatures as _Daphnia pulex_, _Cyclops quadricornis_ and _Rotifera_ should be introduced into ponds.
Snails (_Gasteropoda_) may be roughly divided into three classes, according to the shape of their shells: (1) Flat-shaped coils (type _Planorbis corneus_); (2) Oblong-shaped, somewhat like a trumpet (type _Limnæa stagnalis_); and (3) Ear-shaped (type _Limnæa auricularia_). _Limnæa auricularia_ is particularly suitable for deep waters, and _L. pereger_, whose shell is of type 2, is a most valuable addition to the food supply in any fish pond. It is one of the commonest of our fresh-water snails.
Mussels (_Conchifera_) are another valuable article of food. There are a great many different kinds, and the larger ones should, as a rule, be avoided. _Sphæriidæ_ and _Pisidia_ are probably the best.
In many cases it is advisable to attempt the introduction of some flies which are not present. There are several cases in which the May-fly has been successfully introduced, and also the Grannom. Small _Ephemeridæ_ seem to me preferable to any other flies.
With regard to suitable plants for comparatively deep water in ponds or lakes, lakewort and stonewort grow on the bottom, and do not, as a rule, attain any considerable height. White and yellow water-lilies also grow in fairly deep water; the water-lobelia is also an excellent plant for ponds.
In streams some of the best plants are water-crowfoot, water-starwort, and the great water moss. Anacharis should not be introduced into any water, either pond or stream, unless it can be kept down easily. It will otherwise become an unmitigated nuisance.
Marginal plants are a very important consideration, and plenty of them should be grown. Water-celery and water-cress are perhaps the best food-producing marginal plants that can be grown. Bullrushes and brooklime are also good, but the bullrushes must be planted judiciously.
Having stocked his water with suitable vegetation and food, the next matter which should engage the attention of the amateur, is what fish he had better introduce. He should, where there is a fair chance of success, introduce a trout of some sort, as they give better sport than coarse fish.
The introduction of salmon into a river is not likely to be attempted by the amateur, but the head of salmon frequenting a river is undoubtedly affected in the most marvellous manner by artificial means. In Canada and the United States this is particularly remarkable, but the operations are conducted on a gigantic scale.