In most garden ponds the shrimp that you will see is a small American species introduced to this country during the last century called Crangonyx pseudogracilis (kran-gone-icks sue-doh-gras-i-lis). Unlike lots of species introduced to places outside their normal range, this one doesn’t seem to have done much harm in Britain.
The more familiar river freshwater shrimp Gammarus pulex does occur in ponds with inflowing streams and springs, but isn’t very likely be very happy in most garden ponds.
People believe that freshwater shrimps are good indicators of whether a pond is polluted or not. Actually this isn’t true: they are quite tolerant of all sorts of pollutants, so they can live in good ponds and bad.
Good quality ponds will often have shrimps, especially if the pond has been around for a while. They don’t fly so may take many years to arrive at a pond – perhaps as eggs or tiny young ones stuck to birds feet.