Some upper salt marsh pools provide ideal breeding sites for natterjack toads, when the water in them becomes fresh enough. After the main spring high tides, rainfall and water draining from the land gradually make the upper salt marsh pools fresher, natterjacks select and lay their spawn in ponds which are no longer too saline for eggs and tadpoles to develop. These pools can be very productive but, because of their ephemeral nature, all the spawn or tadpoles may often be lost.