The lights themselves are the spirits New Engelica uses to create their familiars. They are small concentrations of semi-sentient magical energy. They are too small to capture with nets. But in theory you could capture one in a glass jar. Further, if War-Magi are close (say 100 meters) the spirits might automatically attach themselves to small organisms like fish (making the fish appear as if they glow). Long term effects of spirit attachement to organisms is being studied by the mages at Al'Gruin but there seem to be no special effects upon the host organism.

However, it should be said that these spirits don't obey you. At all. They merely glow a faint color (blue in case of water spirits). So far only New Engelican War-Magi managed to control and command spirits through the application of music. (ForKhorne is also busy researching familiars though he might use something else).