Monk Montgomery, the first ever electric bass player, used his thumb and was originally a double bass player.
James Jamerson used his index only ( aka The Hook) and was also a former double bass player.
Carol Kaye, the most recorded artist ever, play with a pick (plectrum) and was a jazz guitar player when she started electric bass.
That said Jack Bruce was a graduated classical cellist, not a double bass player. 😉