had been plying
-había estado ejerciendo
Past perfect progressive conjugation of ply. There are other translations for this conjugation.

ply

By the late 17th century, when this map was made, French fishermen and sailors, many of them Basques, had been plying these waters for more than 150 years.
Hacia finales del siglo XVII, cuando se produjo este mapa, los pescadores y marineros franceses, muchos de ellos vascos, habían estado surcando estas aguas durante más de 150 años.
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