It appears that the movement of French prisoners from Cadiz to the Canary Islands took place in 1810 after the British joined in Cadiz's defence. Perhaps they were moved to ease the supply situation in Cadiz which was under siege, as well as, to combat the ship or jail fever which struck the prisoners (described as being from Dupont's army onboard the hulks) early in that year. Certainly the British wanted them moved from there.
The plan was for the Spanish to move 1,000 prisoners to the islands with 1,000 going to the Balearic Islands and 4,000 to England.
See pages 495, 503, 522 and 534.
See page 87