Well, this is a long-run investment. When you move to new Island, old Island generates gold for you on daily basis, you can claim the gold anytime. To increase the amount of gold from these old Islands; hire crews. On the main screen -> tap on your pirate -> swipe right or head to your old Island -> tap on the Island -> tap on the + icon and hire crews. Hiring crews will increase the amount of gold. However, this investment will benefit you in future. Just keep investing in your old Islands and you are good to go!