First look at the map of Atlantic winds and currents in summer. (Winter conditions are similar.) What does the map tell you about probable trade routes (and communication routes in general) between Spain (at the upper right of the map) and its American colonies in the age of sailing ships? (The large, black arrows show winds; smaller, blue-grey, arrows indicate currents.)
Next consider the graph that shows the tonnage of shipping from Spain to the Spanish colonies, and back, from 1500 to 1650. (The graphs shows official, legal shipping; the amount of unofficial, contraband trade grew over the 150 years covered by the graph, especially after about 1600.) What are the broad movements of the graph? How many plausible reasons -- changes in both Spain and the American colonies -- can you think of to explain those movements?
Finally read carefully the letter of 1553 from a merchant in Seville to his partner and agent in Lima. What does it tell you about the operation of the transAtlantic trade from Spain? What were the fears of merchants? How were trading companies or enterprises staffed?