From 1958, the Union Castle shipping line gradually reduced its service calls to the island. Curnow Shipping, based in Avonmouth, replaced the Union-Castle Line mailship service in 1977, using the RMS (Royal Mail Ship) St Helena.

Answer this question, if possible (if impossible, reply "unanswerable"): What year did the Union Castle shipping line slowly start to reduce their service calls to the island?
1958