Complex sentence: Because the reservoir is the water supply for Boston, approximately 80% of the 37 mile shoreline is usually open to angling from April 1 through November 30, depending on ice conditions.
Split into simpler sentences: Because the reservoir is the water supply for Boston, not all areas around it are open to the public. Approximately 80% of the 37 mile shoreline is usually open to angling from April 1 through November 30, depending on ice conditions.