Complex sentence: Corrections are released in Admiralty Notices to Mariners on a weekly basis, they are indexed by a serial number and the week and year of release, an annual summary is also released shortly before the start of the "sailing season".
Split into simpler sentences: Corrections are released in Admiralty Notices to Mariners weekly, indexed by a serial number and the week and year of release. An annual summary is also released shortly before the start of the "sailing season".