Complex sentence: They began arriving on carriers in the 1980s, serving until the 1990s when they were replaced by Group II aircraft in first-line service, although some ended up in US Navy Reserve, being used to track drug smugglers.
Split into simpler sentences: They began arriving on carriers in the 1980s, serving until the 1990s when they were replaced by Group II aircraft in first-line service. Some some ended up in the US Navy Reserve, being used to track drug smugglers.