Complex sentence: They tend to be parts of the nation's navy, though they may also be operated by secret services and masquerade as civilian ships such as fishing trawlers, which could be reasonably expected to remain in a certain area for a long time.
Split into simpler sentences: They tend to be parts of the nation's navy, though they may also be operated by secret services. Naval trawlers masquerade as civilian ships such as fishing trawlers, which could be reasonably expected to remain in a certain area for a long time.