Final answer:
The St. Lawrence River is the main outlet of the Great Lakes to the Atlantic Ocean, while electric barriers are used to combat invasive species threats through man-made canals such as the Chicago Shipping and Sanitary Canal.
Step-by-step explanation:
The Great Lakes' main outlet to the Atlantic Ocean is through the St. Lawrence River, which is North America's most important deepwater ship route connecting the Great Lakes to the Atlantic. Man-made waterways such as the Erie Canal and the Chicago Shipping and Sanitary Canal have historically provided important links for commerce and transportation.
However, these canals also pose ecological risks, as illustrated by the Asian carp's dispersal threats to the Great Lakes' ecosystems. To curb this threat, measures like electric barriers have been employed in the Chicago Ship and Sanitary Canal, although there are ongoing concerns regarding the effectiveness of these measures and proposals to completely sever the connection to Lake Michigan.