Great Lakes ports and harbors are connected by a complex network of shipping lanes, locks and navigation channels that allow ships to travel over 3000 km from Duluth, MN, to the Atlantic Ocean. Environmental impacts associated with shipping lanes include shoreline erosion from wave action caused by passing ships, substrate disturbance due to propeller wash in shallow areas, habitat degradation from ice-breaking and winter navigation and pollution from ship discharges.
Mapping shipping lanes as a Great Lakes stressor
We obtained shipping lanes data developed by the US Army Corps of Engineers,1 port locations from the Great Lakes Maritime Research Institute,2 and international shipping route data for1996-2000 from Colautti et al.3
To estimate traffic by route, we recorded the first, second and third ports of call for each ship, and summed the number of ships traveling between each pair of ports.
We used the network analyst tool in ArcGIS to identify 1,369 potential shipping routes between port pairs, of which 281 were used by the international ships in our data set.
We assumed ships traveled using the shortest route.
The most-traveled routes by all international ships included the east-west routes through Lake Ontario and Lake Erie and the north-south route in western Lake Huron. The most common trips originated in Thunder Bay (ON), Duluth-Superior (MN-WI), and Hamilton (ON), and were bound for overseas ports.
Great Lakes navigation routes correspond to nautical charts published by the National Ocean Survey and recommended to commercial shippers operating in the Great Lakes.4 We added a 2.5-km buffer to each side of the shipping channel to account for variation in route fidelity, wake effects and ship discharges.
Spatial distribution of shipping lanes as a stressor in the Laurentian Great Lakes (Inset: Western Lake Superior)