OOooh I love rumours...
Well BC is a pretty big part of the West Coast of North America, and with Rivtow folded in, Seaspan is definitely the big boy in these parts. There is some competition in the ship berthing sector with SMIT, which appears to have no intention on leaving, securely positioned in the Fraser, Vancouver, Kitimat and Prince Rupert. There is a rumour of a another ship berthing startup on the lower mainland, but I have not heard much since the initial rumour a couple months back.
Rivtow did not add much in the Seaspan empire from what I can tell - it was severely fledgling for some time, but it still had a pretty good base from its days as a massive BC maritime entity, with them gone, the nearest competitor such as North Arm, Sea Link or Island Tug and Barge, and the likes are just not financially backed enough to out-maneuver Seaspan's experienced corporate players, with their fingers already in numerous business sectors. So there is no real rumour, Seaspan is the dominate player in BC's portion of the West Coast marine transport business. But then again if they don't manage aggressively their markets, small players might take bites out of their empire, by capitalizing on under-served customers, allowing some now smaller entities, to possibly grow.
NTCL, from what I understand, is gravely ill, and it would certainly not be out of character for Seaspan management to roll out big financial moves, given their past predatory history, and swallow this entity up - mind you I don't think there is too much corporate appetite for dealing with aboriginal run companies and their varying priorities and management. Plus there is several other companies in the north that are already pretty experienced and active in that market - so I think overall this would only be a rumour.
On the American west coast, the Americans have some pretty serious marque names on the West Coast which cannot be ignored, because of their established base, namely Foss and Crowley, Lynden. But, there is a litany of transportation companies that would compliment Mr Washington's already massive empire, so who knows what might be true there.
So overall, I don't think there is merit in those wide ranging rumours, Seaspan, big on the BC coast or not, still has to drum up customers and service them well before the business model works. Although they are private and seems to be well financed - therefore highly maneuverable - they are still limited in their options, although they are able to bring resources to bear faster to serve potential customers' wishes.
My two bits.
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