If you have not yet had a chance to see this viral video from CBS News of dolphins swimming en masse, here it is! I have seen large pods of dolphins gather around the fishing vessel when deep sea fishing, presumably hoping for a reject from one of the fisherman. I have also seen them following cruise ships and other ocean vessels, but I have never witnessed that large of a group. When pods of dolphins come together, it is called a herd or aggregation, which sounds less exciting than what the media have been calling it: a "stampede". This particular aggregation consisted of ~2000 dolphins swimming alongside a whale watching boat off the coast of California.
Some researchers suggest that the purpose of these aggregations is for cooperative feeding. When prey numbers were highest, the numbers of dolphins aggregating was also highest. But they may also aggregate for other reasons according to experts at Sea World.
In my search for why dolphins would gather in aggregations, I came across this amusing music video created by Dolphin Communication Project that presents the various species of dolphins: (I apologize in advance if the tune gets stuck in your head)
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Sources:CBS News (video)Dolphin Communication ProjectSea WorldAggregation behaviour:http://adsabs.harvard.edu/abs/2003ASAJ..114.2300B