Do they have a long digestive track?
You said gills. What has a long digestive tract got to do with it? Gills are used for respiration and not digestion. Dolphins are much better adapted to live in the oceans than Humans are to eat meat. After all, dolphins don't develop health problems due to their respiratory system, whilst Humans who eat meat tend to develop cancer and artherosclerosis to a much higher degree than vegetarians.
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