It never ceases to amaze me what basic biology has only recently been properly described. There have been two conflicting views on the distribution of Grampus dolphins: that they are restricted to continental margins, or that they were cosmopolitan and coastal and oceanic waters. Jefferson et al. (2013) reviewed 8009 sightings and captures from 1950–2012 and arrived at the map above. The dolphins are most common between 30° and 45° and while they appear to strongly favor the continental margins, they can occur in coastal and deep oceanic water at lower densities. They appear to be absent from the Black Sea, Arabian Gulf and the Gulf of Thailand.
Jefferson, T. et al. (2013) Global distribution of Risso’s dolphin Grampus griseus: a review and critical evaluation. Mammal Review 44 56–68.