This article is from the Dolphin
FAQ, by Jaap van der Toorn
There is no evidence to support that statement. A recent study, comparing the survival of dolphins in captivity from 1940 through 1992 showed no significant difference in ASR between the "captive population" and the Sarasota Bay population. The ASR for the captive population was 0.944 (life expectancy: 17.4 years). Also in captivity dolphins have reached ages over 40 years.
R.J.Small & D.P.DeMaster (1995) Survival of five species of captive marine mammals. Marine Mammal Science 11(2):209-226.