The endangered Southern Resident killer whales (Orcinus orca) of the northeast Pacific region use two main types of vocal signals to communicate: discrete calls and whistles. Despite being one of the most-studied cetacean populations in the world, whistles have not been as heavily analyzed due to their relatively low occurrence compared to discrete calls. The aim of the current study is to further investigate the whistle repertoire and characteristics of the Southern Resident killer whale population. Acoustic data were collected between 2006-2007 and 2015-2017 in the waters around San Juan Island, Washington State, USA from boats and from shore. A total of 228 whistles were extracted and analyzed with 53.5% of them found to be stereotyped. Three of the four stereotyped whistles identified by a previous study using recordings from 1979-1982 were still occurring, demonstrating that whistles are stable vocalizations for a period of more than 35 years. The presence of three new stereotyped whistles was also documented. These results demonstrate that whistles share the longevity and vocal tradition of discrete calls, and warrant further study as a key element of Southern Resident killer whale communication and cultural transmission.