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There is increasing focus on the impacts of whale-watching on marine mammal species around the world. In the inland waters of Washington State and British Columbia, a region known as the Salish Sea, killer whales (Orcinus orca) are protected by regulations to help mitigate the acoustic and physical disturbance from small vessels. Most recently, the State of Washington implemented a commercial whale-watching licensing program that restricts viewing hours of whale-watching vessels within one-half nautical mile of endangered Southern Resident killer whales to four hours a day during three months of the year. During the development of these regulations, an identified data gap was regarding the “sentinel effect”, a question of whether the presence of commercial whale-watching vessels can help mitigate speed and distance incidents by private vessels via modeling correct viewing behavior and/or by directly contacting vessels to prevent such incidents from occurring. This pilot study conducted five months of boat-based and shore-based data collection on vessel incidents in the proximity of two killer whale populations in the Salish Sea. A negative binomial model predicts a significant decrease in mean estimated incidents per hour by private vessels in the presence of commercial whale-watching vessels, while there was no significant difference in the presence of boater education vessels. This study provides the first quantitative support for the sentinel effect, a finding that should be analyzed further in ongoing studies and incorporated into the adaptive management of vessel regulations, especially those that restrict only commercial whale-watching vessels.