1. Underwater visual census (UVC) using SCUBA is a commonly used method for assessing reef ﬁsh communities. Evidence suggests, however, that ﬁsh avoid divers due to the sound of bubbles produced by open-circuit SCUBA, and avoidance behaviour is more pronounced as ﬁshing pressure increases. Despite the potential for producing biased counts and conclusions, these behavioural eﬀects have rarely been quantiﬁed, especially when assessing the eﬀectiveness of marine protected areas (MPAs). 2. To test the magnitude of avoidance behaviour, we surveyed ﬁsh populations inside and outside two MPAs in Guam, using two diving systems: standard open-circuit (OC) SCUBA and a closed-circuit rebreather (CCR) that produces no bubbles. Data were collected using a diver-operated stereo-video system (stereo-DOV), which provided counts of relative abundance, measures of ﬁsh length and the minimum approach distance of the diver to a ﬁsh. 3. Inside MPAs, ﬁsh surveys conducted with CCR recorded similar community metrics to ﬁsh surveys conducted with conventional OC SCUBA. In contrast, outside the MPAs, the bubble-free diving system recorded 48% more species and up to 260% greater ﬁsh abundance. These diﬀerences reﬂected the ability of a diver wearing the silent CCR unit to sample the larger, most heavily targeted species that are shy of divers in ﬁ shed areas. This diﬀerence was also large enough to change some results from ‘reject’ to ‘accept’ the null hypothesis of ‘no signiﬁcant diﬀerences exist between ﬁshed and protected areas’. 4. The use of CCR for ﬁsh surveys clearly minimizes behavioural biases associated with ﬁsh avoiding open-circuit SCUBA divers. We recommend the use of this bubble-free diving system for surveys assessing reef ﬁsh populations, especially in areas where ﬁsh are heavily targeted by spearﬁshing. If ﬁsh behaviour is not accounted for, surveys using SCUBA could result in erroneous conclusions when comparing ﬁshed and protected areas. While the behaviour of ﬁsh towards divers is rarely mentioned in conclusions from studies using UVC, it is an important source of bias that should be acknowledged and minimized where possible.