Personality is an area of increasing interest in fish ecology because of its potential to affect important ecological processes. Assessing personality traits, such as boldness, usually involves a combination of tests and repeated trials in controlled laboratory environments. However, distress from transportation, handling and extended time in artificial laboratory settings may affect behaviour, increase stress and disrupt natural processes such as feeding. As such, there is a need for a simplified way to assess boldness within a field setting. Here, using a juvenile salmonid as a model, we describe a modified open field test that can easily be applied close to natural habitats. A startle response following sudden exposure to light in a novel environment was used as a measure of boldness in brown trout in two case studies. We showed that boldness was significantly correlated to size with smaller individuals being bolder than larger individuals. In a secondary study, we assessed whether the method could be used to investigate differences in boldness relating to migration timing of sea trout and found that early migrants were bolder than late migrants. We hope this method offers an easy approach to measuring boldness on site and could be particularly useful in situations where transport is unfeasible.