The southern blue devil (Paraplesiops meleagris) is a small (max. length 36 cm) demersal reef fish endemic to the Great Southern Reef. A photo-identification capture-mark-recapture technique based on the unique detail of the head markings of individual fish was developed and utilised to monitor four discrete populations on offshore reefs near Adelaide, South Australia. The 14-year program revealed novel information on the species’ life history and behaviour, including that population sizes were relatively small (range 9–42 per reef) and dominated by larger fish of 20–30 cm, juvenile recruitment was infrequent, and the larger fish displayed relatively slow growth rates, high site fidelity, and small home ranges. Paraplesiops meleagris has a combination of life history and behavioural traits which not only renders it vulnerable to localised population depletions from which it may take a long time to recover, but also makes it an ideal species for protection within no-take marine protected areas and for tracking population changes at the level of individuals which is rare for a fish species. At a broader scale, the southern blue devil may prove to be an important sentinel species for monitoring long-term changes in the marine environment.