There have been few contemporary extinctions in the sea, which suggests marine species are either less vulnerable to extinction than terrestrial species, or marine extinctions may have gone unnoticed. We consider whether a large conspicuous reef fish, the giant humphead parrotfish (Bolbometopon muricatum), is likely to have declined across much of its geographical range. Informal knowledge was used to determine the status of the giant humphead parrotfish. We first surveyed fishers' knowledge of this parrotfish at 12 lightly exploited islands in the Lau group, Fiji and then compiled a global inventory of available information. In the Lau islands, Fiji, fishers reported this parrotfish as previously abundant, but it had not been caught at six islands since at least 1990 and was considered rare at another four islands. The parrotfish had been captured recently (since the 1990s) at three islands where fishers did not target parrotfishes regularly. A compilation of giant humphead parrotfish records provided by local scientists at 39 locations in 31 Indo-Pacific nations suggested this fish is locally common only inside areas where fishing is prohibited and that it is currently globally rare. Local densities of the giant humphead parrotfish were negatively correlated to a categorical scale of fishing pressure across six Indo-Pacific locations. The retrospective discovery of local disappearances and global rarity of a distinctive and formerly prominent reef fish is consistent with the hypothesis that the capacity to detect disappearances of exploitation-vulnerable species in the sea is lower than expected.