A new species that is "nearly indistinguishable" from the white marlin has been discovered, and this has scientists wondering just which fish they've been counting, according to an article published in the Bulletin of Marine Science.

The roundscale spearfish, whose existence was confirmed by a team of scientists from the Guy Harvey Research Institute and NOAA, so closely resembles the white marlin that both anglers and scientists have possibly been including it in white marlin population assessments for years.

"We don't know much about roundscale spearfish, particularly how abundant they are," said Eric Prince of NOAA Fisheries Service and co-author of the study, in a press release. "This unexpected finding adds an unknown level of uncertainty to our previous estimates of white marlin population size, and certainly suggests that the magnitude of roundscale spearfish misidentification and possible 'contamination' of white marlin landings data needs to be examined in greater detail."

The only way to distinguish the species is an expert eye or, failing that, a DNA analysis.

"The existence of the roundscale spearfish was confirmed by analyzing the shape of its mid-body scales, which are slightly more rounded at one end compared to the scales of all other Atlantic billfish species, and by analyzing its DNA which turns out to be very different from other billfish species," said Mahmood Shivij, the article's lead author and director of the Guy Harvey Research Institute, in a press release.

The white marlin is already under review for possible placement on the endangered species list. The fish was reviewed once before for the list in 2002 and rejected.