A fishy case of mistaken identity

An endangered fish in the North Atlantic could be in even more trouble than previously thought because it turns out that for a long time it has been mixed up with another, less-threatened species.

White marlin are magnificent, fast-swimming, ocean-going fish that can weigh in at over 80kg and grow up to 3m long, and each one has an amazing long spear on their nose. But they are not doing well. They are the prized target of a multimillion dollar sport fishery in America and caught accidentally by longline fisheries aimed at tuna and swordfish.

A few years ago it was estimated that the number of White Marlin left in the wild has been driven down to only around 12% of the number needed to maintain a healthy population. But it now seems they could be doing even worse still, because scientists have been confusing them with another species that looks like it, called the Roundscale Spearfish.

That's the finding published in the journal Endangered Species Research by an international team of marine biologists co-led by Laurence Beerkircher from the NOAA Fisheries Service and Mahmood Shivji from the Guy Harvey Research Institute both in the US.

By sifting through DNA samples from the past decade they found that almost 30% of fish reported in fisheries statistics as White Marlin, were in fact Roundscale Spearfish, an enigmatic look-a-like fish that we know very little about and only made itself known to science in 2006 when a genetic study identified it as a separate species.

The team also used computer simulations to work out how the abundance Roundscale Spearfish will alter assessments of White Marlin populations in the past and future. Unfortunately, the crucial piece of missing information is whether spearfish have always made up 30% of the catches, or if that proportion has changed over time. Without that information, it leaves scientists with a very hazy picture of how both these fish are doing making it extremely difficult to manage and conserve them properly.

Twice now the White Marlin has been proposed for protection under the US Endangered Species Act (which would put an end to the lucrative trophy fishery), and twice it has been turned down based largely on population studies suggesting they have been increasing in abundance. But now that the Roundscale Spearfish has come onto the scene, we can't be at all sure if those increases really happened.

We may sometimes think of taxonomy - the science of species identification - as rather an old-fashioned pursuit confined to the dusty corridors of museums. Far from it, this study highlights just how important it is for us to understand what species there are in the world, especially the many ocean-dwelling species that we are busy hunting towards extinction.