Skates on thin ice
When ichthyologist Samuel Iglésias spent 2 years scouring French fish markets and docks, he wasn’t exactly looking for a two-for-one deal on the catch of the day, but he found one. Iglésias recognized that a species of highly endangered fish was actually composed of two separate species, each in greater danger of extinction than the single species they’re lumped together as—including one that could become the first fish species driven to extinction from fishing.
Iglésias examined more than 4,000 specimens of the European common skate (Dipturus batis), a large cartilaginous flat fish that resembles rays, the flattened pectoral fins of which sometimes appear as a traditional French dish—skate wings with black butter.
D. batis populations in the Eastern Atlantic have plummeted over the last 4 decades, decreasing some 90–95%...
But if the outlook is grim for D. batis, it’s even worse for the two species that the mistaken classification comprises. In particular, Dipturus intermedia, the larger of the two, is in dire straits thanks to the fact that it takes 20 years to reach sexual maturity and has relatively few offspring every year, and Dipturus flossada isn’t likely doing much better. “It’s a terrible situation to be in,” says Andrew Griffiths, population geneticist at the Marine Biological Association of the United Kingdom. “We had one critically endangered fish species and now we have two.”
But these impending extinctions aren’t the fault of fishermen alone. Yesterday’s taxonomists share some of the blame.
A century ago, scientists were unsure about the taxonomic status of large skate species that inhabit European waters. Some thought that D. intermedia and D. flossada were separate species, some did not. “Resolution” came in 1926 when R.S. Clark published a paper classifying the European common skate under the name D. batis. Clark’s mistake—which Griffiths says is an understandable one given the taxonomic confusion surrounding skates at the time—stood for more than 80 years. As a result of this mistake, conservationists overestimated the reproductive potential of D. batis because the two species were managed as one, leading to today’s dire situation.
Iglésias and his coauthors point to several differences that substantiate the splitting of D. batis into two species. Morphological differences, such as eye color, size at sexual maturity, and teeth shape, are distinct in the two separate skate species. His team also noted genetic differences in a particular stretch of the skates’ nuclear DNA. “Samuel and his colleagues did an extremely rigorous analysis of these species,” says Nick Dulvy, fisheries conservationist at Simon Fraser University in Canada.
Surprisingly, the two species aren’t even that closely related, based on Iglésias’s genetic data. “It turns out that these species aren’t even sister species,” says Duly, who is also co-chair for the IUCN shark-specialist group. “They’re more like cousins.”
Recent changes in European Union policy are one bright spot for the skate species. In late 2008, the EU mandated that European fishermen throw back any common skates they mistakenly caught; in 2009, those rules were strengthened and broadened to prohibit the catching of the fish. Of course, now that the paradoxically named common skate appears to be two species, the policy picture gets more complicated, according to Sonja Fordham, EU policy director at the nonprofit coalition The Shark Alliance. “The loophole that’s created by this being two species instead of one needs to be closed now.”
Even with the new regulations, D. intermedia and flossada may still be doomed by the fact that they take so long to reach sexual maturity and only produce 30–40 eggs per year, according to Iglésias. “What is sure is that if we stop fishing the species completely, they might have a chance,” he says. “If the species can recover, it will take several centuries.”