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Good news: Inbreeding may not doom vaquita porpoise to extinction after all

There are only about 10 vaquita porpoises left in the world. But a new study offers them a glimmer of hope — if they can avoid being killed by fishermen.

(CN) — There are only about 10 vaquita porpoises left today in the world. But there's a glimmer of hope on the horizon for the small marine mammals. A study published Thursday in the journal Science finds that inbreeding won't necessarily doom the vaquitas — meaning that if the porpoises manage to procreate, they stand a fighting change of surviving as a species.

"Vaquitas have a very good chance of rebounding from their catastrophic population decline — if they are completely protected," said lead study author Jacqueline Robinson, a researcher at the Institute for Human Genetics at the University of California, San Francisco.

Vacquitas are shy; unlike dolphins, they don't swim up to boats and ham it up for tourists. They are among the smallest variety of cetaceans, the order that includes dolphins and whales. They are are found only in the Gulf of California, the narrow inlet between mainland Mexico and Baja California, which helps explain why they weren't identified as their own species until 1958.

It wasn't long after that researchers realized the animals were being decimated by gillnets used by fishermen. Gillnets — a long wall of mesh sitting at the bottom of the water — are designed to allow fish to stick only their heads through the holes in the netting, thus trapping the fish around their gills. Unfortunately, the nets can also entrap whales, dolphins, porpoises and sea turtles, killing them. Gillnets have been illegal in the region since 2015, but fishermen still use them illicitly.

Up until recently, the conventional wisdom among scientists was that once a species' population gets low enough, inbreeding is a serious threat to its survival. But a number of recent examples have cast doubt on that assumption.

Robinson had previously studied the Channel Island fox, an endangered species that lives on six of the eight Channel Islands off the coast of Southern California. The small fox, already a rare site, experienced a near-catastrophic population decline in the 1990s and 2000s, to the point where there were only a few dozen left.

"Through captive breeding, those foxes were able to rebound quickly without signs of genetic ill health, or inbreeding depression," said Robinson. In other words, the foxes were inbreeding but not experiencing life-threatening genetic mutations.

When some of Robinson's colleagues — including Christopher Kyriazis of the University of California, Los Angeles — saw Robinson's paper on the Channel Island foxes, they suggested she take a look at the vaquitas, which were down to even smaller numbers than the foxes. Perhaps the vaquitas had a hope of rebounding in population size, just as the foxes had.

The researchers sequenced the full genome of 19 vaquitas, some living, some now dead, comprising three generations. They then used the data to create computational simulations of what vaquita babies will look like in the next 50 years.

"I was a little surprised," said Robinson, when she saw the results, "because there’s only around 10 vaquitas left. That doesn’t provide a whole lot of hope. I thought the simulations wouldn’t be optimistic as they were."

The 10 or so remaining vaquitas are already fairly similar, genetically. This has been seen by some as a disadvantage, in terms of their chances of survival, since any offspring the vaquitas do produce in the near future will be, by definition, the result of inbreeding.

"There’s an assumption that low genetic diversity is bad, but we find that it’s the opposite," said Robinson. "The vaqutias' historic rarity is an advantage, in this particular instance." Harmful mutations that might threaten the health of the porpoises have "already been purged" from the population. "The mutations are not kicking around in the population now," she said.

Assuming no more vaquitas are killed by gillnets, the species survives in 94% of the simulations ran by Robinson, Kyriazis and the rest of the team. If the death rate from gillnets declines by 80%, the species' chance of survival drops to just 38%.

The study may offer some hope for other near-extinct species.

"I wouldn’t go so far as to make sweeping generalizations about the capacity to rebound," Robinson said. "But we now have many examples of species recovering from low numbers. It’s possible — even for a single breeding pair. The odds aren’t great, but it's possible."

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