Small populations are usually threatened by inbreeding depression, where rare harmful recessive mutations become common enough to appear in offspring. The severity depends on history. A large population crashing suddenly carries a heavy hidden load of such mutations; one that has been small for a long time has already expressed and removed many of them.
Sequencing of vaquita genomes, including archival samples, showed the second pattern. Genetic diversity is the lowest measured in any cetacean, but the burden of strongly deleterious variants is correspondingly low, consistent with a naturally small population of a few thousand over the last several thousand years. Simulations projected a high probability of recovery provided that deaths in gillnets ended promptly.
The finding does not make the vaquita safe. The modelled recovery depends on removing the fishing mortality that continues in the reserve, and with so few animals, chance events alone could still end the species.