Summary
The authors sequenced whole genomes from 20 vaquitas and used them to build simulations of recovery. They found that the species' long history of being rare left few harmful gene variants that inbreeding could expose, so low genetic diversity should not by itself prevent recovery. In the simulations, the outcome depends on how much gillnet mortality is cut, with 6% of runs going extinct within 50 years if bycatch stops, 27% with a 90% cut and 62% with an 80% cut.
Why it matters
Some people had argued that the vaquita was doomed by its genes whatever was done about fishing. This study gives a quantified answer that points back to bycatch as the deciding factor.
Key findings
- 20 genomes (19 newly sequenced, mean coverage 60X) from samples taken in 1985 to 1993, 2004 and 2016 to 2017 all show uniformly low heterozygosity.
- Bycatch stopped entirely: 6% of simulated runs go extinct within 50 years; mean population in 2070 is 298.7 (S.D. 218.2) in the runs that persist.
- Bycatch cut by 90%: 27% of runs go extinct; cut by 80%: 62% of runs go extinct.
- Vaquitas carry the fewest deleterious heterozygous alleles per genome among the cetaceans compared, 0.33 times the orca's count for deleterious variants.
- Estimated inbreeding load (2B) is 0.95, against a median of 6.2 for mammals; the simulations start from about 10 animals.
Caveats
The results come from simulations that rest on stated assumptions, and the starting population of about 10 is itself uncertain; assuming 5 animals raises extinction risk. The simulations leave out reduced adaptability and disease risk linked to low genetic diversity, and two parameter changes (a higher mutation rate, a shorter calving interval) raised extinction to 55% at a 90% bycatch cut.