The first chromosome level genome sequences for great hammerhead and shortfin mako sharks have shown that both species have experienced major population declines over a 250,000-year history. Low genetic diversity and signs of inbreeding are concerning for Critically Endangered great hammerhead sharks, whose overfished populations have plummeted. With higher levels of genetic diversity, shortfin mako sharks may be more resilient to environmental change; that is, if the current fishing pressure on them is substantially reduced.