MEXICO CITY — Scientists working to prevent the extinction of an elusive porpoise called the vaquita put out to sea last month, anxious about what they would — or would not — find.

It has been almost two years since the last count of vaquitas, when scientists estimated that only 30 remained in the Gulf of California, their only habitat.

Since then, the illegal fishing that has decimated the species has outpaced law-enforcement efforts. Seven vaquitas have died or been killed, and experts fear that more have been entangled in gill nets and have drowned.

“Every time I go to look for vaquitas, I worry it will be my last time to see them or that we may not even be able to find them,” Barbara Taylor, a biologist at the National Oceanic and Atmospheric Administration, wrote in an email.