They get killed by propellers. They get snagged in gill nets and drown. But such deaths are humane compared to fibropapilloma tumors. The disease often starts in the eyes, and spreads. Particularly gruesome are tumors growing from the corner of the mouth.
Large tumors interfere with both breathing and eating.
Eventually, blinded and wearing a collar of tumors, the turtle finds swimming difficult. The turtle becomes emaciated and dies. Notice the small triangle at the center of this turtle's profile, and the Y-shaped pattern formed around it. This pattern is how we can identify this individual quickly. If you compare this picture and his 1991 profile carefully , you will see more points of correspondence.