Please answer the following question: Question: What does Ellen think caused Martin's heart condition? If there is no answer, please output "Insufficient information to provide an answer.". Movie title: Jaws: The Revenge Context: In Amity, Long Island, Ellen Brody (Lorraine Gary) still lives with her son, Sean (Mitchell Anderson), who is a police deputy. Ellen's husband, Martin -- the former chief of police -- died several years ago of a heart attack; Ellen is convinced his heart condition was caused by fear of the great white sharks he battled in the 1970s. One night just before Christmas, while on patrol in the harbor, Sean is ambushed by a great white shark disguised as a piece of driftwood. Ellen is devastated at her youngest son's death. Her eldest son, Michael (Lance Guest) arrives with his family -- wife Carla (Karen Young) and daughter Thea (Judith Barsi) -- arrive from the Bahamas, where he studies conch migration as a marine biologist. Ellen orders him to change careers as she no longer wants any of her family to have anything to do with the ocean. Instead Michael persuades her to come with them back to the Bahamas for Christmas. He promises that the ocean water is too warm for great white sharks, a species that Ellen is convinced has a vendetta against the Brodys.On the plane to the Bahamas, Ellen meets Hoagie (Michael Caine), a relaxed pilot with a gambling problem. They begin a flirtation. Michael and his colleague, Jake (Mario van Peebles) attach radio transmitters to conch shells. Carla makes ugly iron sculptures for the local government. Meanwhile, the shark that killed Sean has followed the family from Amity to the Bahamas. Ellen senses its presence, and the shark attacks the boat Michael uses as his headquarters. Michael elects not to tell anyone of his encounter with the shark but he and Jake decide to track the shark with their equipment. They put a radio tag on the shark but are not able to follow it through the ocean depths.Assuming the shark has left the area, Michael goes back down to study conch shells again. The shark appears out of nowhere and attacks Michael's submersible craft. Michael escapes into a sunken ship but the shark follows him. He barely escapes but insists on going down to look for the shark...
Answer:
His fear of the great white sharks he battled in the 1970s