请以json格式概括《老人与海》的主要情节。
user
```json
{
  "title": "老人与海",
  "author": "海明威",
  "main_plot": {
    "old_man": "圣地亚哥",
    "location": "古巴海岸",
    "adversary": "巨大的马林鱼",
    "duration": "84天未捕到鱼",
    "key_event": "在第85天捕获一条巨大的马林鱼，经过三天两夜的搏斗最终成功捕获",
    "consequence": "鱼被拖回途中被鲨鱼群袭击，最终只剩鱼骨"
  },
  "theme": "人与自然的斗争，表现人的勇气和坚持"
}
```
assistant