LitCharts assigns a color and icon to each theme in Life of Pi, which you can use to track the themes throughout the work.
Survival
Religion and Faith
Storytelling
Boundaries
Summary
Analysis
One day the lifeboat drifts into a mass of floating trash. Pi picks out a corked, empty wine bottle. There is a refrigerator, but all the food inside is horribly rotten. Pi writes a message about his predicament, seals it in the bottle, and throws it back into the water.
This is another attempt at communication with the outside world, although a pretty hopeless one. Pi does get a sign from humanity, but it is only trash and waste.