You can catch someone's "trash", ie something got dropped in the water and it goes into a global "trash" pool that can be found by fishing or washed up on the shore as beach trash. Most of it is useless, but you look to have gotten a lucky find. Congrats! FYI, this is a good way of catching dead swans and mallards.

Any time you get a "fail" state on fishing, this can happen. Either throw the trash back into the pool, or throw it onto land so nobody has to see it again.
Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.