> A bit of plastic — to a fish gut - is no different than a pebble, chunk of coral, bit of bone, etc.

There's reasonable empirical evidence that plastic accumulation in fish causes them to reproduce less than they otherwise would[1]. The prevailing theory is that most plastics leach at least some of their chemicals in seawater.

[1]: https://www.scientificamerican.com/article/from-fish-to-huma...