Plastic isn't 'bad' or 'good' - it is a both wonder material, strong, light and resistant (hence why it lasts) and without it we wouldn’t have a lot of things we rely on everyday. The problem is our over-use and poor disposal of it, which isn’t so wonderful
A study in 2017, trying to understand why and how microplastics are getting in the marine food chain, found that when coral were offered plastic, sand and bacteria-covered plastic + sand they actually preferred the naked-plastic. They even retained the plastic for longer compared to the other food. They suspect that some types of plastic contain chemicals which stimulate the corals - which are tiny animals related to jellyfish - to feed. Or in other words, plastic is tasty for corals.
Only certain types of fish, including mackeral and tuna, have dangerous levels of mercury and only if you eat them nearly every day. Though the Atlantic Mackeral caught here in Cornwall, contain a lot less Mercury than the Spanish Mackeral eaten elsewhere and are very safe and healthy to eat.