Many times, fishing boats catch fish they do not mean to catch, called "bycatch". All kinds of fish can become bycatch, and they are usually thrown back into the sea after they have died. Drift net fishing sometimes catches creatures like seals, dolphins, whales, and sea turtles. This kind of fishing made people complain. In the 1980s, it was guessed that 18 miles (30 km) of nets were lost every night, tangling up boats and animals.