A result of harmful fishing practice where part of the catch is discarded and not used is called overfishing

supertrawling

bycatch

longlining

I think its either by catch or over fishing

When part of the catch is discarded and not used is called a "bycatch". This occurs when other, usually smaller fish, get caught by accident by fishermen hoping to catch other kinds of fish.

A result of harmful fishing practice where part of the catch is discarded and not used is called Bycatch.