One of the difficult aspects of managing a commercial fishery which targets a wild population is that you don't know what fish will be caught. A fisherman may have little control on what he catches because generally many different species may live in the same fishing area, and within those species there may also be a range of sizes.
In mixed fisheries of this sort, discarding is a serious problem. Large quantities of young fish can be taken in trawls and seines, and discarded dead over the side. The stock would be larger and the fishery more productive if these fish grew to a larger size and also had an opportunity to spawn to safeguard future generations.
To help this process, fisheries managers define minimum landing sizes for the main commercial species. They also regulate key features of the design of fishing gears so that smaller fish below minimum landing size can escape to grow and join the spawning stock in future years.
Fishing gears can also catch non-target species of fish, birds and mammals. Research into ways to reduce this incidental by-catch continues. Designs of gear that avoid the capture of certain species or separate species during the capture process are being developed.