This could be something as simple as a seagull becoming entangled by apiece of discarded nylon fishing line from a recreational angler, to the destruction of centuries-old deep sea corals by heavy trawling gear. In order to decide whether a particular fishing method has unacceptable levels of environmental harm we need to consider the following factors:
Bycatch
Bycatch is the term used to describe all organisms that are inevitably caught when fishing for a specific species or group of species, for example, when trawling for prawns a large volume of other marine organisms such as fish, crabs, squid and even turtles may be caught. Prawn trawling is considered one of the most wasteful fisheries. Bycatch is not always discarded, for example, kingklip and monkfish are regularly caught by deep-water hake trawlers, and have high market values. However, these species are not abundant enough to allow dedicated fisheries for them and are thus managed as a bycatch of the hake fishery.
Incidental mortality
This refers to the deaths of non targeted marine life that is unintentionally caught during fishing activities. This could include seabirds such as albatrosses that drown when they swallow baited hooks on a longline, or turtles and sharks that drown in a prawn trawler´s net.Many of these species play important roles in maintaining the health of marine ecosystems and are extremely vulnerable to even low levels of mortality. In many cases these unnecessary deaths can be prevented or reduced if the correct mitigation measures are put in
place.
Selectivity
How successful is a fishing method at catching what it intends to catch? If a certain type of gear is very target specific it can be considered a better fishery than one where alot of the catch is wasted (e.g. Tuna pole fishing for Yellowfin tuna and Albacore vs. bottom trawling for prawns).
Habitat damage
This refers to the physical destruction of marine habitats. Some marine habitats are more fragile than others and can be permanently damaged from fishing gear, such as trawling over a coral reef. Destroying bottom habitats not only reduces the number of refuge or shelters for fish but disrupts the overall productivity of the system. Habitat damage can include human activities other than fishing, for example, undersea mining, or developments in or near estuaries that destroy important nursery areas for many types of fish. More than 50% of the world´s total marine catch (approximately 81 million tons) is harvested using towed fishing gear.