What is bycatch?

Most fisheries are unselective to some degree, in that they incidentally catch other species along with their target catch during the process of fishing. This non-target catch is known as "bycatch".

Bycatch species tend to be associated with certain fisheries (for example sea turtles with tropical shrimp trawl fisheries). Nevertheless bycatch, in terms of diversity of species caught and ratios of bycatch to target catch, can vary significantly over the geographic region and over time.

Bycatch reflects variations in marine communities, types of fishing methods used, and changes in the target catch of fisheries. For example, in some parts of Asia, former shrimp trawl fisheries have become "multi-species" fisheries where a much greater proportion of the catch is used.

In some cases bycatch is predictable and straightforward to control, and is therefore easier to avoid. In other cases, bycatch is unpredictable and very difficult for fishermen to control. In the latter, bycatch reduction is harder to achieve.

Bycatch also varies according to its impacts. These are not always immediately evident. For example, even a low rate of bycatch can have an ecological impact if fishing effort is high and bycatch species are vulnerable to over-fishing, for example if they are species that are long-lived and slow to reproduce, such as sharks and marine turtles.

A significant proportion of the world's bycatch is discarded back into the ocean.

The existence of high levels of bycatch in today's fisheries is, in part, a reflection of how the oceans' resources are harvested. Fisheries typically target one single species, often towards the top end of the marine food chain.

Aside from the ecological impacts of removing these generally large "piscivorous" (fish-eating) target species, the fishing methods employed often lead to the incidental capture of other large species such as cetaceans, sea turtles and seabirds.

Alternatively, trawlers are used to target species lower down marine food webs, such as shrimp or prawns. However, due to their fine-meshed nets, trawlers can capture high levels of non-target species unless fitted with bycatch reduction devices.

Focusing fishing effort on a narrow range of species is not the most efficient way to harvest marine resources. And, from an ecological perspective, it "seems, intuitively, a very unlikely way to preserve ecosystem structure and function". A more common sense approach would be to spread fishing pressure across the food web, thereby utilising the biomass (protein) found at a range of trophic levels. Obviously, care would still have to be taken to avoid the bycatch of juvenile fish and vulnerable or endangered species. Equally, overall fishing pressure should not be allowed to exceed recommended scientific limits. But learning to process and market a diverse range of species may be a more ecologically sustainable approach in the long term.