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.
 
"A key part of resolving the bycatch problem depends on rethinking our approach to fishing, and modifying this approach to be more in line with ecological, rather than economic, principles."

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

Impacts of bycatch
Bycatch is not automatically a problem in all fisheries. If bycatch is minimal, does not deplete populations of vulnerable species or undermine the productivity of fish stocks, it doesn't necessarily cause ecological harm. Unfortunately, on a global scale, bycatch is significantly exacerbating the threats posed by the commercial over-exploitation of the oceans. According to the latest estimates of the UN Food and Agriculture Organisation 7.3 million tonnes of bycatch are discarded annually by the worlds fisheries.

This was not always the case. The tremendous growth of the fishing industry in the last few decades has meant not only expanding fishing fleets, but the development of vessels which are larger, faster and able to cover greater areas of ocean. Unfortunately, these vessels are often less selective than their predecessors. And as fisheries are rapidly reaching their limits of exploitation, wastage of marine life is coming under greater scrutiny.
 
Bycatch is bad for wildlife
High levels of bycatch can affect entire marine communities, reducing biomass and altering the ecological structure and diversity of the oceans. The impacts of bycatch on vulnerable "charismatic" species have been particularly well documented, and have perhaps received greatest public attention. Often long-lived and slow to mature with low natural mortality and low reproductive output, populations of several marine mammals, sea turtles, sharks and sea birds (amongst others) have declined due to incidental capture. Some species in these groups are highly endangered, for example the vaquita porpoise of the Gulf of California.

As those species vulnerable to high levels of mortality are selectively removed, food chains and predator-prey relationships are affected. The long-term consequences of these changes are not fully understood.
 
Bad for People
Fishing supports the livelihoods of 450 million people and, as the global population expands, pressure on fish stocks is likely to increase. Over-exploitation will lead to an eventual loss in profit and job opportunities for fishing companies and their employees. Yet for artisanal and subsistence fishers in developing nations, declines in fish stocks can mean hunger and a loss of livelihood where few other options exist.

Industrial vessels operating in shallow coastal areas of developing countries can have considerable impacts on food security. Even when targeting different stocks altogether, high incidental capture of artisanal fish species can quickly undermine the productivity of traditional fishing grounds.

Bycatch not only affects marine ecosystems but also the millions of people who rely on healthy fish stocks for food and employment. The economic costs of discarded bycatch for the fishing industry can be considerable. Bycatch may contain commercial species: often juvenile individuals with little market value. High juvenile mortality due to incidental capture has implications for the productivity of stocks (both target and non-target) potentially leading to declining yields and premature closures of fisheries. In several cases this has caused conflict between fishermen.