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dolphin caugth in driftnet

The victims

Different types of fishing practices result in different animal/species being killed as bycatch: nets kill dolphins, porpoises and whales, longline fishing kills birds, and bottom trawling devastates marine ecosystems.

It has been estimated that a staggering 100 million sharks andrays are caught and discarded each year. Tuna fisheries, which in thepast had high dolphin bycatch levels, are still responsible for the death of many  sharks. An estimated 300,000 cetaceans (whales, dolphins and porpoises) also die as bycatch each year, because they are unable to escape when caught in nets.

Birds dive for the bait planted on long fishing lines, swallow it (hook included) and are pulled underwater and drowned. Around 100,000 albatrosses are killed by longline fisheries every year and because of this, many species are facingextinction.

fishing boat recovering net

Bycatch is bad for people

Fishing supports the livelihoods of 520 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. 

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. 




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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. 

The highest rates of incidental catch of non-target species are associated with shrimp trawling. In 1997, the FAO documented the estimated bycatch and discard levels from shrimp fisheries around the world. They found discard rates (bycatch to catch ratios) as high as 20:1 with a world average of 5.7:1.

Shrimp trawl fisheries catch two percent of the world total catch of all fish by weight, but produce over one third of the world total bycatch.

Trawl nets in general, and shrimp trawls in particular, have been identified as sources of mortality for cetacean and finfish species. When bycatch is discarded (returned to the sea) it is often dead or dying.

Why bycatch occurs

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. 

Some fishing methods, such as driftnets, target species towards the top end of the marine food chain, such as tuna and swordfish. 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. 

In contrast, bottom trawlers are used to target species lower down marine food webs, such as shrimp or prawns. However, due to their fine-meshed nets and the way they are dragged along the seabed, bottom trawlers can capture high levels of non-target species.

Bycatch has severe environmental, social and economic impacts

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, this situation is rarely the case. High levels of bycatch globally are significantly worsening the threats posed by the commercial over-exploitation of the oceans. According to the latest estimates of the UN Food and Agriculture Organisation 20 million tonnes of bycatch are discarded annually by the world’s 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 often use fishing methods that are less selective than their predecessors. As fisheries are rapidly reaching their limits of exploitation, wastage of marine life is coming under greater scrutiny.

How to reduce Bycatch

Concern about bycatch led fishermen and scientists to find ways of reducing unwanted catch. There are two main approaches.

One approach is to ban fishing in areas where bycatch is unacceptably high. Such area closures can be permanent, seasonal, or for a specific period when a bycatch problem is registered. Temporary area closures are common in some bottom-trawl fisheries where under-sized fish or non-target species are caught unpredictably. In some cases fishermen are required to relocate when a bycatch problem occurs.

The other approach is alternative fishing gear. A technically simple solution is to use nets with a larger mesh size, allowing smaller species and smaller individuals to escape. However, this usually requires replacing the existing gear. In other cases, it is possible to modify gear. The "bycatch reduction device" (BRD) and the Nordmore grate are net modifications that help fish escape from shrimp nets.

bycatch reduction device for fish

BRDs allow many commercial finfish species to escape. The US government has approved BRDs that reduce finfish bycatch by 30%. Spanish mackerel and weakfish bycatch in the South Atlantic was reduced by 40%. However, recent surveys suggest BRDs may be less effective than previously thought. A rock shrimp fishery off Florida found the devices did not exclude 166 species of fish, 37 crustacean species, and 29 species of other invertebrates.

In 1978, the National Marine Fisheries Service (NMFS) started to develop turtle excluder devices (TEDs).A TED uses a grid which deflects turtles and other big animals, so they exit from the trawl net through an opening above the grid. US shrimp trawlers and foreign fleets which market shrimp in the US are required to use TEDs. Not all nations enforce the use of TEDs.

For the most part, when they are used, TEDs have been successful reducing sea turtle bycatch. However, they are not completely effective, and some turtles are still captured. NMFS certifies TED designs if they are 97% effective. In heavily trawled areas, the same sea turtle may pass repeatedly through TEDs. Recent studies indicate recapture rates of twenty percent or more, but it is not clear how many turtles survive the escape process.

The size selectivity of trawl nets is controlled by the size of the net openings, especially in the "cod end". The larger the openings, the more easily small fish can escape. The development and testing of modifications to fishing gear to improve selectivity and decrease impact is called "conservation engineering."

Longline fishing is controversial in some areas because of by-catch. Mitigation methods have been successfully implemented in some fisheries. These include:

  • weights to sink the lines quickly, or not using frozen bait which tends to float before it defrosts.
  • streamer lines to scare birds away from baited hooks while deploying the lines
  • setting lines only at night with minimal ship lighting (to avoid attracting birds)
  • limiting fishing seasons to the southern winter (when most seabirds are not feeding young)
  • and not discharging offal while setting lines.

However, gear modifications do not eliminate by-catch of many species. In March 2006, the Hawaiʻi longline swordfish fishing season was closed due to excessive loggerhead sea turtle by-catch after being open only a few months, despite using modified circle hooks.

On a global level, probably the only effective way to address the problems of bycatch is to control fishing effort. This will be best achieved through the creation of marine reserves. Nonetheless, in the case of highly mobile species such as seabirds and cetaceans, the only effective way of preventing bycatch is to discontinue the use of particularly damaging fishing methods.

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