At a bustling quayside, hundreds of shiny silver-grey tuna fish, frozen and wild-eyed, are being hoisted from a fishing boat in huge nets. Poured into metal trolleys, the haul is wheeled into a factory where it will be thawed, gutted and tinned in a slick, malodorous operation.

Welcome to the Seychelles, best known as an idyllic tourist destination but also home to one of the world’s largest tuna processing plants. Much of it, especially skipjack (Katsuwonus pelamis), is destined for the UK market.

But the renowned tranquillity here hides a fierce conservation storm over the quantity of fish being caught and the methods used to catch them. These were the key topics at a conference attended by scientists, industry leaders and environmentalists in the Seychelles last December. And they could equally form the core of a classroom debate on sustainability.

On one side are scientists who claim that stocks of tuna in the Indian Ocean - including skipjack, bigeye (Thunnus obesus) and yellowfin (T. albacares) - are currently healthy, but believe a crisis is looming. They want urgent action to cut the fishing fleet’s capacity. On the other side are senior industry figures who want capacity to be frozen at its current level.

Then there are Greenpeace and other environmental groups, which have attacked the industry for “over-fishing” and are particularly concerned about the bycatch (unwanted haul) - especially endangered sharks, turtles and rays - which die on board or in the nets. An estimated 60,000 sharks alone are killed in this region each year.

The groups blame this terrible waste on fish aggregating devices (FADs), man-made rafts that attract tuna - along with many other fish - into nets that are up to 2km long. The FAD-using vessels are called purse seiners because their nets (or seines) close like a purse when the fish are caught.

However, Laurent Dagorn, at the French Institute of Research for Development, who studied 50 FADs and tagged sharks in the Indian Ocean in 2011, argues that the devices can be environmentally friendly if bycatch sharks are released quickly while still alive, and if nets are not set on small tuna schools that attract proportionately more sharks than larger schools.

Another method, involving the use of bait and stimuli such as lights to attract sharks away from FADs and nets, lured an average of half of all sharks to safety, he says. Dagorn also proposed nets with “escape panels” and non-meshed “eco-FADs” after his research showed that almost twice as many sharks died after becoming entangled in FADs outside the nets.

Greenpeace, however, does not want FADs to be used at all. Simon Clydesdale, Greenpeace UK’s tuna-fishing spokesman, says: “We welcome attempts to mitigate the effect of FADs, but even if they cut bycatch by 50 per cent that would still not be enough. The industry has to face the reality that consumers’ attitudes are changing.” Clydesdale says that all major UK supermarkets are committed to selling FAD-free tuna, but supplier John West says no supermarket has asked for all its tuna to be FAD-free.

At the heart of the issue is the industry’s struggle to meet growing global demand for tuna while attempting to maintain sustainable fish sources.

Tuna caught by traditional and sustainable “pole and line” fishing is increasingly popular with consumers but satisfies only about 5 per cent of the market. Twenty-nine per cent of John West’s tuna for the UK is now caught in this way and the company has pledged to source all its UK sales from pole and line andor FAD-free boats by the end of 2016.

Yet some environmentalists say FAD fishing is sustainable if regulated. “There is no need for boycotts, just good management,” says Paolo Bray, founder of the Dolphin Safe project and Friend of the Sea charity.

The FAD fisheries want to achieve certification from the internationally recognised Marine Stewardship Council but first they have to meet rigorous sustainability criteria. There are positive signs. Crew training has improved and bycatch has fallen in the past few years as “non-entangling” FADs have been introduced in the Indian Ocean - more than 1,200 last year alone, according to the industry.

But the debate is fragmented even over the percentage of bycatch. Industry sources say it is about 5 per cent, but the environmental lobby claims it is about 10 per cent.

Now there has been a call for independent professional observers to be allowed on all purse seiners, and Victor Restrepo, chair of the International Seafood Sustainability Foundation’s scientific advisory committee, wants “improvements” to fishing practices to be independently verified.

The industry has made progress in recent years, but the debate shows no sign of going away.

David Harrison is a freelance journalist and former environment editor of The Sunday Telegraph and The Observer. For more information visit these websites: www.iss-foundation.org; www.friendofthesea.org; www.msc.org; www.greenpeace.org; www.wwf.org.uk; www.john-west.co.uk; www.mwbrands.com

What else?

Explore the idea of recycling with ariommac’s PowerPoint with sound. bit.lyRecyclingPowerPoint

Investigate how energy sources affect the environment using tinaquick’s PowerPoint-based resource on renewable energy. bit.lyEnergySources

Get pupils researching the plight of endangered species and creating a podcast in a set of lessons from mcfi.

bit.lyEndangeredSpeciesLesson

Debate the pros and cons of sustainable fishing using this lesson plan from National Schools Partnership.

bit.lySustainableFishing

Top 10

Green issues

Try these resources on issues such as access to water, migration, development and the work of charities.

The human impact

Explore risk, vulnerability and the importance of tackling climate change.

Marine conservation

The Sea Watch Foundation’s resources tackle marine food chains, rubbish and pollution.

Water for all

An interactive resource featuring stunning images.

Sow the Seed project

This resource from Oxfam encourages active global citizenship.

Migration and development

A resource from the British Red Cross on the causes and effects of migration and development.

Chocolate trade game

Trace the journey of the cocoa bean from tree to chocolate bar in this game.

Natural disasters

An activity on flooding and how natural disasters affect development and resources.

Please wash your hands

This assembly on hygiene highlights the fact that in some parts of the world hand-washing is a matter of life and death.

Recycled wristbands

Instructions on how to make charity wrist bands using recycled plastic bands.

Charity assembly

Help pupils consider how other people live and survive and how they can support charities.

Find these resources and more at: bit.lyTESGreenTheme

Spotlight on...

A Green Mouse

Whether you are a teacher of MFL, art, English or chemistry, encourage children to learn new words and to develop a love of language.

TES partner A Green Mouse provides free resources for children to support their listening skills in French and Spanish, to get them saying the words aloud and to relate language learning to everyday life. All the resources can be used on tablets, computers, mobile devices and whiteboards, making them as accessible as possible.

Visit A Green Mouse’s profile on TES Resources to find out more: bit.lyAGreenMouse.