Harassed by Israeli gunboats, Palestinian fishermen are chasing dwindling shoals in the heavily guarded six-mile fishing limit off the Gaza coast. Alex Renton goes aboard to see what it's like to be a fisherman in some of the world's most contested waters

Sometimes the sea calms at sunset: and so it is here in the bottom corner of the eastern Mediterranean. The little fishing boat has been tugging on its anchor rope like an excited puppy. But now, as the waves ease, the deck steadies.

We're going to catch sardines. It's an all-night trip. So in the afternoon we'd loaded the boat with £190-worth of diesel, sweaters, lots of cigarettes, water, pitta bread and some nuts and dates. That's it, I asked? No VHF radio? No safety equipment? No lifejackets?

The fishermen laughed. "Lifejacket? In Gaza, there's no need for a lifejacket!" That's the kind of joke that goes down well here. Skipper Abu Nayim, big, sun-dried, smiling, sniffed the wind. "It's from the north. Good for sardines," he said. "Yalla! Let's go."

Now we are waiting. The generator chatters, powering a string of arc lights hung around the boat. These will dazzle and seduce the fish; in six hours we'll drop a net to gather them in from under the keel. The crew relax on salt-crusted rugs – eating dates, teasing each other, smoking. Two of the men step up to the foredeck to prostrate themselves and pray.

It's nearly dark. A couple of Israeli F-16 jets make twin scratches across the glow in the southeast, above the Egyptian border. "They own all the world," mutters old Abu Nayim. But, for now, this feels like the most peaceful place you could find on this crowded coast, where there live some of the most disputatious people in the planet. There's not much to tell you that this is a very risky way to catch fish.

Under fire: an Israeli gunboat fires on a Palestinian fishing boat/ Photograph: Gianluca Panella for the Observer

Then, as night closes in, trouble starts. First to the north: the machine gunfire is hardly audible to me, but the fishermen give each other a look. "It's routine," one says to reassure me. "The Jews, always making a noise!" says Abu Nayim, his tone more bored than aggrieved. But his son, 22-year-old Mukhtar, is more anxious. He was on the boat on Monday when the Israeli navy put four bullets into the fibre-glass tender – a felucca – that's bobbing behind us. He warns Gianluca, the photographer, who has his telephoto lens out – "If they see you poking that out, they will shoot at you."

Mukhtar points towards the orange lights of the Israeli city of Ashkelon, 10 miles away. "There, you can see the Israeli gunboat. It's circling the fishing boat." There's another crackle of gunfire. I can't see much. We settle back on to the deck. The fishermen try their mobile phones, to find out what's happened. But, though we're just five miles from the shore, there's no signal. They think the gunboats may have some way of blocking it.

Now it is fully dark, but the sea around us is a cartoon royal blue, turned to daylight by the arclights. Little flashes of silver appear on the surface. Baby sardines – the outliers of the big shoal already turning in the water below. I count 14 more boats, all spread in a chain along the sea, carefully short of the six-mile limit that the Israelis have imposed.

They bob at anchor, their lights festive against the dark ocean. We're close to the limit, as close as Abu Nayim dares, because the further out to sea the more sardines there are. "To go over six miles is death," says Abu Nayim. Ideally he would be out at around 11 miles, in early November, with the last flush of the sardine season on. But Israel has not permitted any Gazan boat beyond six miles since 2006. Once, when he may have drifted over, an Israeli gunboat tossed a live grenade on to the nets. In the past month the fishing boats of Gaza have come under fire 10 times.

Half an hour later, we hear the blat-blat-blat of heavy machine-gun fire. It sounds very close. I jump up from the deck and then drop down again, in case it is us being fired at. Not that the flaky fibreglass of the hull would do anything to stop a bullet. But we see that the object of the Israeli fire is the boat next door to us in the line, Captain Nihad's. It's about 300 yards away, maybe a little further out. There's a roaring of engines. The fishing boat's tiara of brilliant lights is lurching crazily.

Under the stars: Abu Nayim rests while the bright arc lights around his boat dazzle the sardine shoals below. Photograph: Gianluca Panella for the Observer

I look at Mukhtar's handheld GPS and check it against the map app on my phone. Both agree: we're 5.7 nautical miles northwest of Gaza City. Nowhere near the border zone, and within the six mile limit.

There's another one-second blast of machine-gun fire. I can see the Israeli boat, so close to the fishing boat that its huge bow-wave gleams white in the lights. It's circling the fishermen at high speed, the wake throwing the fishing boat around. There's shouting, through a megaphone. "The Israelis are insulting them," says my interpreter. He doesn't want to say the words. "They curse the prophet. They call the fishermen 'son of a dog'. Tell them to go back to Palestine." Another burst of gunfire. We wonder if we'll be next. Everyone in the crew, even the two boys, takes time to tell me not to be frightened. I've already informed them I am a bad swimmer and I don't want to take my clothes off – if the Israelis decide to arrest us, we'll be ordered to strip naked and swim to their boat.

We watch our neighbours move 200m away from the limit line. Closer to us. This seems to satisfy the gunboat, which moves off. Is that it? Probably, until the next patrol boat's shift starts. "But now Nihad is too close to us, their light will attract our fish," says Abu Nayim.

How does he feel about the patrol boat? "I don't let it get to me, I'm used to it. But Nihad will have lost the fish under his lights."

At midnight, the moon is high above us. The crew awake and someone brews up coffee. Then Abu Nayim takes the felucca off with a couple of lights to keep the fish at the surface while we lay 100m of weighted seine net around it. As soon as it's deployed we will start to pull it in. Our lights are switched off, and I can see Orion spread against the sky. Here it's not the Hunter, but the Seeker.

Now Abu Nayim stands on the foredeck of the tender, shouting encouragement. "Pull! Pull! Keep it flat!" All the crew are leaning over the side and the gunwale is nearly under. They chant a prayer to the Prophet to fill the net.

Caught out: fisherman Abu Nayim with his son Khaled, and baby grandson Mohammed. Photograph: Gianluca Panella for the Observer

After 30 minutes of heaving, the catch comes in. It is a disappointment. The sardines and baby squid picked from the bottom of it would hardly fill two garden buckets. Abu Nayim's sunny face has turned old and tired. The lights go on again for another try. Mohammed, Abu Nayim's adopted son, brews us all coffee. "This is the worst since we went to Egypt," he grumbles. "We caught no fish at all, then. We were chased away by the Egyptian navy boats, they fired small rockets at us. It cost us 4,000 shekels (£700). " Mohammed is 20. He has a baby son and his wife is pregnant. He's lucky, he says, that he has a Saturday job as a waiter to fall back on. There will be no profit from this trip.

Dawn breaks at 5am; we surf home over an oily grey swell. It is a race to get into Gaza City's port: the earlier boats will catch a better spot in the fish market. But the boys can see no one has caught much. For 14 hours' work, we have two boxes of squid and a bucket of sardines.

"Perhaps 50 shekels [£9]," says Abu Nayim, as we take the boxes of squid up to the seafront market. "Often we used to land 200 boxes of sardines, in the top of the season. 15kg each box." He makes a face – he has seven crewmen's work and the £190 fuel bill to cover. On the pavement the boats lay out their catch for the merchants to bid on. The boxes of silvery sardines, of little tuna and squid lie along the pavement for 20 yards. "Once they were piled five high, and they were on both sides of the road," says Abu Nayim.

"A fisherman does not feel sad when he doesn't catch," he says, as we turn towards his house. "Because one day he will." "Fil mishmish," mutters young Mohammed, overhearing. It means, literally, "When the apricots arrive"; "In your dreams." A favourite Gaza expression.

The bucket of sardines goes home with Abu Nayim. There are six sons, all married, all living with Abu Nayim and his wife Naima. With the grandchildren, there are 30 people to feed.

Not so long AGO, Gaza had a thriving fishing industry. In 1994 the Oslo peace accords with Israel granted the Palestinian enclave, which was formed largely by refugees from the 1948 war that followed the founding of Israel, rights to fish up to 20 nautical miles offshore. That supported a fishing industry, according to a study done by the United Nations Food and Agriculture Organisation (FAO), of some 4,000 boat-owning families. In 2004 they landed nearly 3,000 tonnes of fish. It was crucial to the nutrition of the 1.7 million people of the Gaza Strip, more than half of whom were dependent on food aid, even then.

In 2005 Israel, which had conquered the Strip in 1967, withdrew from the Strip unilaterally. But, under international law and in the eyes of the UN and Britain, Israel's occupation continues and so does its legal responsibility for the land and its people. In 2006 the radical group Hamas, which is a declared enemy of Israel, took control of Gaza: cross-border attacks on Israel escalated. In retaliation Israel stopped the flow of Gazan workers over the border and started what is in effect a blockade of the Gaza Strip that continues to today.

Israel unilaterally reduced the fishing zone to six miles offshore, with a further no-go zone along in border areas. This cut off 60% of the fishing grounds – the best waters, where most of the sardines, the mackerel and the valuable bottom fish lie, according to the FAO. In 2008, after Israel invaded the Gaza Strip in retaliation for rocket attacks, the fishing limit was reduced to just three miles; it was returned to six miles a year ago, but Israel periodically changes it back to three miles, usually in reaction to a violation of the ceasefire deal. Human rights groups and NGOs such as Oxfam point out that there is no connection between the fishermen and the militants who fire the rockets – and that "collective punishment" of a people is an infringement of the Geneva Conventions.

Three miles or six, the industry's collapse was inevitable. Fishing inshore is poor, and there's an added danger because of Gaza's failed sewage system. That was built to serve 400,000 people, and it has collapsed because of war damage and lack of materials to maintain it. Eighty-nine million litres of raw or partially treated waste water go straight into the sea every day. Last year the fishermen's catch was less than half what it had been 10 years before. They are now the poorest section of society, according to the UN, with 95% of fishing families dependent on food handouts. Oxfam runs the sort of food-voucher and job-creation programmes normally seen in famine zones. It says Israel has a duty to allow fishing to the 20-mile limit – and to stop using "excessive force" to patrol the zone.

Fishing is a harsh trade at any time, but here it is made rather more risky by the Israeli navy. As I witnessed, it makes its own arbitrary rules about the fishing zone, and exacts savage punishments for those who break them. The warning fire we heard and saw is usual, even for boats well within the limit. Sometimes the fishing boats will be water-cannoned with sewage, and the gunfire directed at generators, outboard engines and tenders. Four days after our trip last month, Gazan fishermen had their gear confiscated when fishing, by their account, just 1.5 miles off the beach. Three days before we went to sea a 23-year-old was injured by shrapnel at three miles, after a gunboat had fired at his engine.

Last year Abu Nayim's son Hassan, 35, was ordered by the Israelis to stop fishing immediately and leave his nets and tenders: when he came back later the boats and outboard engines had been shot to pieces and the net was gone. The damage cost $2,000 to put right. Hassan says he was just three miles out, in a spot where the patrol had previously let him fish without complaint. "I said: 'I have children to feed!' The Israeli said: 'Go to Gaza, let Hamas feed them.' Why do they do it? Are you a threat? I asked Hassan. "They don't like us and what they do is part of the siege of Gaza."

Arrests are common. Often fishermen will be taken to Israeli ports, blindfolded and handcuffed and questioned under "aggressive interrogation" (the UN's phrase) and then, after what may be several days' detention, charged a fee to be transported back to the border. Fishing boats and gear have been impounded for months and sometimes taken to pieces. Another fee – usually 1,000 shekels (£175) – is charged for their return. No fisherman has ever been charged with an offence – and no legal challenge for compensation for loss of earnings has ever had any success.

There are injuries, of course. A count is kept by Al Mezan, a Gaza-based human rights NGO that logs infringements of law and damage to property (it is trusted by the UN and other humanitarian agencies, who cross-check the data). In the year to November 2013, Al Mezan reported 136 Israeli attacks against fishermen in Gaza's waters. These injured eight fishermen, including a child. In 2013 Israeli forces arrested 18 fishermen, confiscated seven boats, and damaged or destroyed fishing equipment on 23 separate occasions. At least five Palestinian fishermen, all judged "non-combatants" by the UN, have died at sea in confrontations with the Israeli gunboats since 2010.

Israel is under constant threat from terrorist attack. But it is hard to see what good the harassment of the Gaza fisherfolk can serve. They are generally unpolitical, and, like most Gazans, far from fervent supporters of Hamas (only 20% of Palestinians are, according to polls). While the sea holds obvious opportunities for arms smugglers or terrorist attacks, there is no recent record of any Gaza boat having been found carrying guns or terrorists.

I put some of these issues to the Israeli Defence Force, who fixed up a briefing with "a senior naval officer". He was adamant that the policy was to enable fishermen to do their work, so long as they didn't cross the lines. If they did cross, then using live fire was justified, given proven security issues. But why were boats inside the six-mile zone harassed and fired at, even when at anchor? He told me the patrol boats would not approach any "unsuspicious" boats on the right side of the line. And the policy worked: it had foiled terrorist attacks. I asked for a recent example. "If you couldn't find one, then we are on the right track!" On the events of my night with Abu Nayim, and the arrests of other fishermen that week, the officer said he could not go into operational specifics.

When you ask Gazans what the Israeli motive is for this or that puzzling aspect of the six-year-old blockade of the Strip – the near-total ban on cement, for example – people usually just shrug and laugh. "Who can ever understand?" But speculation on the fishing is that the rich waters beyond the six-mile limit may now be being used by Israeli boats, though none have been seen, and I could get no view on this possibility from the UN.

Whatever the rationale, the policy crushes a people who have made their living from the rich Mediterranean coast for centuries. Abu Nayim knows his great-grandfather was a fisherman and has no reason to doubt that his great-grandfather was, too. They lived just north of the wall that now separates the Strip from Israel, at a place called Errabiya.

Now 60, Abu Nayim was born in Gaza, in the Beach Camp set up for the 1948 refugees. He still lives there. But when he was young Israel allowed Gazans to leave the Strip to work. From the age of 12 he and his father used to go back to their village, paying a fee to the Israelis who had taken their house and land to be allowed to fish. They long-lined from a row-boat they built. The families became friends. They attended each others' weddings.

"For me, it was hard, going there," Abu Nayim says. "But it was good to feel that the benefit of my family's land was going into my pocket." But in 2006, with the arrival of Hamas and the kidnap of Israeli soldier Gilad Shalit, the border was closed. For the people of Gaza, then numbering 1.5 million, normal life ended. Not since they were children had any of the young men I went fishing with left the "prison" – that's David Cameron's term.

Breakfast at Abu Nayim's house was truly great. Hunger helped. As we men sat on sofas drinking coffee, the women baked fresh pitta. Then they fried last night's sardines with chilli and garlic. (Note for foodies – Naima told me she first rolled them in dried dill, powdered coriander and cumin, then in flour; before frying them fast in very hot vegetable oil.) We ate the fish with our fingers, tearing the flesh from the backbones with our teeth. The bread we dipped into a typical Gazan salad, a mix of chopped tomato, tahini, lemon and parsley.

After the food, we talked – about fishing, children, jobs and, of course, Israel's blockade of Gaza. I said I was shocked to see how little had changed since I last visited the Beach Camp in 2008, just before the Israel Cast Lead invasion, mounted to stop the rocket attacks. That terrible time brought around 1,200 Palestinian deaths and 13 Israeli ones. It devastated Gaza's infrastructure.

Things are much worse, everyone agreed. Fewer jobs. More "unhappy people". Gaza is now undergoing power cuts of 12 hours on most days. Supplies of fuel and much else had been smuggled in through tunnels from Egypt, but those had mainly been closed since the Muslim Brotherhood regime fell in July and the Egyptian army took over. The price of diesel had doubled. Abu Nayim said he and his sons were worried that they would not be able to pay back the money they had borrowed to put the new lighting rig on the boat.

"You're from Britain," says the eldest son, Nayim. "It's your fault. You invited the Jews to come from Europe to here, to take our land." I demurred: it was more complicated than that. "What about the Balfour declaration?" he asked – referring to the note signed in 1917 by Britain's foreign secretary AJ Balfour, declaring that Britain favoured the establishment of a Jewish state in Palestine. I thought about telling him that Balfour was my grandmother's uncle, and that my own great-uncle fought Israeli terrorists in Jerusalem in 1947, as a British policeman. But what I said was: "It's true, we are part of the history. We are responsible, too."

Abu Nayim's eyes are red from all the salt and cigarette smoke, and I can feel his longing for his bed. We thank him and Naima for their hospitality and the breakfast. "You like the sardines?" asks Mohammed, as we say our goodbyes. "Here we say they are very good for the sex." Everyone laughs. The women are not in the room. "In my country, we say oily fish are good for the brain," I say. "In Gaza," says young Mohammed, "the brain is not important." Everyone roars. It's a very Gazan joke.