Bringing fresh attention to its blockade of the Gaza Strip, Israel on Tuesday turned back a boat attempting to deliver three tons of medical supplies to Gazans.

After a radio message asking the small ferry to turn back was ignored, the Israeli Navy boarded the boat and redirected the vessel to the Israeli port of Ashdod. Reuters quoted a police source as saying that the activists aboard, members of the US-based Free Gaza movement, would "likely be deported."

"Yesterday evening the Israeli Navy contacted the boat while at sea clarifying that it would not be permitted to enter Gaza coastal waters because of security risks in the area, and the existing naval blockade," the Israeli military said in a statement, adding that humanitarian aid would be sent to Gaza "subject to authorization."

The naval blockade – part of a wider Israeli effort to seal off the tiny coastal strip controlled by the Islamist militant group Hamas – not only prevents such shipments, it is also devastating a key Gazan industry and source of food: fishing.

Citing security concerns and fears of arms smuggling, Israel has progressively tightened the blockade over the past 15 years. Once a thriving enterprise, Gaza's fishing industry is now on the verge of collapse. Fishermen are cut off from the heavily populated shoals, and have seen total revenue drop by half in less than a decade.

"We are witnessing a huge crisis where the livelihoods of thousands of fishermen, associated laborers, and their dependents have been decimated by Israel's blockade and closure," says Erminio Sacco of the United Nations' Food and Agriculture Organization (FAO).

Following the Oslo peace accords, signed in 1994 between Israel and the Palestinian Authority (PA), Israel permitted the fishermen to go 20 nautical miles (NM) out to sea.

This was restricted to 12 NM in 2002, after the outbreak of the second Palestinian intifada in 2000.

This area was further limited to the current 3 NM when the Islamic movement Hamas wrested control of Gaza after an intense fight with its rival Fatah led to a collapse of a unity government headed by Western-backed Palestinian Authority President Mahmoud Abbas.

Those fishermen who go further out risk being arrested, shot, and killed, or having their boats destroyed or confiscated. However, human rights organizations have reported that fishermen have been attacked even within the 3-NM zone.

Click here to read about fisherman Mohammed Hassuna, who says he and his crew were recently surrounded by Israeli Navy boats, shot at, forced to strip, and swim in frigid water to the Navy gunboat, where they were handcuffed, blindfolded, and their feet chained.

Catch dropped by two-thirds since 2007

According to the UN Office for the Coordination of Humanitarian Affairs (OCHA), Israel's restrictions undermined the sardine season, which started in March and peaked in mid-April.

The bulk of sardines are located beyond six NM, with the UN estimating that a distance of 12-15 NM off Gaza is the minimum required to access the larger shoals of fish for maximum economic benefit. Shoals closer to shore have been depleted and unable to replenish themselves.

"During March 2007, 248 metric tons of fish were caught. In March 2008 this figure dropped to 121 tons and in March this year, the catch was only 89 tons," says Mr. Sacco.

A total annual catch of 2,700 tons was caught in 2008, down from nearly 4,000 tons in 1999, according to Gaza's General Syndicate of Marine Fishers.

OCHA states that at the end of the 1990s, Gaza's fishing industry was worth about $10 million annually. This represented approximately 4 percent of the Palestinian economy.

Nezar Ayyash, from Gaza's fishing syndicate, which has 3,500 registered fishermen, says this figure was halved between 2001 and 2006.

"It has become too expensive for many fishermen to take the bigger boats out to sea, so only some smaller boats venture out," says Sacco.

The cost of one fishing trip can vary between $125 and $625, depending on the size of the vessel, nets, and crew; many fishermen cannot cover these costs.

Fishing employed 45,000 Gazans

About 45,000 Gazans once worked in fishing and its associated industries, including repairs, onshore support, or as merchants.

With Gazans having an average family size of seven, the fishing industry used to help support many times more of Gaza's 1.4 million residents. It also supplemented a diet critically short of animal protein.

Gaza faces chronic unemployment, poverty, and malnutrition in part as a result of Israel's blockade, which now allows only food and medicine – but not as much as aid workers say is necessary to sustain the population. Everything from pasta to catheters have been turned back, frustrating aid workers who have been unable to obtain a list of permitted items. A ban on steel and cement, which Israel says can be used to fortify tunnels along the Gaza border that are used for smuggling, has prevented many Gazans from rebuilding after the war, with some resorting to mud bricks for their homes.