The pair were diving in Bravo Crater, a basin 75m (246ft) deep and 1.5km (0.9 miles) wide in the north of the island chain. The water column there is relatively low in radiation, with amounts comparable to background levels in most of the world. But the sediment on the bottom tells another story – to this day, it has high concentrations of radioactive plutonium, americium and bismuth, higher than anywhere else in the Marshall Islands. This is where, on the morning of 1 March 1954, the US conducted its largest ever thermonuclear test.
Over six decades later, Palumbi and his colleague were awed by what they saw. The centre of the crater is still relatively barren, with just a thick layer of silt. But at the edges, they found a hidden refuge, where rainbow shoals of small fish circled boulder corals the size of small cars, and the distinctive torpedo-like forms of blacktip and grey reef sharks were omnipresent.
“It’s mind-blowing,” says Palumbi. Despite battling the effects of radiation, which is thought to have created a population of mutant sharks missing their second dorsal fins, the reef was very much alive. And the fish were giants – at least, compared to those you would find in places that are regularly plundered for their fish.
This is the most obvious consequence of abandoning fishing – there would be more fish, and they would be much bigger than modern generations are used to.
A rapid response
Back in March 2006, George W Bush – the then-US president – was watching television at the White House. According to popular rumour, on the programme that day was a PBS documentary about the Northwest Hawaiian Islands, a remote archipelago in the Pacific. He was apparently so enchanted, that he immediately began looking into ways to protect them. With the help of an obscure, century-old law, he created the Papahānaumokuākea Marine National Monument – now the largest marine conservation area in the world.
Unlike vast expanses of other marine protected areas, which still allow fishing – no-take zones represent just a fifth of this category – the new reserve imposed a total ban.
The impact was almost immediate. “We started to see effects after about one and a half years,” says John Lynham, a professor of economics at the University of Hawaiʻi who specialises in ocean recovery. There was more marine life around overall, with the speediest recoveries from species that were previously harvested the most heavily, he says. Amazingly, yellowfin and bigeye tuna were among the first to respond – although they’re apex predators and adults average at least 6ft (1.8m) in length, they’re fast-growing.
Like at Bikini Atoll, other notable reprises have been total accidents. Take the advent of World War Two in September 1939. For the next six years, the North Sea was almost entirely devoid of fishing. With large, sturdy designs and clear, open decks, fishing trawlers were relatively easy to convert into minesweepers – warships that scoured the oceans for mines and discharged them. Along with the dangers posed by mines, warships and bombing to civilian fleets, this meant there were very few active fishing vessels for the entire duration of the war.