Here is an excerpt from a Guardian article that was published last year (by Tim Burrows) about Grimsby, England:

In Grimsby’s 1930s heyday, fishermen used to head to Freeman Street as soon as they were off the trawler, straight to the Lincoln or the Corporation Arms to spend their bountiful earnings. A century previously, Grimsby had been a fairly sleepy fishing village, but by the 1890s it was on the way to becoming the biggest fishing port in the world. In the mid 20th-century, trawlers were bringing in 500 tonnes of fish a day.

Today, Grimsby still has a thriving indoor market (paid for by the EU and the Enrolled Freemen of Grimsby, an organisation that dates back to the 13th century), but the further north towards the docks you walk, the emptier and more dilapidated things get. A local businessman says sex workers wait around at night for lorries to take them to the deserted docks. “It’s a legacy of the old fishing days.”

There is scant legacy to be found elsewhere. After a long decline, the fishing industry died in the mid 1980s, its owners selling their trawlers to companies in Aberdeen or Japan. Unlike Hull across the river, currently basking in its year as Capital of Culture, Grimsby is the Humber city that never was.

More than 70% of people in Grimsby, England voted to leave the European Union in the 2016 "Brexit" referendum. It was one of the highest shares in the country. But with one of the highest unemployment rates in the country, that outcome is not all that surprising.

Supposedly, at its peak, there were eight onshore jobs for every one at sea in Grimsby. And like all thriving cities, there were economies of agglomeration, which resulted in things like the largest ice factory in the world. The fishing fleets needed crushed ice -- and lots of it.

The Grimsby story is, of course, not a unique one. You just have to replace fishing with some other industry. Many cities have managed to diversify their economies either out of necessity or because they saw the writing on the wall. But for others it has been a real struggle.

It's one of those things that is perhaps simple, but far from easy.