Gliding through Venice, its brocaded velvet seats occupied by a sullen pair of tourists, the boat is almost everything a gondola should be: black, sleek and gleaming, with a genial man in stripes rowing it expertly to the canal-bank.

Just one thing is missing from this quintessentially Venetian scene, and while it is passes unnoticed by most visitors it is an absence that aficionados see as a cruel blow to the city’s heritage.

On the stern, where there should be a curved piece of iron recalling the skilled movement of the gondolier’s oar – or, say some romantics, the shape of a lion’s mane – there is nothing. “Shall I put it back on?” asks Stefano, the gondolier, bending down to pick the iron stern ornament up from where it is lying, discarded, beside the seats. “This morning there was acqua alta [high water] and I had to take it off,” he says. “It’s a necessity.”

A glance at the hundreds of gondolas ferrying tourists through La Serenissima reveals that Stefano, who would not give his surname, is far from alone in his abandonment of the ferro di poppa. As the acque alte occur more frequently, largely due to rising sea levels, gondoliers are having more and more trouble getting their boats under bridges – and, as the highest part of the boat, the stern iron is becoming increasingly problematic.

“I take it off and put it back on again,” says gondolier Vittorio Manfré, 56, whose stern is properly adorned with what is known as a risso (curl). “It’s right that gondolas should be like this, but we also have to find ways of being able to work. [High water] is happening much more often and the water is coming much higher than it used to.”

According to the Venice municipality’s lengthy regulation governing the use and appearance of gondolas, a steel risso is an essential feature. The only compromise “allowed but not recommended” is to have a hinged device so the gondolier, “exclusively” when rowing beneath a particularly low bridge in particularly high water, can flip it down. To keep within the regulations, it must be flipped back up again immediately to restore Venetian dignity.

That, however, is clearly not being done by a large number of gondoliers. Saverio Pastor, from the El Felze association of gondola-making artisans, says that while 10 years ago some gondoliers used a hinge-operated risso, “almost all” now chosen the more drastic option of taking them off altogether.

When Pastor sees a gondola without a risso, “I see a beauty which has been spoiled. Because [the stern] is the most beautiful part of the gondola,” he says. For him, the removal of the risso altogether is an aberration, but he also has sympathy with those who must contend with the changes in water levels.

“It comes down to a serious problem of life in the city,” he says.

The increased frequency of high water incidents is clear: according to figures on the Venice city council’s website, there have been 125 acque alte this year, seven of them reaching more than 110cm above normal sea level. Somewhat unusually, they continued throughout the summer months.

In 1983 there were 35, with only one reaching over 110cm. In 1993 there were 44. Last year there were 156.

Last month the local newspaper La Nuova Venezia reported that, faced with a widespread flouting of the regulations, city police had begun handing out the first fines for gondoliers breaking the rules on rissos. The city police did not respond to the Guardian’s request for confirmation.

But for Stefano, a gondolier for 13 years, a fine would be ludicrously unfair. “We take it off when we have to,” he says. “It’s absurd this thing.”