One day in 2012, Nick Shields was sitting in a dive bar in Westport, Connecticut called the Black Duck when he made a simple observation that would change his life. "I saw five women order vodka sodas, one after the other," he recalls.

At the time, Shields was in the beer industry, managing a brand called Haffenreffer that his great-grandfather created in 1870. But in 2012 beer had started losing ground to wine and spirits. So as Shields sat at the bar, the vodka-swilling women became his muse to create something entirely new: a drink that was as light as vodka but rooted in beer-making methods. What emerged after plenty of trial and error was the first-ever spiked seltzer, made with fermented sugar rather than malted barley.

He named it, easily enough, SpikedSeltzer, and the rest is carbonated alcohol history: Sales skyrocketed, copycats followed, and Shields in 2016 sold the brand to Anheuser-Busch InBev, which took it national last summer.

This week SpikedSeltzer will pass another milestone as it unfurls its largest ad campaign, including a TV spot, digital ads and outdoor marketing featuring wrapped buses and trains. The effort makes liberal use of the brand's original mermaid logo, but adds a new tagline, "Follow Your Siren." The phrase is another way of saying follow your intuition, kind of like Shields did back in 2012. The agency is Bullish, which the brand hired late last year.

Ads take cues from 1920s-era art deco posters that were commonly used to plug travel destinations and Champagne brands. The animated 15-second TV spot shows a woman diving into a pool where she swims with a mermaid. U.K.-based artist David Doran created visuals for the campaign.

The campaign comes as SpikedSeltzer tries to beat back competition from a plethora of brands crowding the alcoholic seltzer category that Shields created.

SpikedSeltzer, while still growing strong, has been surpassed in sales by some of the copycats, including White Claw Hard Seltzer by Mark Anthony Brands, known for Mike's Hard Lemonade; and Truly Spiked & Sparkling by Sam Adams-maker Boston Beer Co., according to retail sales data from IRI. MillerCoors is also in the game with Henry's Hard Sparkling Water, which debuted early last year and is backed with a new national campaign that began this month.

SpikedSeltzer ranks 14th in the broader flavored malt beverage category with sales exceeding $25 million in the 52 weeks ending March 25, according to IRI, which does not include bar sales.

Shields still oversees the brand, which kept its Norwalk, Connecticut offices after the sale to AB InBev. Shields, who co-founded the brand with financial industry professional Dave Holmes, agreed to the deal because the brewer brought distribution and marketing resources that SpikedSeltzer could never gain on its own, he says. He also likely made a lot of money with the sale, although he would not disclose figures.

AB InBev took the brand national last year. It now has 500 wholesalers, up from 30 when SpikedSeltzer was independent. But there has been a learning curve. "Of the 470 wholesalers that were added, quite a few are suspicious about any new-fangled product and they were dipping their toes in the water last summer," Shields says. "Now the training wheels are off and it's no holds barred…because the category itself is growing leaps and bounds."

The brand is staking a premium positioning with fancy flavors like West Indies Lime, Indian River Grapefruit and Cape Cod Cranberry. It checks in at 6 percent alcohol-by-volume, compared to the 4.2 percent of an average light beer.

Shields, who studied fermentation science at the University of California, Davis, went through nearly 100 batches before perfecting the formula. While most beer is made with malted barley, the seltzer starts with sugar, which is fermented and infused with cold-pressed citrus essence from a variety of fruits. A 12-ounce can has 140 calories. By comparison, a Bud Light has 110 calories. About 70 percent of buyers are women.

From the beginning, Shields says he knew he wanted to give the brand a nautical look. "At first we thought about Neptune [but] he seemed to be too aggressive," he says. "We wanted the look to be feminine and strong and a powerful mystical feature. So the mermaid, she fit." Signaling the brand's aggressive ambitions, he pointed to Starbucks, which also uses a mermaid logo. "I would love to ride their coattails," he says.

Correction: Due to incorrect information provided to Ad Age, a previous version of the story stated the campaign was modeled after 1940s-era art deco posters. The correct era is 1920s.