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  (Photo: Reuters/Carlo Allegri)
  People dressed in green watch the 251st annual St. Patrick's Day Parade in New York March 17, 2012.

Vegetarians and vegans alike have balked at the realization that Guinness sometimes contains trace amounts of fish bladders on St. Patrick's Day this year.

The thick, creamy Irish stout joins corned beef and cabbage as staples during annual St. Patrick's Day celebrations. However, this year, a blog revealed the ingredients used to make Guinness and included isinglass- a part of a fish's bladder.

While only tiny amounts of isinglass can be found in Guinness, the substance is a form of collagen culled from a dried swim bladder- an internal fish organ that helps regulate buoyancy in water, according Smithsonian.com's Food&Think blog.

When it comes to the beer, isinglass has long been used to quicken the process of removing any unwanted leftovers such as solid particles from the brewing process, called fining.

The use of isinglass as a fining agent has come as a shock to many, but Guinness is not the only beverage to contain the fish bladders. Furthermore, as many other beers and wines use gelatin instead of isinglass; those beverages are not vegan either.

On Twitter, St. Patrick's Day revelers and beer drinkers posted reactions to Guinness' use of isinglass.

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"Didn't want to know that isinglass is found in Guinness… gross," wrote Amber.

Becky added, "To those who don't know, Guinness is filtered through isinglass, which is a fish's bladder. Enjoy your fishy drink."

Although some beer enthusiasts may be deterred from the dark traditional beer because of isinglass, it has been a part of Guinness' brewing process since the 1800s.

Last year, Guinness responded to vegan and vegetarian's reactions to the use of isinglass and defended their use of the fining agent. While acknowledging that minor traces of isinglass may end up in the finished product, the agent is the best suited to the fining process.

"As part of our ongoing efforts to further improve the way we make our products, we are seeking alternatives to isinglass," Guinness said during an email correspondence with a wine and spirits website, Barnivore. "To date, we have not found any alternative that is as effective and as environmentally friendly as isinglass for our product."