The toxic algae bloom that has carved a trail of dead animals and triggered a putrid stench along western Florida's coastline has drifted further north, killing hundreds of thousands of fish in the Tampa Bay region.

The legions of dead fish were reported in a 20 mile (32 kilometre) stretch of coastline from Clearwater to St. Petersburg, environmental officials with Pinellas County told the Tampa Bay Times on Saturday.

County workers roamed beaches and trawled offshore to collect the fish carcasses to head off decomposition as some beachgoers turned back.

Rotting fish and the strong odor of the algae has previously repelled locals and imperiled Florida's vital tourism sector for much of the summer.

The toxic algae has claimed countless fish, hundreds of sea turtles, dozens of bottlenose dolphins and even a 26 foot (8 metre) whale shark in the past few months.

The toxic algae stretches in varied density for about 120 miles (190 kilometres) of coastline, the Florida Fish and Wildlife Conservation Commission said.

In August, Governor Rick Scott declared a state of emergency and released funds to help with the massive cleanup effort and help businesses recover from lost profits.

The algae has affected the coast in some way for 10 months - and has become a key political issue in the midterms for Scott, a US Senate hopeful.

A red tide is a natural phenomenon that develops miles offshore before making its way to the coast, where it feeds on a variety of pollutants, including phosphorus and nitrogen from fertilizer, along with other runoff and wastewater.

The toxins can aerosolize in the wind that drifts ashore, triggering respiratory problems or worsening conditions such as asthma.

What is not clear is whether climate change and pollution from humans near the shore has made this outbreak severe and prolonged. Scientists have found that the algae thrive in warmer waters and increased carbon dioxide levels.

Until this past week, the red tide lurked south of Tampa Bay, the Times reported. But samples of high concentration of the algae have been found in waters near Clearwater Beach in the past few days.

(Chris O'Meara/Associated Press)(Chris O'Meara/Associated Press)

The sudden approach of the algal bloom and dead fish washing ashore surprised beachgoers on Saturday. Andres and Veronica Bernal told the Times that they had checked county websites for alerts before leaving Tampa in the afternoon.

Their two children were horrified to see dead fish littered on the beach. They opted to play in the sand instead as the smell of rotting fish lingered.

Scientists are trying to figure out why, exactly, the current red tide along the Gulf Coast has been so protracted and deadly to wildlife. State officials and scientists point out that, at base, this is a natural phenomenon.

Fish die-offs were noted by Spanish explorers in the 1500s and have been well documented since the 1840s.

The crisis has become a political issue in the upcoming midterms as Scott challenges Senator Bill Nelson for his seat. Both men have blamed the prolonged crisis and delayed responses on each other.

They have also attacked each other over the severity of a different type of algae that is choking rivers and plaguing Lake Okeechobee, the state's largest freshwater lake.

Darryl Fears, Lori Rozsa, Joel Achenbach and Kate Furby contributed to this report.

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