Frigid weather was largely to blame for making 2011 the second-deadliest year on record for Florida’s endangered manatees.

Of the 453 dead manatees recovered in state waters, state wildlife biologists determined that just over a quarter of them were killed by “cold stress.’’ It was the third year in a row that bad weather helped drive up the annual death total.

In 2010, a record high 766 manatees were found dead, with at least 282 killed by cold water. In 2009, 429 carcasses were found, with 56 deaths blamed on cold weather.

That’s a troubling jump from a five-year average of 30 cold-related deaths, said Gil McRae, director of the Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute.

“We are concerned about the number of manatee deaths the past three years, including those resulting from exposure to cold weather,” said McRae. “Over the next few years, we will use data from monitoring programs to better understand any long-term implications for the population.’’

The number killed by boats, 88, was slightly up from last year but close to recent averages and lower than the all-time high of 97 set in 2009.

Manatee deaths have risen sharply over the past decade, but so has the population. Last year, state biologists conducting an annual aerial survey tallied more than 5,000 of them, a record high ironically caused by a cold front that had corralled much of the population around power plant outfalls and springs that offer warm water refuges for the temperature-sensitive sea cows.

Pat Rose, executive director of the Save The Manatee Club, credited protection measures mandated by state and federal wildlife agencies for keeping the death toll from being even worse during a near-record cold snap early in the year.

When Florida Power & Light decided to upgrade an aging power plant in Brevard County, for instance, the utility was required to install some $5 million in heating equipment to help maintain a warm-water refuge in the Indian River Lagoon near Titusville that draws hundreds of manatees every year. The temporary system, paid for by utility customers, will be in place until the new plant comes online in three years.

Despite a crackdown on the state’s million-plus boaters with slow and no-entry zones, deaths from watercraft strikes have continued to trend upwards and remain the biggest long-term threat to the survival of the slow-moving sea cows, said Rose. The 2011 total was up slightly from the previous year’s 83, but lower than several years that topped 90 over the last decade.

“It’s too high and it’s still very bad for the population but it’s not grossly accelerating,’’ Rose said.

Lee County in Southwest Florida accounted for the highest number of boat deaths, with 14. Brevard County and Pinellas County tied for second with seven boat deaths. Miami-Dade and Broward both recorded two boat-strike deaths. Monroe County had five. Brevard led for overall manatee deaths with 100, including 28 from cold stress.