NEW ORLEANS—Four conservation groups say they'll sue the National Marine Fisheries Service because it's taking too long to analyze shrimping's effects on threatened and endangered sea turtles.

The groups contend that shrimp nets in the Gulf of Mexico kill more than 45,000 sea turtles a year and want the agency to bar shrimp trawling there until it completes its analysis.

They also want to make Louisiana enforce federal rules requiring turtle escape hatches in shrimp nets, which shrimpers say let much of their catch escape as well. State law now bars agencies from doing so.

The letter sent Wednesday began a 60-day settlement period required before suing under the Endangered Species Act.

The division of the National Oceanic and Atmospheric Administration doesn't comment about pending or active litigation, spokeswoman Allison Garrett said in an email. She said the analysis is underway.

"NOAA has known there is a problem with the number of sea turtles stranding and being killed in the Gulf since the spring of 2010," said Eric Bilsky, an attorney with Oceana, one of the groups involved. "And so, when we are coming on the spring of 2014, 'We're working on it' is not enough.

"We need to know when it's going to get done and we need it done soon. And we need the analysis not just to be an analysis; we need to know what specific steps NOAA is going to take to make sure that sea turtles are not getting killed anymore."

Reports of turtle strandings increased from fewer than 100 a year from 2002 through 2009 to about 600 in 2010 and hundreds a year since. Last year 545 were reported through Aug. 25, according to the NOAA Fisheries website. However, because a closer watch has been kept since the Deepwater Horizon oil spill in 2010, the significance of the numbers is unclear.

The Center for Biological Diversity, the Turtle Island Restoration Network and the Sea Turtle Conservancy joined the letter of intent sent Wednesday to Commerce Secretary Penny Pritzker and three NOAA officials.

Clint Guidry, president of the Louisiana Shrimp Association, responded by email: "Two words ... 'NOT GUILTY.' "

He said shrimpers do all they can to protect sea turtles without killing their own business.

In a telephone interview, Guidry cited a June 2013 report for the Gulf States Marine Fisheries Commission—which regulates fishing in the Gulf—about Kemp's ridley sea turtles, the bulk of those found dead in shallow waters since the 2010 spill.

The report said that although the number of nests at the turtles' main nesting beaches fell 35 percent from 2009 to 2010, when 12,377 were counted, they rebounded to preliminary counts of 19,368 in 2011 and 20,197 in 2012.

"Whatever we're doing is working," Guidry said. "The turtle populations are coming up unbelievably."

But Bilsky said, "There shouldn't be hundreds of sea turtles washing to shore if what we're trying to do is catch and eat shrimp. That's just wrong."

Kemp's ridleys had been nearly wiped out by the mid-1980s, when fewer than 1,000 nests were laid each year. All six sea turtle species found in U.S. waters are endangered or threatened.

The escape hatches called turtle excluder devices "are not a silver bullet," said Randy Pausina, assistant secretary of Louisiana's Department of Wildlife and Fisheries. He said other measures could include closing shrimping in specific areas when turtles are known to be there.