The Georgia Aquarium says a government agency's denial of its permit to import 18 beluga whales from Russia was arbitrary and capricious, but the government argues the aquarium failed to meet the requirements of a law meant to protect marine mammals.

The aquarium in September 2013 filed a lawsuit asking a judge to overturn the denial of its June 2012 application by the National Oceanic and Atmospheric Administration's National Marine Fisheries Service, known as NOAA Fisheries. Lawyers for the two sides faced off Friday in federal court in Atlanta.

Each side accused the other of twisting the facts, with a lawyer for the aquarium saying the government had "cooked the books" on whale population numbers and a lawyer for NOAA Fisheries accusing the aquarium trying "to confuse the court."

The two sides have asked U.S. District Judge Amy Totenberg to make a decision on the merits of the case, based on court filings and oral arguments, without holding a trial. Totenberg asked questions of both sides and seemed troubled by "an extremity of data poverty" concerning beluga population numbers.

The 1972 Marine Mammal Protection Act prohibits the capture of marine mammals in U.S. waters and by U.S. citizens elsewhere and also doesn't allow the import of marine mammals and marine mammal products into the U.S. But it has some exceptions, including one that allows animals to be caught and imported for public display by applicants meeting certain qualifications.

The 18 belugas the aquarium seeks to import originate from the Sea of Okhotsk in northern Russia and were collected by scientists there in 2006, 2010 and 2011. They currently live in the Utrish Marine Mammal Research Station in Russia.

The aquarium says new whales are needed to diversify the gene pool in the captive beluga population in the U.S., which currently numbers 29, and to increase research and education opportunities. Some of the whales would live at the Georgia Aquarium while others would be loaned to aquariums in Chicago and Connecticut and Sea World facilities in Florida, Texas and California.

NOAA Fisheries invented new standards, broke with agency practice and ignored data, said George Mannina, a lawyer for the aquarium.

In its application, the aquarium used the concept of "potential biological removal," which is defined in the Marine Mammal Protection Act as "the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its optimum sustainable population."

That standard is inherently precautionary because it is quite conservative, Mannina said. NOAA Fisheries has regularly applied that standard in the past and ignored data offered in this case that shows the stock of belugas in question is not only not decreasing but is actually increasing, he said.

NOAA Fisheries found that the aquarium's import of the whales would exceed that standard, but the agency doesn't generally use that standard to assess import permits and that's not the main reason the aquarium's permit application was denied, said Department of Justice lawyer Clifford Stevens.

A main reason for the denial was the aquarium's failure to show the import would not likely have a significant adverse impact on the population of belugas where they were captured, Stevens said. The use of the potential biological removal standard in the aquarium's application assumes there is no other human-caused mortality of the beluga stock at issue, but the agency found that is not the case, he said.

NOAA Fisheries initially indicated it would approve the permit but then changed its mind without explanation, Mannina said. The agency's staff was leaning toward granting the application upon initial review, but that changed as questions were raised and they started to look closer, Stevens said.