The number of endangered Hawaiian monk seal pups and juveniles in the Northwestern Hawaiian Islands has increased slightly this year, scientists reported after a recent research mission.

National Oceanic and Atmospheric Administration teams reported 121 monk seal pups were born this season, compared with 103 pups in 2013 and 111 pups in 2012.

"This is a pretty significant increase for an endangered species," lead scientist Charles Littnan of the Hawaiian Monk Seal Research Program said during a news briefing Monday at Pearl Harbor.

The vessel Oscar Elton Sette returned Friday from a 21-day research and recovery voyage during which scientists monitored and assessed the endangered population. Littnan described the mission as a success, including large-scale recovery efforts as well as the observation of the uptick in the number of monk seal pups and juveniles.

"It’s been a population that has been characterized by pretty long-term decline," he said.

Food availability and environmental changes with the ocean’s warming surface waters are possible factors contributing to the increase.

The monk seal research program typically conducts two missions a year to address the declining population. Scientists will review data collected from the missions to determine the causes of the higher survival.

Although there was a slight increase in survival among young pups and juveniles, Littnan said the monk seal population has declined at a rate of just under 1 percent a year over the last three years.

The overall Hawaiian monk seal population is just over 1,100: about 900 monk seals in the Northwestern Hawaiian Islands and roughly 200 monk seals in the main Hawaiian Islands.

During the recent mission, researchers relocated 11 female monk seal pups from the French Frigate Shoals and Midway Atoll to Lisianski and Laysan islands, where they have a higher chance of survival.

"This is probably one of most intensive field seasons or cruises we’ve had in terms of both the research component but, more importantly, efforts to help recover and save the species," said Littnan.

Teams travel to the islands every year to help reduce monk seal deaths. Adult female monk seals are sustaining the monk seal population, but they stop reproducing as they get older.

"We need to make sure that the young females grow up to be moms," he said. Efforts are focused on young female pups to make sure they survive for the first two to three years.

Researchers said the first three years of a pup’s life are critical because that’s when most deaths occur. They need to survive long enough to grow and use their bodies to compete successfully for food against other predators like ulua or sharks.

"It’s a long, hard road for them when you’re young," said Littnan.

Scientists also disentangled five seals from marine debris and removed more than 2,000 pounds of marine debris from the ocean.

Researchers also recovered two emaciated female pups and transported them to the monk seal hospital, Ke Kai Ola, in Kona.

A 5 1⁄2-month-old pup identified as KF04 and named Mele­ana — which means "continuous song" in Hawaiian — was born at Kure Atoll.

The other, born at Laysan Island, is about 4 months old. Identified as TF90, staff named her Pua‘ena o ke Kai, which means "fiery child of the sea."

The pups have since been tube-fed blended fish to help them recover and gain weight. Michelle Bar­bi­eri, a veterinarian and conservation medicine officer at the Marine Mammal Center, said they are now eating on their own.

"They’re doing really well," she said. "They’re alert and active and agile in the pool."

Meleana and Pua‘ena o ke Kai are expected to be at the hospital for the next four to six months and eventually released back to the Northwestern Hawaiian Islands.

The research team released four monk seals — three young females and a young male — back to the islands during the recent mission after the seals were rehabilitated at the hospital. The once emaciated seals, identified as PN10, Y5PP, BF00 and BF02, underwent eight weeks of feeding at the facility.

Scientists are tracking their movement with tags that deliver signals to satellites.