Translocation and disease monitoring of wild Laysan ducks

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The Laysan duck (Anas laysanensis), also known as the Laysan teal because of its small size, is a critically endangered waterfowl species that once occurred widely across the Hawaiian Archipelago. For the past 150 years, however, it was restricted to a single population on Laysan, a 4-square-kilometer (1.5-square-mile) island with a hypersaline shallow lake. Laysan is part of the Hawaiian Islands National Wildlife Refuge in Papahanaumokuakea Marine National Monument.

Evidence suggests that the Laysan duck's disappearance from the rest of the Hawaiian Islands was partly due to the introduction of predatory non-native rats during human colonization of the Hawaiian Islands about 1,000 years ago. Rats never became established on Laysan Island. However, in the 1800s, people who came to Laysan to harvest guano introduced rabbits that largely denuded the island's native vegetation and led to the extinction of several native species, including the Laysan rail (Porzana palmeri) and Laysan millerbird (Acrocephalus familiaris familiaris). Fortunately, Laysan ducks survived this onslaught, and subsequently the rabbits were extirpated from the island in the early 1900s.

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Small, isolated island populations have high extinction risks. On Laysan, numbers of the duck fluctuate and have seldom exceeded 600 individuals. The Laysan duck is vulnerable to extreme weather, diseases, introduction of mammalian predators, and global sea level rise. In 1993 and 1994, there was a large die-off of Laysan ducks attributed to emaciation and infestation with a worm, Echnuria uncinata. Recognizing that the Laysan duck was highly vulnerable to extinction, the U.S. Fish and Wildlife Service (FWS) and U.S. Geological Survey (USGS) developed a partnership to translocate these ducks to Midway Atoll National Wildlife Refuge. After careful consideration, the agencies chose Midway Atoll because they judged that is has the most promising logistical feasibility and potential to support translocated ducks. The translocation was a team effort led by the USGS Pacific Islands Ecosystem Science Center and involving the collaboration of the USGS National Wildlife Health Center-Honolulu Field Station and the FWS. The team took great care not to translocate the internal parasite, Echinuria, via Laysan ducks to Midway Atoll, where it has not been documented.

To maximize the chances for success, we chose the healthiest candidates for translocation. Biologists with the project trapped the ducks on Laysan, gave the birds a complete physical exam, and treated them for Echinuria worms prior to transport. In October 2004, 20 juvenile and pre-breeding island ducks went on a 2- day, 600-km (370-mile) Pacific voyage by boat from Laysan to Midway Atoll (USGS 2005). In October 2005, an additional 22 ducks made the same trip. All birds survived the translocation with nutritional and veterinary support.

Understanding mortality factors and occurrence of disease is important in managing threatened and endangered species. Post-release monitoring with the aid of radio telemetry helped us determine the fate of the translocated birds and monitor their health during October 2004-2007. Identifying the causes of mortality and disease allows for exploration of management options to address the problems and enhance recovery of the species. The refuge staff sent all carcasses suitable for examination to the USGS National Wildlife Health Center-Honolulu Field Station for complete examination to determine cause of death. In other cases, suspected causes of death were apparent from field signs.

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To date, the identified causes of Laysan duck mortality on Midway are varied. Causes of duck mortality on Midway have included egg-bound females, trauma, yolk sac infection, emaciation, and botulism. Field evidence also suggests mortality from attacks by large seabirds and vagrant birds of prey. Fortunately, we have yet to document the presence of Echnuria on Midway, although biologists continue to monitor for the disease. Avian pox lesions spread by introduced mosquitoes on Midway Atoll are common in the native Laysan albatross (Phoebastria immutabilis) but have not been observed in the ducks, probably because the virus that causes pox is specific to particular types of birds. All mortalities observed in Laysan ducks on Midway are similar to those in any wild waterfowl population.

Despite these mortality incidents, the Laysan duck continues to flourish on Midway Atoll, and the population has increased after only three breeding seasons. Juvenile recruitment has exceeded adult mortality during the first three breeding seasons, and number of eggs laid per female on Midway is higher than of those on Laysan. This reflects the suitability of Midway's habitat for Laysan ducks. Furthermore, the translocation has established a second population of the species and more than doubled its range from four to nine square km (1.5 to 3.5 square miles). All of the 42 founding birds survived the transport to Midway and 90 percent survived their first year post-release, similar to the survival rate on Laysan Island. The ducks bred successfully after the first year of release, and they produced the first generation of fledglings in 2005. On Laysan, one-year-old ducks typically do not successfully breed, so the productive first year at Midway Atoll was a pleasant surprise. The total population size of Laysan ducks on Midway has grown from the original 42 translocated birds to an approximate count of at least 192 post-fledgling juveniles and adults (see Table). Interestingly, we placed the ducks translocated to Midway Atoll onto its two islands (Eastern and Sand), and ducks now routinely fly between the islands. On Laysan, the ducks rarely fly over the ocean.