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Protected Resources Division
Population Size and Threats
Population Status/ Stock Assessment
The prolonged and steep decline of the Hawaiian monk seal population has occurred more or less continuously since the 1950's. However, there have been some relatively recent encouraging developments, including:
- Web story: NOAA and Their Partners Celebrate Year of the Monk Seal
- promising advances in juvenile seal survival enhancement
- and local community engagement
The best estimate of the current total Hawaiian monk seal population is 1,400 seals – about 1,100 in the Northwestern Hawaiian Islands (NWHI from Nihoa to Kure Atoll), and about 300 in the main Hawaiian Islands (MHI from Niihau to Hawaii). The most recent annual population assessment shows that the Hawaiian monk seal, bucking past trends, has increased in numbers by 3% annually for the past three years. While numbers have increased since 2013, the long-term decline in abundance at the six main NWHI sites (French Frigate Shoals, Laysan, Lisianski, Pearl and Hermes, Midway and Kure) remains concerning.
Click to enlarge graphs
Hawaiian Monk Seal Population Index in the Northwestern Hawaiian Islands
The abundance index shows the long-term trend in Hawaiian monk seal populations. While numbers have declined dramatically since monitoring began in the 1950s, recent years show stability or even increases. (Note, this graph depicts only the portion of the monk seal population in the NWHI from French Frigate Shoals to Kure Atoll, where data extends back many decades.)
Number of Seals in Recent Years
Recent years show an increase in the total Hawaiian monk seal population (encompassing all animals across the species' range). Only recent years are shown for which there was sufficient survey data at all monk seal sites throughout the Hawaiian Archipelago. This positive trend is a change after decades of decline.
The overall decline has been moderated by the increasing population of seals in the MHI. Although Hawaiian monk seals were only rarely reported in the MHI over most of recorded history, since 1990, an increasing number of seal sightings and births have occurred in the MHI. Sightings in the MHI increased from 77 individually identifiable monk seals in 2005 to just under 300 in 2016. This increase is due in part to intrinsic population growth, and also to the increased monitoring effort identifying individual seals. Documented annual births in the MHI have increased since the mid-1990s, with 37 births reported in 2016. The small but increasing population of seals in the MHI is perhaps the most promising aspect for Hawaiian monk seal recovery, but this growing seal population in areas that are heavily populated by humans is creating a new set of recovery challenges as discussed below.
Threats to Recovery
Despite the fact that Hawaiian monk seals are one contiguous species, the subpopulations in the NWHI and MHI face different threats. In the NWHI, primary threats include food limitation for juveniles, shark predation on juveniles, entanglement in marine debris, male seal aggression on females and juveniles, and shoreline habitat loss. Threats in the MHI include disease and various types of human-induced impacts, such as disturbance at haul-out areas, fishery interactions, feeding and other interactions that cause habituation to humans, and most recently, intentional killings.
Northwestern Hawaiian Islands
- Food Limitation - Limited food intake by juvenile seals is the dominant factor driving the steep population decline in the NWHI. In the NWHI, seals must compete for food with large populations of other apex predators, such as large jacks (Carangids) and sharks. Shifts in ecosystem productivity, caused by global climate change and/or cyclical changes, may also be contributing to food limitation.
- Shark Predation - Predation by Galapagos sharks on pre-weaned or recently weaned seal pups at French Frigate Shoals has become a major cause of injury and mortality for this population. This is a unique and relatively recent type of seal mortality that appears to result from atypical behavior of a limited number of Galapagos sharks that prey on pre-weaned and recently weaned pups right on shore, often within just a few inches of water. For more information on NOAA's monk seal pup protection efforts at French Frigate Shoals, visit here.
- Entanglement - Hawaiian monk seals have one of the highest documented entanglement rates of any pinniped species. Marine debris and derelict fishing gear are chronic forms of pollution affecting monk seal habitat in the NWHI. The number of monk seals found entangled each year has generally remained unchanged. While marine debris removal efforts, undertaken by various agencies within NOAA, the US Coast Guard, and various other partners have removed over 700 metric tons of debris since 1996, accumulation rates of marine debris in the NWHI appear to remain constant.
- Male Aggression - A significant cause of female and juvenile monk seal mortality, and overall population decline during the 1980s and early 1990s, was injury and death caused by aggression from multiple male seals (especially at Laysan, Lisianski and French Frigate Shoals). NOAA Fisheries Service has found that removal of specific aggressive males appears to be an effective method to address this threat. PIFSC also works to intervene with individual males who aggressively attack and injure recently weaned and juvenile seals. This includes harassment or removal of the aggressor and treating injured seals as appropriate. This threat continues to be a concern, even though it tends to be episodic, geographically limited and largely manageable provided necessary resources are available.
- Habitat Loss - The loss of terrestrial habitat is a significant issue in the NWHI, which are mostly low-lying atolls subject to beach loss from storm erosion and sea level rise. Some habitat loss, such as the disappearance of Whale-Skate Island at French Frigate Shoals, has already been observed, and sea level rise over the longer term may threaten a large portion of the resting and pupping habitat in the NWHI.
Main Hawaiian Islands
- Human Impacts - Impacts caused by humans (disturbance, injury and death) are a primary threat to the small, but growing population of Hawaiian monk seals in the MHI. Beaches that are popular for human recreation are also increasingly used by monk seals for "hauling out" (resting) and molting. Female monk seals are also increasingly pupping on popular recreational beaches. These "pupping events" entail mother-pup pairs remaining on the beach to nurse for up to 7 weeks, during which time they are particularly vulnerable to human disturbance.
- Fishery Interactions - Due to recent fishing restrictions, hookings and entanglements in active fishing gear have become virtually non-existent in the NWHI. However, in the MHI, the growing seal population has led to increased fishery interactions. Over the past two years, several seals have required removal of embedded recreational fishing hooks. Twelve hookings were reported in 2009 and at least two seals drowned in lay gillnets over the reporting period.
- Human-Seal Interactions - Intentional feeding and/or other direct interaction, such as swimming with juvenile seals, has recently become a serious concern for the MHI population. These human-seal interactions have increased over the past two years, and relocation of "conditioned" seals to remote locations has been required in at least three cases.
- Disease - Recent MHI monk seal deaths have heightened concern about monk seal exposure to diseases not previously encountered, such as leptospirosis and toxoplasmosis. There is also the threat of emergent diseases which have yet to make it to Hawaii. The lack of antibodies in monk seals to these diseases makes them extremely vulnerable to potential infection. At least two seals died during 2009-2010 where toxoplasmosis was identified as the most likely cause of death. As of the end of 2016, 8 monk seals have died as a result of toxoplasmosis, and even this is likely an underestimated.
- A Cat-astrophic Threat for Seals - Toxoplasmosis Infographic (March 2018, pdf 2.36 MB)
- Intentional Killing - Finally, intentional killing of seals is an extreme example of negative human impacts, and unfortunately appears to be a growing problem in the MHI. Over the past two years, at least three seals have died from apparent gunshots, and foul play could not be ruled out as the cause of death for at least three other seals.
Threats Fact Sheet (May 2010, pdf 505kB)