The Hawaiian monk seal (Monachus schauinslandi) is the only endangered marine mammal occurring entirely within U.S. waters. Its current population is estimated at 1,300 to 1,400 seals, a decrease of 60 percent since the 1950s. Counts declined about 5 percent per year from 1985 to 1993 and then remained relatively stable through 2002, although beach count indices indicate that numbers have declined during the last two years. Population trends are influenced by highly variable dynamics in the six main reproductive subpopulations in the Northwestern Hawaiian Islands (NWHI). Overall pup production has decreased in recent years, and juvenile survival is also decreasing in varying degrees in several subpopulations. The largest subpopulation is at French Frigate Shoals, where counts of non-pups have dropped by 60 percent since 1989, and age distribution has become severely inverted due to high juvenile mortality. Future abundance trends for the species will likely depend upon whether predicted losses at French Frigate Shoals are countered by gains at other sites.
Monk seals occur throughout the Hawaiian archipelago, and although most monk seals occur in the NWHI, a small but increasing number of seals haulout and pup in the main Hawaiian Islands. The seals' terrestrial habitat is usually confined to isolated beaches that are necessary for resting, molting, parturition, and nursing offspring. The species marine distribution and foraging habitat are also limited, primarily due to its nonmigratory behavior and reliance on demersal and epibenthic prey species occurring in the waters surrounding the NWHI.
Past and present sources of anthropogenic impacts to monk seals include hunting, disturbance at beach and nearshore habitats (e.g., past military activities), entanglement in marine debris, shark predation, adult male monk seal aggression resulting in mortality of adult females and immature seals of both sexes, and environmental effects on habitat and prey resource availability. Other factors possibly associated with disease are currently under investigation. Assessment and mitigation of factors limiting population growth is an ongoing challenge and a primary objective of the Hawaiian Monk Seal Recovery Program.