Hawaiian Monk Seal
Restricted almost entirely to the northwestern Hawaiian Islands, the Hawaiian monk seal population declined by 50% between the late 1950's and the late 1970's, to 1,406 individuals in 1993 and perhaps 1,300 individuals in 1995 (National Oceanic and Atmospheric Administration 1996; Fig. 1). Progress in managing the recovery of this endemic species varies among the six main breeding islands. The status of the species as a whole is indicated by the annual mean beach counts of seals and the number of pups born (Fig. 2; Table). The total number of births has been highly variable since the early 1980's, increasing between 1985 and 1988, declining 35% in 1990, and increasing during 1991-1992. However, mean counts of seals (excluding pups) at the five main breeding sites have fallen steadily from a peak of 575 in 1986 to 378 in 1993, largely as a result of declines at French Frigate Shoals, the largest rookery (Ragen and Lavigne 1997). In addition, high mortality of females at Lisianski and Laysan islands is due to male mobbing behavior, where multiple males simultaneously attempt to mate with a single female. Efforts were undertaken to reverse this trend through rehabilitation of undernourished juvenile females at French Frigate Shoals and elimination of mobbing-related mortality.
Fig. 1. Hawaiian monk seal and pup at Kure Atoll, Northwest Hawaiian Islands. Courtesy T. Gerrodette, National Marine Fisheries Service
Recent trends in monk seal survival suggest a link between population size and large-scale natural perturbations in ecosystem productivity (Polovina et al. 1994; Fig. 3). Declines in Hawaiian monk seal, seabird (Fig. 4), and lobster reproductive success in the late 1980's appear to have been linked to a large-scale climatic event. From the mid-1970's to late 1980's, the central North Pacific experienced increased vertical mixing, with a deepening of the wind-stirred surface layer into nutrient-rich lower waters and probable increased injection of nutrients into the upper ocean. Resulting increased productivity likely provided a larger food base for fish, lobsters, and eventually seals and seabirds in the region.
The declines in seabird and monk seal reproduction and survival in the late 1980's and early 1990's may have been caused by a return to less productive oceanographic conditions. The effects of climatic change on the large marine ecosystems of the oceanic Pacific are, however, very poorly understood and thus cannot be definitively linked to changes in animal populations.
Fig. 3. Monk seal pup survival (percent) and mixed-layer depth (in meters). Decadal declines in mixed-layer depth may have caused a reduction in primary production and a concomitant subsequent reduction in monk seal prey items that led to a decline in pup survival (Polovina et al. 1994; data from J. Polovina and T. Ragan, National Marine Fisheries Service, Honolulu).
Fig. 4. Red-footed booby reproductive success and mixed-layer depth (in meters). Decadal decline in mixed-layer depth may have caused a reduction of primary production and subsequent reduction of prey items for seabirds that in turn led to a decline in seabird reproductive success (Polovina et al. 1994; data from J. Polovina, National Marine Fisheries Service, Honolulu, and E. Flint, U.S. Fish and Wildlife Service, Hawaiian and Pacific Islands National Wildlife Refuge Complex).