Demographic Consequences of Changed Pupping and Hauling Sites of the Hawaiian Monk Seal

Abstract

Causes of the DeclineThe emergent land at Kure Atoll consists of a single, permanent, vegetated island, Green Island, and several much smaller sand islets. Before human occupation, Green Island was the favored site for pupping and hauling out. The number of seals hauling out on Green Island declined after 1960 when the U.S. Coast Guard (USCG) built a loran station there ( Fig. 1). Dogs and recreational beach activities of the 2CL25 men at the station, primarily beachcombing for Japanese glass fishing floats, caused the seals to abandon the preferred Green Island beaches and haul out on the sand islets instead (Kenyon 1972). The converse effect has been observed on Tern Island at French Frigate Shoals, a multi-island atoll 1,350 km to the southeast. For more than two decades while a loran station was in operation on Tern Island, few seals were ever seen, but the number of seals using the island increased sharply after the USCG vacated the station in 1979 (Fig. 1). In 1988, the highest of the weekly s,eal counts on Tern Island was 118, and the MarchSeptember mean was 61.4.The immediate effect of human beach activity on the choice of monk seal hauling sites is thus clear, but the demographic effects of changed pupping and hauling sites are less obvious and require a longer time to express themselves. In the absence of human disturbance, seals use the optimal sites available for their activities; a change to suboptimal sites may lead to higher mortality, lower fecundity, or both. After the establishment of the loran station on Green Island in 1960, fecundity, as indicated by number of pups per adult, did not decline ( Finally, the phocid pattern of maternal care makes the postnatal period a particularly vulnerable stage in the life cycle. Phocids (true seals) have a high rate of energy transfer from mother to pup during a short nursing period, terminated by an abrupt weaning (Oftedal et al. 1987). Hawaiian monk seal pups nurse for 35-40 days, during which the mother is in constant attendance to her pup and does not feed. The amount of energy transferred to the pup during the nursing period is critical since, after weaning, the pup must live on its fat reserves until it learns to feed on its own. Immature survival rates are positively correlated with weaning size in the Hawaiian monk seal (unpublished data) and other phocids (Coulson & Hickling 1964; Hill 1987). Repeated interruption or stress during the nursing period reduces the amount of milk the pup receives and its subsequent likelihood of survival. Disruption of normal maternal care has been a factor in the decline of many threatened and endangered mammals (Oldfield 1988 press). Data are insufficient to assess the relative contribution of these sources of mortality to the population declines at the other islands. Changes Leading to RecoveryAfter the Hawaiian monk seal was declared an endangered species in 1976, management actions were undertaken to assist its recovery. The USCG modified its regulations at Kure Atoll to reduce disturbance to seals. Dogs were ban...

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