Monitoring of Hawaiian Monk Seal Population Continued in 2012 Despite Reduced Resources for Field Surveys

Hawaiian monk seal mother and pup.
Hawaiian monk seal mother and pup.

PIFSC scientists annually survey the endangered Hawaiian monk seal in the Northwestern Hawaiian Islands (NWHI), where most of the population resides and where the abundance of monk seals is declining. And in recent years, survey effort has extended to the main Hawaiian Islands (MHI), where monk seals are increasing. The population surveys are essential for estimating the abundance of seals and assessing trends in survival and factors bearing on welfare and recovery of the seals.

A recent report revealed that in the MHI survey, 138 individually-identifiable seals were documented during 2012. This result is lower than MHI monk seal counts in the previous 2 years, but represents a minimum estimate of abundance, as not all areas of the MHI were included in the 2012 survey; in particular, no information was obtained from Niihau Island and Lehua Island.

Because survey effort is not sufficient to characterize abundance trends of monk seals in the MHI, the best available trend indicator is the intrinsic rate of population growth, estimated at 7.2% per year; the estimate is derived from a life-table analysis. Without knowing the age structure of the MHI subpopulation, it is not possible to estimate the current realized population growth rate. Nevertheless, it appears likely that the MHI subpopulation is increasing robustly. During 2012, 14 births and 6 deaths were documented in the MHI (excluding possible events at Niihau and Lehua Islands). Survival rates in the MHI remained favorable for all ages. Of the 6 documented deaths in 2012, 5 were due (or likely due) to anthropogenic causes. These were known hookings of seals in 2012, the highest number of hookings in a calendar year documented to date. Two seals were documented entangled in active lay gillnets and were either freed or escaped on their own. In 2012, monk seal program staff intervened on 16 separate occasions in the MHI to assist 11 seals at risk from hooking injury or other threats.

In the NWHI, survey effort in 2012 was dramatically reduced as a result of a cutback in funding of the PIFSC monk seal research and recovery program as a whole. Researchers were only in the field from 30 to 44 days at each field site, a reduction of 50% to 80% compared to efforts in recent years. The abundance of the 6 most-studied NWHI subpopulations was estimated at 862 seals in 2012, compared to 909 in 2011. The lower result for 2012 may be due to the reduced field survey effort in 2012. The number of pups identified at the 6 most-studied NWHI subpopulations in 2012 was 111, 30 fewer than in 2011 (141 pups) and the lowest pup count recorded since the 1980s. Again, some portion of this decline may be a result of the low field effort. In recent years, favorable survival of juvenile monk seals has been observed in the NWHI overall, and was still evident at several sites in 2012. However, early survival, especially during the first year after weaning, remained low at French Frigate Shoals, Midway Island and Kure Atoll.

The shortened field season in 2012 resulted in greater uncertainty in the annual estimates of NWHI population abundance, abundance trends, population structure, and survival rates. In addition to reduced data acquisition, the shortened field season meant that researchers had fewer opportunities to intervene and rescue seals from threats to survival such as entanglement in marine debris, shark predation, and aggression by certain male monk seals. In 2011, when the field season was several months long, researchers were able to intervene on 63 occasions to improve the survival of individual seals. In contrast, during the shortened 2012 season, only 16 interventions occurred.