Harlequin Duck
Injury
Harlequin ducks spend most of their time in intertidal and shallow subtidal habitats where
much of the oil was initially stranded. In Prince William Sound, about 150 harlequin duck
carcasses were collected immediately after the spill in 1989. From these birds, it was estimated
that 1,000 harlequins were killed by the initial oiling event, which represented about 7 percent of
the wintering population. In addition to acute effects, harlequin ducks were one of the few
species for which chronic injury related to long-term exposure to lingering oil was documented.
Recovery Objective
Harlequin ducks will have recovered when breeding- and
nonbreeding-season demographics return to prespill levels and when
biochemical indicators of hydrocarbon exposure in
harlequins in oiled areas of Prince William Sound are similar to
those in harlequins in unoiled areas.
Recovery Status
Winter populations of harlequin ducks in Prince William Sound have ranged
from a high of 19,000 ducks in 1994 to a low of around 11,000 ducks in March of 1990, one year
after the spill. The 2000 estimate of wintering harlequin ducks in the Sound was approximately 15,000.
Several post-spill studies were designed to measure the extent and severity of injuries to the Prince William Sound harlequin duck population from the oil spill and assess recovery. Through 1998, oil spill effects were still evident although the extent and magnitude of the injury remained unclear. Supporting studies provided evidence of continuing injury to harlequins through the following mechanisms: 1) invertebrate recovery in upper intertidal and subtidal areas remained incomplete for some species, thereby impacting potential prey base for harlequins; 2) oil persisted in intertidal areas of Prince William Sound where it was identified as a source of contamination of benthic invertebrates; 3) the possibility of external oiling of feathers remained due to lingering surface oil; 4) a biochemical marker of oil exposure (cytochrome P450) was greater in tissues of harlequin ducks captured in oiled areas than in reference areas and 5) overwinter female survival was lower in oiled than reference areas.
More recent studies indicate improving conditions. From 1997 - 2005, age composition and
population trends were compared in harlequin ducks between oiled and unoiled areas of the
Sound. No difference in population trends was observed between areas. Although populations in
the oiled area were no longer declining as they were in the mid 1990s, a positive trend was not
observed. Overall, more males than females occurred Sound-wide which is consistent with other
Pacific populations of harlequin ducks. The ratio of immature to adult males was similar between
areas, thus indicating similar recruitment into both populations. However, there remains a
disproportionately lower number of female ducks in the oiled areas. From 2000 - 2002,
measurements of cytochrome P450 activity and female survival rates were converging between
oiled and unoiled areas. However, in 2005 the P450 biomarker was elevated in ducks from the
oiled areas. Finally, lingering oil still remains in habitats used by harlequins, thereby maintaining
the possibility of chronic effects related to continued exposure.
Evaluation of population trends, survival measures, and indicators of exposure through 2005 indicates a positive relationship among these parameters within harlequin duck populations in the Sound. The evidence suggests that harlequin ducks are recovering, but have not fully recovered from the effects of the oil spill.