That otter numbers at oiled sites in WPWS were higher shortly after the spill than before was an obvious paradox. One explanation is that the spill-caused mortality was masked by an increase in otter numbers that occurred during the 5-year interval between the last pre-spill counts and the spill (Garshelis and Johnson, 2001). Previously, it was thought that all of WPWS had been at carrying capacity, so increases in otter numbers were not expected. However, even

at carrying capacity, otter numbers could increase if the food base increased. Garshelis and Johnson (2001) found that otters retrieved more and larger clams (their primary food in PWS) after the spill than they had at the same sites http://www.selleckchem.com/products/SB-431542.html in the early 1980s and also spent less time foraging, suggesting that food resources had increased. Two studies, using different methodologies, indicated that otter numbers in WPWS continued to increase for several years after the spill. Boat surveys conducted in a portion of WPWS that included both oiled and unoiled areas indicated an annual population growth rate of 2.5% per year during 1991–1996 (Garshelis and Johnson, 2001; Fig. 2). Aerial surveys conducted across a broader area of WPWS during 1993–2009

indicated that numbers continued to grow at an average of 2.6% per year over this longer period; in fact, the population in this region virtually doubled, increasing Anti-diabetic Compound Library datasheet by nearly 2000 otters (Bodkin et al., 2011). Population changes in WPWS after the spill differed by site, with no clear association between rates of change and previous extent of oiling (Johnson and Garshelis, 1995). For example, among three oiled sites, otter numbers increased rapidly at Knight Island but remained stable at Green Island and Applegate Rock during the early 1990s (Fig. 2). During the late 1990s, Urease numbers declined at Knight Island, increased at Green Island, and stayed stable at Applegate Rock (but then declined after 1998) (Garshelis

and Johnson, 2001). At the neighboring unoiled site, Montague Island, boat surveys showed no trend in otter numbers, whereas aerial surveys indicated a sudden rise in 1997 (Fig. 3a). This discrepancy may have been due to the inclusion of a portion of Green Island in the aerial counts of Montague; this portion of Green Island contained a large kelp bed where sizeable but variable numbers of otters often rested. The delineation of study-site boundaries became a significant factor in the assessment of population trends and evaluation of potential effects of the spill (see Garshelis and Estes, 1997). In a study of oil-spill effects on harlequin ducks (Histrionicus histrionicus) ( Esler et al., 2002 and Esler and Iverson, 2010) that was conducted under the same program as the sea otters ( Holland-Bartels et al.