3b). No other areas have been investigated as intensely as NKI, so it is not known whether the more prolonged dip in numbers there was an anomaly or part of a broader trend. Eight boat-based surveys conducted during 1991–2007 around Knight Island found a strikingly parallel trend in otter numbers between the northern and southern halves of the island (Fig. 4a), even though Southern Knight Island (SKI) was much less impacted by
oil than NKI (Table 2), and reproduction (measured as the proportion of otters with pups) was consistently higher at SKI after the spill (Fig. 4b). Notably, a change in distribution of otters around Knight Island was evident prior to the spill: otter numbers declined precipitously at NKI from 1973 to 1984 while increasing at SKI (Table 1). Although it is not wise to infer trends from only two points in time, that is all that was available in this case, and the counts changed markedly over this 11-year period. If this pattern continued after 1984, Olaparib supplier then numbers may have been decreasing at NKI while increasing at SKI at the time of the spill. Uncorrected counts from different platforms (Table 1) are difficult to compare, but changes in the proportion of otters counted within the northern versus southern halves of this island should nevertheless be reflective of real changes in distribution. These apparent spatial Selleckchem TSA HDAC dynamics in the sea otter population complicated interpretations
of population recovery, although parallelism between similar sites with differing impacts is one measure of recovery (Skalski et al., 2001 and Parker and Wiens, 2005). A host IMP dehydrogenase of demographic circumstances has been posed to explain the lack of growth of the NKI otter population during an extended period post-spill. These included higher mortality and emigration from NKI (Bodkin et al., 2002); higher rates of loss despite immigration from source populations (Monson et al., 2011); and immigration of otters from EPWS to WPWS but avoiding NKI (Rice et al., 2007). The variety of these conjectures reflects the lack of direct evidence that mortality, immigration, or emigration were
aberrant at this site. Moreover, other sites that were less oiled also showed no noticeable population increase (Fig. 2). Although a population increase was expected across WPWS as otters recovered from the spill, such an increase was not expected for unoiled sites such as Montague Island, which was the primary control site for several comparative studies. Historically, numbers of otters at Montague have varied widely, but no trend was apparent in counts (n = 9) made during 1959–1984 ( Lensink, 1962, Pitcher, 1975 and Johnson, 1987). A post-spill surge in otters at this site (doubling from 1995 to 1997, based on aerial counts but not boat-based counts), followed by sudden sporadic declines (nearly 50% in 2002, 2007, and 2009, returning to the mid-90s levels; Fig. 3a), were as anomalous as the lack of an increase at some other sites.