Sea otter population dynamics and the Exxon Valdez oil spill: disentangling the confounding effects

1. Oil that spilled after the grounding of the Exxon Valdez in 1989 killed large numbers of sea otters in western Prince William Sound, Alaska, USA. H owever, our boat-based counts of sea otters during 1990-96 at oiled sites w ere as high or higher than boat-based counts in the same area in the early 1980s. 2. Another study reported a significant decline in sea otter numbers after the spill, but our reanalysis of that data produced results very similar to ours. Counts of otters were higher than pre-spill counts in the oiled area ; the only detectable decline was in the northern part of the sound, outsid e the area of oiling. 3. We suggest that otter numbers in the western sound may have been increas ing during the late 1980s, masking the loss due to the spill. Direct eviden ce for such an increase is lacking because no counts were conducted during this period. However, for several years after the spill pup production was higher than normal, which, if characteristic of the period immediately pre- spill, could have spurred a population increase. 4. Heightened pup production may have been caused by increased food supplie s: after the spill, otters obtained more and larger clams per dive and spen t less time feeding per day than in the early 1980s. 5. We postulate that in the early 1980s clams were still recovering from th e uplift caused by the 1964 earthquake, which resulted in massive clam mort ality and habitat change in the western sound. Lingering effects of previou s catastrophic events, like the earthquake and even 19th-century fur harves ts, have hampered attempts to assess the impacts of the oil spill on sea ot ter population dynamics. The effects of uncontrolled and unreplicated envir onmental incidents, even major disasters, may be difficult to assess becaus e of confounding factors.