Dataset: Historical Sea Otter Density Island Survey

At each island, visual counts of sea otters were made from a 4 m open boat as it circumnavigated the island perimeter at a speed of 18 to 25 km per hour, just outside of any existing kelp canopy and/or close enough to shore (50-150 m) to be able to easily see an animal swimming or resting on the surface.  The perimeter of the island was divided into sectors, and these designations were utilized every year a survey was conducted.  Survey teams consisted of a primary observer, data recorder, and boat operator (who also acted as a secondary observer).  As the boat moved parallel to shore and around any offshore islets, observers visually scanned for sea otters both inshore and offshore of the boat trajectory and, in the case of larger sea otter groups or difficult-to-access locations, the boat was paused and binoculars were used to scan the area and obtain accurate counts.  Whenever sea otters were observed and counted, we noted their geo-location, group size, reproductive status (single animals or females with pups), whether the animals were less than or greater than 20 m from the nearest point of land or emergent substrate, and whether each animal was within or outside a surface canopy of kelp.  Here, we report only island-wide abundance estimates for independent (non-pup) individuals, as dependent pup counts were variable due to annual differences in the timing of reproduction.  Because surveys varied from year to year in terms of the percentage of the island’s perimeter surveyed (ranging from ~50% to 100%), we standardized counts to linear density (independent sea otters per km of linear coastline surveyed).

NOTE: Surveys were conducted opportunistically, and thus intermittently, from 1991 to 2015 at each island. As such, some islands were surveyed more frequently than others.