Light levels experienced by foraging Antarctic fur seals, Arctocephalus gazella

This study examined the feasibility of using time depth recorders (TDRs) to measure light levels experienced by a diving marine mammal. TDRs were deployed on 10 female Antarctic fur seals (Arctocephalus gazella) at Bird Island, South Georgia (54° 00' S 38° 02' W) in the 1994/5 austral summer. Depth and light measurements were made during 11 foraging trips which lasted on average (± SE) 7.1 ± 0.7 d. A total of 25657 dives were recorded with a mean dive depth and duration of 18.0 ± 3.6 m and 49.5 ± 6.9 s, respectively. Depending on time of day, fur seals experienced between 6 and 57 % of the surface illumination when diving. Illuminance ranged from full sunlight (102 lux) at the surface to minimum starlight (10-6 lux) at night and during deep daytime dives. The change in il1uminance with depth did not follow a simple exponential decrease as recorded in clear oceanic waters. TDRs recorded strong light attenuation in the top 50 m of the water column. The mean attenuation coefficient was 0.140 ± 0.014 m-1 which was in the upper range of values measured by ship surveys at South Georgia. When fur seals made shorter and shallower dives TDRs also recorded greater light attenuation. These findings suggest that Antarctic fur seals forage in relatively turbid waters containing large concentrations of phytoplankton and 1 or dense aggregations of krill. Night-time foraging and deep daytime diving indicates that fur seals are well adapted to locating prey in low illumination.