Using time-depth-light recorders to measure light levels experienced by a diving marine mammal

This study examined the feasibility of using time-depth-light recorders (TDLRs) to measure light levels experienced by a diving marine mammal. TDLRs were deployed on ten female Antarctic fur seals (Arctocephalus gazella) at Bird Island, South Georgia (54°00'S 38°02'W) in the 1994-1995 austral summer. Depth and light measurements were made during 11 foraging trips which lasted on average (±SE) 7.1±0.7 days. A total of 25,657 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 on average 6-57% of the surface illumination when diving. Illuminance ranged from full sunlight (10⁴ lx) at the surface to minimum starlight (10^-6 lx) at night and during deep daytime dives. TDLRs 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. These findings suggest that TDLRs may be a useful method of obtaining information on the biooptical properties of the ocean where fur seals forage. Development of sensors with appropriate spectral sensitivity and suitable resolution at low light levels is recommended to improve the capability of these devices for the study of marine mammals.