Dive depths of Weddell seals in relation to vertical prey distribution as estimated by image data

Abstract

To better understand the foraging behavior of diving animals it is important to monitor aspects of the animal's environment, including prey distribution, which may influence their behavior. However, prior to recent technological advancements, monitoring the distribution of prey immedi- ately surrounding a diving animal had been impossible. We attached newly developed Digital Still picture Loggers (DSLs) to 8 free-ranging female Weddell seals Leptonychotes weddellii at breeding colonies in McMurdo Sound, Antarctica, from November to December 2000. The DSLs provided depth data and several thousand underwater images taken from the seals' perspectives. Some of these images clearly showed scattered prey-like objects directly in front of the seal. Using image pro- cessing software, the images were converted to a 256 gray-scale and the prey-like objects were iden- tified according to their brightness ratio and counted. Finally, a 'prey index' was calculated for each image and the vertical prey distribution along the seals' dive paths were compared with the seals' dive depth. Seals frequently dived to depths greater than 250 m where the prey index was both higher and exhibited a wider range. We concluded that the seals' dive depths might be affected by the vertical distribution of prey, which appeared to be aggregated in shoals at deeper depths.