MEPS 252:283-288 (2003)  -  doi:10.3354/meps252283

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

Yuuki Watanabe1,*, Yoko Mitani2, Katsufumi Sato2,3, Michael F. Cameron4,5, Yasuhiko Naito2,3

1Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan
2National Institute of Polar Research, and
3Graduate University for Advanced Studies, 1-9-10 Kaga, Itabashi, Tokyo 173-8515, Japan
4Department of Ecology, Evolution, and Behavior, University of Minnesota, 100 Ecology, St. Paul, Minnesota 55455, USA
5Present address: National Marine Mammal Laboratory, Alaska Fisheries Science Center, NOAA, 7600 Sand Point Way NE, Seattle, Washington 98115, USA

ABSTRACT: To better understand the foraging behavior of diving animals it is important to monitor aspects of the animal¹s environment, including prey distribution, which may influence their behavior. However, prior to recent technological advancements, monitoring the distribution of prey immediately 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 seals¹ perspectives. Some of these images clearly showed scattered prey-like objects directly in front of the seal. Using image processing software, the images were converted to a 256 gray-scale and the prey-like objects were identified according to their brightness ratio and counted. Finally, a Œprey index¹ was calculated for each image and the vertical prey distribution along the seals¹ dive paths were compared with the seals¹ 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 seals¹ dive depths might be affected by the vertical distribution of prey, which appeared to be aggregated in shoals at deeper depths.


KEY WORDS: Foraging behavior · Vertical prey distribution · Digital Still picture Loggers · Prey index · Weddell seals


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