Behavior of midwater fishes under the Antarctic ice: observations by a predator

Abstract.

Few details are known of the habits of Antarctic midwater fishes, especially those living below the heavy pack ice and shore-fast ice, are known, because they are difficult to capture or observe at depth. We used video sequences with synchronized positional data recorded by Weddell seals to describe the vertical distribution, diel movements, trends in abundance, and swimming behavior of two ecologically important fish species in McMurdo Sound, Antarctica. Pleuragramma antarcticum occurred in loose aggregations (individuals 2–4 m apart) and migrated daily between mean depths of 252 m at night and 346 m by day (vertical transit of 94 m). Their depth during November was correlated with surface light intensity even in the absence of a daily sunset. Interannual variations in local abundance indicated that the Pleuragramma population did not remain stationary. Large Dissostichus frequently occurred at shallow depths (12–180 m) and showed a significant change in depth with time of day. Dissostichus encounters followed a diel cycle having a mean depth of 93 m, a nighttime minimum of 17 m, and a daytime maximum of 168 m. Their depth was not correlated with surface light intensity. Dissostichus were present even when Pleuragramma, their principal prey, were scarce. When chased by a seal, one Dissostichus sustained a speed of 3.4 m s^–1 for a period of 24 s. This use of a marine predator as a guided, high-speed sampling device for its midwater prey provided clarification and new insights into the behavior, interactions, and ecology of species that have been especially difficult to study.