Dive patterns of tagged right whales in the Great South Channel

Abstract Right whales were tagged in 1988 and 1989 with radio and sonic telemetry tags as part of a multidisciplinary investigation of right whales and their habitat in the Great South Channel region east of Cape Cod. The tags yielded data on the durations of 6456 dives and 6482 surfacings, as well as 23,538 measurements of the depth of a diving whale. Log-survivorship analysis of the 1988 data showed a clear separation between the durations of dives between blows within a single surfacing sequence or bout (intea-bout dives) and longer dives between surfacing sequences (interbout dives) at 27 s, which was also applied to the 1989 data. Inter-bout dives averaged 127.3 s, and were significantly longer in 1988 than in 1989. Inter-bout dives were significantly longer during the day than night in 1988, and longer at night in 1989. The average intea-bout dive duration was 11.8 s, with 1989 dives longer than those in 1988. Surface durations averaged 6.2 s, and were also significantly longer in 1989. Dive depths were recorded only in 1989. Mean dive depth was 7.3 m, and only 12 dives went deeper than 30 m. The typical right whale dive pattern in 1988 included relatively short surfacings, long dives during the day, and shorter dives at night. This correlated with strong diel vertical migration by the dense zooplankton patches on which they were presumed to be feeding based on indirect evidence-from near the surface at night to near the bottom during the day. The 1989 pattern included longer dives during the night, as well as some exceptionally long surfacings. Zooplankton in 1989 did not migrate vertically, and remained near the surface day and night in right whale feeding areas. Right whale dive patterns in the Great South Channel are closely correlated with the horizontal and vertical distributions and movements of dense patches of their zooplankton prey.