Diel periodicity in both sei whale vocalization rates and the vertical migration of their copepod prey observed from ocean gliders

The daily activity cycles of marine predators may be dictated in large part by the timing of prey availability. For example, recent studies have observed diel periodicity in baleen whale vocalization rates that are thought to be governed by the diel vertical migration of their zooplanktonic prey. We addressed this hypothesis by studying associations between sei whale (Balaenoptera borealis) vocalization rates, oceanographic conditions, and the vertical distribution of the whales’ prey, the calanoid copepod Calanus finmarchicus, during May 2005 in the southwestern Gulf of Maine using an array of autonomous ocean gliders. Each of the four gliders was equipped with sensors to measure temperature, salinity, and chlorophyll fluorescence. Three of the four gliders carried a digital acoustic recorder and the fourth carried a 1-MHz acoustic Doppler current profiler. We observed strong diel periodicity in the acoustic backscatter measured by the current profiler that we attribute (based on a corroborating shipboard study) to the diel vertical migration of C. finmarchicus. Sei whale vocalization rates also exhibited diel periodicity, with more calls detected during the daytime when C. finmarchicus was observed at depth. We found no evidence to suggest that the observed patterns in sei whale calling rates were attributable to diel periodicity in background noise or acoustic propagation conditions. Sei whales are adept at foraging on nearsurface aggregations of C. finmarchicus; therefore we expect that the whales were feeding at night. We hypothesize that calling rates are reduced at night while the whales are feeding, but increase with social activity during the day when copepods are either more difficult or less efficient to capture at depth. The gliders’ persistence during adverse weather conditions experienced during the study allowed continuous collocated observations of whale vocalization behavior and oceanographic conditions that have not been previously possible with traditional shipboard techniques. Both whalers and scientists have observed diel patterns in baleen whale feeding behavior for decades. Ingebrigtsen (1929) described sei whale (Balaenoptera borealis) skim feeding on copepods by noting, ‘‘it is especially in the evening and early in the morning, when the copepods are most at the surface, that ‘skimming’ takes place.’’ Likewise, Nishiwaki and Oye (1951), Nemoto (1957), and Kawamura (1974) observed diel patterns in stomach fullness of blue (Balaenoptera musculus), fin (Balaenoptera physalus), humpback (Megaptera novaeangliae), and sei whales harvested in the North Pacific and the Southern Ocean that suggested many prey species are not captured during the daytime. These authors speculated that the diel vertical migration of copepods and euphausiids governed the availability of prey for relatively shallow-diving baleen whales, and that the whales fed primarily during the evening and early morning when their prey was near the surface (interestingly, nighttime feeding was not considered by these authors, perhaps because whales could neither be observed nor harvested at night). Diel vertical migration in herbivorous zooplankton is a well-studied phenomenon that is classically described as zooplankton migrating from depth to the surface at night to feed on phytoplankton and then returning to depth at dawn to escape visual predators (Lampert 1993). This behavior has been documented in a wide variety of marine zooplankton, including those considered important prey species for baleen whales, such as Calanus finmarchicus (Durbin et al. 1995; Dale and Kaartvedt 2000; Baumgartner et al. 2003), Meganyctiphanes norvegica (Kulka et al. 1982; Tarling et al. 1999; Tarling 2003), Euphausia superba (Kalinowski and Witek 1980; Demer and Hewitt 1995), and Euphausia pacifica (Bollens et al. 1992; De Robertis et al. 2000). Recent long-term acoustic recordings of blue whales have revealed diel variability in vocalization behavior that may be related to diel feeding patterns established by the vertical migration of prey (Stafford et al. 2005; Wiggins et al. 2005). These studies raise the interesting possibility that Acknowledgments We thank P. Fucile, R. Singer, J. Partan, K. Ball, and L. Freitag for development of the digital acoustic recorder; J. Lund for glider operations and assistance in the field; H.C. Esch for assistance with the acoustic analysis; M. Patrician for assistance with the corroborating study of acoustic backscatter; J. Lynch and A. Newhall for guidance with the acoustic propagation modeling; K. Houtler and I. Hanley for help at sea aboard the R/V Tioga, and F. Wenzel and J. Nicolas of the Northeast Fisheries Science Center for marine mammal sighting support aboard the National Oceanic and Atmospheric Administration (NOAA) ship Albatross IV. We greatly appreciated the helpful comments and criticisms of two anonymous reviewers. Wind speed and wave height data were procured from the NOAA National Data Buoy Center. This study was funded by the Woods Hole Oceanographic Institution Ocean Life Institute with additional support from the Office of Naval Research. Limnol. Oceanogr., 53(5, part 2), 2008, 2197–2209 E 2008, by the American Society of Limnology and Oceanography, Inc.