Calambokidis John

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Calambokidis
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  • Article
    Diving behavior and fine-scale kinematics of free-ranging Risso's dolphins foraging in shallow and deep-water habitats
    (Frontiers Media, 2019-03-12) Arranz, Patricia ; Benoit-Bird, Kelly J. ; Friedlaender, Ari S. ; Hazen, Elliott L. ; Goldbogen, Jeremy A. ; Stimpert, Alison K. ; DeRuiter, Stacy L. ; Calambokidis, John ; Southall, Brandon L. ; Fahlman, Andreas ; Tyack, Peter L.
    Air-breathing marine predators must balance the conflicting demands of oxygen conservation during breath-hold and the cost of diving and locomotion to capture prey. However, it remains poorly understood how predators modulate foraging performance when feeding at different depths and in response to changes in prey distribution and type. Here, we used high-resolution multi-sensor tags attached to Risso's dolphins (Grampus griseus) and concurrent prey surveys to quantify their foraging performance over a range of depths and prey types. Dolphins (N = 33) foraged in shallow and deep habitats [seabed depths less or more than 560 m, respectively] and within the deep habitat, in vertically stratified prey features occurring at several aggregation levels. Generalized linear mixed-effects models indicated that dive kinematics were driven by foraging depth rather than habitat. Bottom-phase duration and number of buzzes (attempts to capture prey) per dive increased with depth. In deep dives, dolphins were gliding for >50% of descent and adopted higher pitch angles both during descent and ascents, which was likely to reduce energetic cost of longer transits. This lower cost of transit was counteracted by the record of highest vertical swim speeds, rolling maneuvers and stroke rates at depth, together with a 4-fold increase in the inter-buzz interval (IBI), suggesting higher costs of pursuing, and handling prey compared to shallow-water feeding. In spite of the increased capture effort at depth, dolphins managed to keep their estimated overall metabolic rate comparable across dive types. This indicates that adjustments in swimming modes may enable energy balance in deeper dives. If we think of the surface as a central place where divers return to breathe, our data match predictions that central place foragers should increase the number and likely quality of prey items at greater distances. These dolphins forage efficiently from near-shore benthic communities to depth-stratified scattering layers, enabling them to maximize their fitness.
  • Article
    Killer whales and marine mammal trends in the North Pacific : a re-examination of evidence for sequential megafauna collapse and the prey-switching hypothesis
    (Blackwell, 2007-10-26) Wade, Paul R. ; Burkanov, Vladimir N. ; Dahlheim, Marilyn E. ; Friday, Nancy A. ; Fritz, Lowell W. ; Loughlin, Thomas R. ; Mizroch, Sally A. ; Muto, Marcia M. ; Rice, Dale W. ; Barrett-Lennard, Lance G. ; Black, Nancy A. ; Burdin, Alexander M. ; Calambokidis, John ; Cerchio, Salvatore ; Ford, John K. B. ; Jacobsen, Jeff K. ; Matkin, Craig O. ; Matkin, Dena R. ; Mehta, Amee V. ; Small, Robert J. ; Straley, Janice M. ; McCluskey, Shannon M. ; VanBlaricom, Glenn R.
    Springer et al. (2003) contend that sequential declines occurred in North Pacific populations of harbor and fur seals, Steller sea lions, and sea otters. They hypothesize that these were due to increased predation by killer whales, when industrial whaling's removal of large whales as a supposed primary food source precipitated a prey switch. Using a regional approach, we reexamined whale catch data, killer whale predation observations, and the current biomass and trends of potential prey, and found little support for the prey-switching hypothesis. Large whale biomass in the Bering Sea did not decline as much as suggested by Springer et al., and much of the reduction occurred 50–100 yr ago, well before the declines of pinnipeds and sea otters began; thus, the need to switch prey starting in the 1970s is doubtful. With the sole exception that the sea otter decline followed the decline of pinnipeds, the reported declines were not in fact sequential. Given this, it is unlikely that a sequential megafaunal collapse from whales to sea otters occurred. The spatial and temporal patterns of pinniped and sea otter population trends are more complex than Springer et al. suggest, and are often inconsistent with their hypothesis. Populations remained stable or increased in many areas, despite extensive historical whaling and high killer whale abundance. Furthermore, observed killer whale predation has largely involved pinnipeds and small cetaceans; there is little evidence that large whales were ever a major prey item in high latitudes. Small cetaceans (ignored by Springer et al.) were likely abundant throughout the period. Overall, we suggest that the Springer et al. hypothesis represents a misleading and simplistic view of events and trophic relationships within this complex marine ecosystem.