When cormorants go fishing: the differing costs of hunting for sedentary and motile prey

Abstract

Cormorants hunt both benthic (sedentary) and pelagic (motile) prey but it is not known if the energy costs of foraging on these prey differ. We used respirometry to measure the costs of diving in double-crested cormorants (Phalacrocorax auritus) foraging either for sedentary (fish pieces) or motile (juvenile salmon) prey in a deep dive tank. Short dives for sedentary prey were more expensive than dives of similar duration for motile prey (e.g. 20% higher for a 10s dive) whereas the reverse was true for long dives (i.e. long dives for motile prey were more expensive than for sedentary prey). Across dives of all durations, the foraging phase of the dive was more expensive when the birds hunted motile prey, presumably due to pursuit costs. The period of descent in all the dives undertaken appears to have been more expensive when the birds foraged on sedentary prey, probably due to a higher swimming speed during this period.

Figure 1

The relationship between adjusted oxygen uptake and dive duration in five cormorants foraging on either sedentary or motile prey. Oxygen uptake is adjusted to mean duration of the surface period (13.4 s), mean dive depth (5.6 m) and mean bird mass (2.1 kg). Regressions: sedentary prey, solid, =1.68x+60.4, R²=0.23; motile prey, dashed, =2.66x+37.6, R²=0.38. Grey lines indicate 95% confidence limits. The region of non-significance (see text) is the range of dive durations where the elevations of the two regressions are not significantly different. Inset: mean adjusted oxygen uptake+s.e.m. for dives for sedentary prey (filled bars) of 10, 20 and 30 s and for dives for motile prey (open bars) of 10, 20, 30 and 40 s.