Table 1.
Dive and foraging statistics.
Figure 1.
Foraging dive profile and echograms showing sea-floor and organisms ensonified by the whale.
(A) Dive profile of a female Blainville's beaked whale (MdH22 tagged on 10/03/04) showing the vocal phase of the dive (thick grey line), the location of the buzzes (black circles) and the seafloor (red dots). (B, D) Echograms with a range span of 15 m indicating the distance from the whale to echoic organisms on consecutive clicks. Echoes appear as dots coloured by the signal-to-noise ratio of the echo. In these examples, the echo counts are 198 echoes/60 clicks (B) and 6 echoes/35 clicks (D). The red thick lines on the left of the plots are the clicks emitted by the whale. (C) Echogram with a range span of 150 m showing reverberant reflections off the sea-floor which was 90–120 m below the whale.
Figure 2.
Time budget of seven whales in 11 tag deployments with at least two complete foraging dives. The bars represent the proportion of time invested in: inter foraging dive intervals (IFDI, light grey), silent transport to and from foraging depths within deep foraging dives (dark grey) and searching (i.e., echolocating) for prey (black).
Figure 3.
Whale foraging activity in relation to the sea-floor.
(A) Dive profile of a male Blainville's beaked whale (MdH1 tagged on 16/05/08). Thick grey lines indicate the vocal phase of the dives (echolocation) and grey circles represent foraging attempts (buzzes). Black dots show the location of the sea-floor and the vertical dashed line at 20:45 h indicates sunset. The whale approached the sea-floor in most dives despite diving to a wide range of depths. (B) Detailed view of part of the first dive when the whale follows the sea-floor contour to forage. (C) Deviation of the sea-floor depth, in the directions taken by tagged whales, as a function of dive time. The sea-floor deviation is the instantaneous sea-floor depth minus the mean sea-floor depth over each dive. The negative slope of the regression line indicates a tendency for whales to forage down-slope.
Figure 4.
Day and night depth profiles of echo and foraging parameters.
(A) Echo-index, indicative of the relative density of echoic organisms insonified, (B) proportion of time spent echolocating per depth bin, (C) proportion of buzzes produced per depth bin, (D) buzzes per minute spent in each depth bin. Lines and bars correspond to the mean and standard error, respectively, of the above parameters for day (grey) and night (black) dives. These plots represent pooled data for all dives from all tag deployments in 50 m depth bins except the echo-index, which was not possible to quantify for two tags (6 dives) due to high flow-noise levels. (E) whale depth and altitude above the sea-floor for each buzz recorded within 60 s of a sea-floor echo (black and grey dots for night and day time buzzes, respectively) and histogram of whale-altitude in 50 m bins for those buzzes (top black bars).
Figure 5.
Whale start of clicking in relation to biological and physical parameters of the study area.
(A) Depth at which tagged whales (n = 9 individuals, identified with different symbols) start echolocating in 50 foraging dives performed at different hours of the day. (B) Echosounder profile showing the depth distribution of the biomass in the study area off El Hierro during the day-night transition. The colours indicate echo intensity in dB re 1 m−1. The vertical dashed line indicates the time of sunset and the red areas correspond to echoes from the sea-floor. The white bands result from outages in echo reception due to air bubbles produced by manoeuvres of the boat. The circadian vertical migration of the deep scattering layer is evident. (C) Profile of the dissolved oxygen concentration (ml/l) in the same waters.