Diving behaviour of harbour porpoises, Phocoena phocoena

Abstract

The diving behaviour of seven free-ranging harbour porpoises (Phocoena phocoena) was examined using time–depth recorders. In total, 8167 individual dives were recorded over 254 h. The longest period of data collection from an individual was 106.1 h. Mean dive depths and durations ranged from 14 ± 16 to 41 ± 32 m, and from 44 ± 37 to 103 ± 67 s, respectively. The maximum recorded dive depth and duration was 226 m and 321 s. This performance may not represent the maximum capacity of harbour porpoises but rather the maximum depth of the study area. Individual dives had similar rates of descent and ascent, which ranged from 1.1 ± 0.6 to 2.3 ± 1.4 m/s, and from 0.9 ± 0.6 to 2.1 ± 1.4 m/s, respectively. Two porpoises with monitoring periods >2 days demonstrated a diel pattern in their diving, making fewer, but deeper dives at night. Comparison of the diving behaviour of harbour porpoises with data on the depth of 140 porpoises entanglements in groundfish gill nets in the Bay of Fundy showed these seven porpoises made between 22 and 70% of dives to depths (range 20–130 m) where the majority of entanglements were reported.

Comparison of echolocation behaviour between coastal and riverine porpoises

“…In the later part of the approach phase of the present study, the inter-pulse interval changed by approximately 5.8 ms during 2.6 s indicating an estimated change in sonar range of 4.4 m. If the porpoise locked its sonar on a target at this time, the approaching speed would be 1:7 m s À1 . This is higher than the assumed cruising speed of 0:89 m s À1 and close to the upper limit of the descent and ascent speeds of wild harbor porpoises (Westgate et al, 1995). The animal might increase swimming speed during the final stage of approaching a potential target and reduce the speed thereafter.…”

“…5 is the distance travelled with no triggering of the A-tag. Here we assume a mean swimming speed of 0:89 m s À1 based on finless porpoise data (Akamatsu et al, 2005a), on body size (Sato et al, 2006), and on harbor porpoise swimming speeds during diving (Westgate et al, 1995). The distance swam without the A-tag being triggered is usually less that the calculated sonar range (Fig.…”

A recovery of harbour porpoises (Phocoena phocoena) in the southern North Sea? A case study off Eastern Frisia, Germany

“…It should be noted, however, that our calculations of g (0) and hence the resulting absolute densities are rather rough estimates, since the diving patterns of the porpoises in the study area are not known. Yet, the results of an investigation in Canadian coastal waters confirm that porpoises spend about 40% of their time in the water column of 0-1 m, making the assumption realistic that a general factor of 2-3 might be applicable for the availability bias (Westgate et al 1995). It is true that visibility has varied across all flights and even within one survey, depending on algae-blooms, reflections from clouds and turbidity.…”

Echolocation-based foraging by harbor porpoises and sperm whales, including effects of noise and acoustic propagation

“…Their echolocation signals are much more narrowband than those of most larger toothed whales; porpoise echolocation clicks are about 150 psec long, with peak frequency between 120 -140 kHz and a -3 dB bandwidth of about 10 -15 kHz (Au, 1993). They tend to forage in shallow, coastal waters (less than a few hundred meters deep) (Westgate et a/., 1995; Read and Westgate, 1997;, and they consume species of fish that tend to be found at or near the sea floor (Fontaine et al, 1994;Santos et al, 2004;Akamatsu et al, 2007). Consequently, they may forage in a highly cluttered acoustic environment.…”

“…They tend to forage in shallow, coastal waters (less than a few hundred meters deep) (Westgate et al, 1995;, and they consume some species of fish that tend to be found at or near the sea floor (Fontaine et al, 1994;Santos et al, 2004). Consequently, their foraging environment is highly cluttered, and sensitivity to Doppler shifts in returning echoes or Doppler compensation like that of CFDoppler bats could help porpoises more easily detect moving prey against a background of stationary clutter.…”