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Acoustic behaviour of sperm whales nursery groups in the waters of Ischia, Italy

Authors:
Barbara Mussi
Barbara Mussi
Angelo Miragliuolo
Angelo Miragliuolo
Daniela Silvia Pace at Sapienza University of Rome
Daniela Silvia Pace
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Abstract

Sperm whale (Physeter macrocephalus) nursery groups are poorly know in the Mediterranean Sea. Since 2002, 8 sightings of sperm whale social groups were listed in the deep waters of the canyon of Cuma (Island of Ischia, Italy). Animals were detected by the means of towed hydrophones and 820 minutes of recordings were collected. Maximum group size was of 12, including two juvaniles and one calf. Whales were observed from August to October between 320 and 750 bathymetric lines. The principal behaviours seen were slow travel (2,5/3 knots) and rest at the surface. In the 2004, both acoustical and behavioural activities were recorded, since whales stayed in the above mentioned area for 20 days. Adults spent most of their time diving while juvenile individuals remained at the surface (or just below it) for at least one hour or more. Although adult whales appeared to dive scattered in a wide area (1370 km2), they immediately joined juveniles when the research vessel tried to approach them, showing a clear protective behaviour. Principal surface activities included: spy hop, tail slap, side fluke, fluke out, fluke float and breach. Recordings of acoustic vocalizations showed long sequences of regular clicks and social sounds as codas, chirrups and squeals. Due to the high probability of error in naked eyes estimation, IPI was calculated in order to better define the length of the clicking whales. Results (9.13-11.36) confirm the absence of large bulls in the groups, probably composed only by females with juveniles and calves. The existence of sperm whale nursery groups in the study area over periods of years is an important finding since may represent the evidence that reproductive and breeding activities occur within the Tyrrhenian Sea. Where and when they mate, as well as they produce offspring is still unknown.
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ACOUSTIC AND BEHAVIOUR OF SPERM WHALE NURSERY GROUPS
IN THE WATERS OF ISCHIA, ITALY.
Mussi, B., (1) Miragliuolo, A. (1) & Pace, D. S. (2,3).
(1) Delphis MDC, via Zaro 22, Forio d’Ischia, Naples, Italy
(2) Università di Napoli “Federico II”, Naples, Italy
(3) Associazione Oceanomare, Via G. Marinuzzi, 74, 00124 Rome, Italy
Sperm whale (Physeter macrocephalus) nursery groups are poorly known in the Mediterranean Sea.
Since 2002, 8 visual sightings of sperm whale social groups were listed in the deep waters of the
canyon of Cuma (Island of Ischia, Italy). Whales were observed from August to October between
320 and 750 bathymetric lines (Fig. 1).
Fig. 1 Sperm whales social groups' routes in the study area
Animals were localized by the means of stereo
hydrophone and 820 minutes of recordings were
collected. IFAW cetaceans' software was used
during the boat surveys: navigation and data
storage were controlled by Logger 2000; whales’
clicks detections were monitored by Rainbow
Click. Real time analysis was performed using SIA
SmartLive 5. Post processing was made with
SpectraLab and Cool Edit software.
Difference in group size between visual and
acoustic detections were found. Visual ones
revealed a range of 1-5 whales (mean 3.02, SD
1.54), whereas acoustic ones allowed to identify
greater group size values (range 1-15, mean 8, SD
5.86).
In 2004 both acoustical and behavioural activities
were recorded, since whales stayed waters of
Ischia for 20 days.
Slow travelling (2.5/3 knots) and clustering at the
surface were the most frequent activities seen, with a relevant number of associated surface
behavioural events (spy hop, tail slap, side fluke, fluke out, fluke float and breach) (Fig. 2).
Adults spent most of their time diving while
juvenile individuals remained at the surface (or just
below it) for at least one hour or more. Although
adult whales appeared to dive scattered in a wide
area (1370 km2), they immediately joined
juveniles when the research vessel tried to
approach them, showing a clear protective
behaviour.
Recordings of acoustic vocalizations showed long
sequences of regular clicks and social sounds as
codas, chirrups and squeals (Fig. 3).
In order to define the length of the clicking whales, IPI (Inter Pulse Interval) was calculated for
regular clicks sequences (789 clicks measured) (Tab. 1).
A sperm click is made up of a number of regularly
spaced sound pulses resulting from multiple
reflection of the initial sound pulse within the head
of the animal. The spacing between the pulses in a
click, termed inter-pulse interval (IPI), has been
demonstrated to be solely a function of the length
of the spermaceti organ (backed by the distal and
frontal sacs) and the speed of sound in spermaceti
oil (Gordon, 1991; Goold and Jones, 1995).
Assuming
the latter to be a constant property and the length
of the spermaceti organ to be related to total body
length, the size of a sperm whale may be estimated
from the length of the pulse interval of its clicks.
Gordon (1991) fitted a polynomial equation to
Clarke’s data (1978) to relate the total body length
to the spermaceti organ length.
Tab. 1, Inter Pulse Interval measurements
Preliminary results confirm the absence of large bulls in the groups, probably composed only by
females with juveniles and calves.
The presence of sperm whale nursery groups in the study area over periods of years may represent
the first evidence that reproductive and breeding activities occur within the Tyrrhenian Sea. Where
and when they mate, as well as they produce offspring is still unknown.
THANKS TO: Humane Society International and RSPCA for funding the research; IFAW for free
cetacean software; Marco Battaglia and Marco Mocellin for field assistance. Special thanks to
Ischia dolphin Project volunteers that helped in collecting data.
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... Seven cetacean species have been regularly observed here. It is a feeding site for fin whales (Balaenoptera physalus) (Mussi et al., 1999), a feeding and breeding ground for striped dolphins (Stenella coeruleoalba), Risso's dolphins (Grampus griseus) (Mussi and Miragliuolo, 2003) and sperm whales (Physeter macrocephalus) (Mussi et al., 2005). Furthermore, the area has been listed in the IUCN Cetacean Action Plan (Reeves et al., 2003) as a critical habitat for the endangered short-beaked common dolphin (Delphinus delphis) Mediterranean sub-population. ...
Occurrence and spatio-temporal distribution of sperm whale (Physeter macrocephalus) in the submarine canyon of Cuma (Tyrrhenian Sea, Italy)
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... The low encounter rate of sperm whale may be attributed to the long dive behavior of the species. This can lead to an underestimation of the relative abundance of this species when using visual instead of acoustic methodologies (Mussi et al. 2005;Pavan et al. 2006). However, the comparison of encounter rates recorded in the other monitored transects using the same methodology reveals a minor presence of this species in this area during the summer months (Arcangeli et al. 2010). ...
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