2008 JCRM Pace Miragliuolo Mussi sperm whale driftnet

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INTRODUCTION
On 9 August at 14.00, a social unit (sensu Whitehead, 2003)
of five sperm whales (Physeter macrocephalus), two adult
females and three juvenile individuals, was found entangled
in a driftnet 40 miles southwest off Capo Palinuro (Southern
Tyrrhenian Sea, Italy). Driftnets are large, floating nets
made of a mesh of monofilament or multifilament line,
generally deployed in open marine waters. They can be up
to 50km long and hang vertically 20-30m from the surface.
They are designed primarily to trap and entangle large fish
such as tuna (Thunnus sp.) and swordfish (Xiphias gladius),
however, left to drift freely, they indiscriminately trap and
kill non-target large pelagic species such as whales,
dolphins, sharks, turtles, rays and seabirds.
Large-scale pelagic driftnet fishing is of considerable
international concern; the United Nations banned the use of
large-scale driftnet operations on the open seas from 31
December 1992 and the European Union prohibited the use
of driftnets of all sizes from the 1 January, 2002.
Driftnets were the main cause of fatal entanglements for
cetaceans in the Mediterranean Sea, with serious
consequences for some populations (Di Natale and
Notarbartolo di Sciara, 1994; IWC, 1994). Between 1986
and 1990, 83% of all recorded cetacean strandings were
attributed to fisheries by-catch, nearly all in driftnets
(Cagnolaro and Notarbartolo di Sciara, 1992). At the peak of
driftnet deployment, an annual by-catch of over 7,000
cetaceans was estimated for the Italian seas alone
(Notarbartolo di Sciara, 1990).
Despite their illegal status, these nets are still in use in
Italy (where both nets and vessels are usually called
‘spadare’) and continue to cause harm and/or the death of
unknown numbers of protected species each year. Of the
large cetaceans, the sperm whale is the most affected by this
method of fishing (Lazaro and Martin, 1999; Notarbartolo di
Sciara et al., 2004). Between 1986 and 2000, 64 sperm
whales were killed in Italy alone as a result of entanglement
in fishing gear (they showed injury or were stranded as a
result entanglement); most, if not all, of these were
attributable to driftnet bycatch (Reeves and Notarbartolo di
Sciara, 2006).
The aim of this work is to describe the behaviour and
acoustic vocalisations (clicks) of an entangled social unit of
sperm whales during the rescue operation conducted by the
Italian Coast Guard scuba-diving team.
Sperm whale clicks are sharp-onset, broadband,
impulsive vocalisations with a frequency of between 5 and
25kHz (Madsen et al., 2002), arranged in various patterns
(usual clicks, slow clicks, codas, creaks, etc.) and used in a
variety of circumstances (Whitehead, 2003). Within sperm
whale social groups, clicks are possibly representative of
intimate interactions among its members and the exchange
of codas is probably the most evident form of
communication (Watkins and Schevill, 1977) aimed at
social interaction. Hence, a detailed analysis of these
sounds, emitted during the rescue procedures, was
performed.
METHODS
A professional underwater camera was used to record the
sperm whales’ behaviour during the disentanglement
process. Out of a total video/acoustic recording of 110
minutes, 91 were visually examined with the aim of
J. CETACEAN RES. MANAGE. 10(2):131–135, 2008
131
Behaviour of a social unit of sperm whales (Physeter
macrocephalus) entangled in a driftnet off Capo Palinuro
(Southern Tyrrhenian Sea, Italy)
D. S. PACE
*#
, A. MIRAGLIUOLO
+
AND B. MUSSI
+
Contact e-mail: danielasilvia.pace@gmail.com
ABSTRACT
Driftnet fishing is notorious for being the major source of fatal entanglement of cetaceans and for its devastating impact on some pelagic
species of the Mediterranean fauna. Of all the large cetaceans, the sperm whale (Physeter macrocephalus) is most affected by this fishing
technique. On 9 August 2004, a group of five sperm whales, two adult females and three juvenile individuals, was found trapped in a driftnet
40 miles southwest off Capo Palinuro (Italy). Their tails were totally immobilised by the net and one animal was completely entangled. All
the animals showed numerous lesions on their bodies. The group was freed by the Italian Coast Guard scuba-diving team during a two-day
rescue operation.
This exceptional case of sperm whale disentanglement was a unique opportunity to study the group’s acoustic and general behaviour
during a particularly stressful event. Out of a total video/acoustic recording of 110 minutes, 91 were examined. During the rescue
procedures, the whales’ behaviour was described as open mouthed, sideways roll, agitation of fluke and pectoral fins, head rubbing, fluke
contact (with head, flippers and back by the liberated animals) and defecation. As expected, the entangled individuals produced different
patterns of clicks, identified as ‘usual clicks’, ‘codas’ and ‘creaks’. Each pattern was associated with specific behaviour.
Despite international and national regulation banning fishing with driftnets in the Mediterranean Sea, driftnets continue to be used
illegally in this sperm whale habitat, posing a constant threat to the species’ survival in the region.
KEYWORDS: INCIDENTAL CATCHES; SPERM WHALE; DRIFTNET; MEDITERRANEAN SEA; BEHAVIOUR; NORTHERN
HEMISPHERE
*
Marine Ethology, University of Naples ‘Federico II’, Italy.
#
Oceanomare, Via Gino Marinuzzi 74, 00124 Rome, Italy.
+
Delphis MDC, Via Zaro, Ischia, Italy.
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analysing the animals’ behaviour during the rescue actions.
For the study, a short behavioural catalogue of seven
activities – open mouthed, sideways roll, agitation of fluke
and pectoral fins, head rubbing, fluke contact (with head,
flippers and back by the liberated animals) and defecation –
was established.
For the acoustical analysis, three categories were
identified: ‘usual clicks’; ‘codas’; and ‘creaks’.
‘Usual clicks’: a series composed of regularly spaced
clicks lasting for several minutes. These have been
interpreted as echolocation clicks, used for locating prey and
orientation (Whitehead and Weilgart, 1990), but may also
serve to keep widely dispersed foraging groups in contact
(Andrè and Kamminga, 2000).
‘Codas’: distinctive stereotyped patterns of clicks
(Watkins and Schevill, 1977) with different rhythms. Each
rhythm defines a distinct coda type (Weilgart and
Whitehead, 1997), these characterise diverse vocal clans
within sperm whale populations. Coda repertoires are
reported to be transmitted culturally within the sperm whale
social unit (Rendell and Whitehead, 2003).
‘Creaks’: patterns of closely spaced clicks with inter click
intervals (ICIs) ranging from 5 to 100ms and lasting from
0.1 to 45s. Creaks are thought to be produced by sperm
whales investigating objects at close range (Mullins et al.,
1988). In social context they have been called codacreaks
(Weilgart, 1990), rapid clicks or chirrups (Goold, 1999).
They have been described as social sounds (Gordon, 1987)
but their exact function is still unknown.
Forty-five minutes of acoustic recordings were examined
using the Rainbow Click software package (see Gillespie
and Leaper, 1997; Jaquet et al., 2001; Leaper et al., 2000).
Codas were then marked and outputs of the digitised sound
data for each click in each coda were used for IPI (interpulse
interval) analysis. A MATLAB routine written by Rendell
and Whitehead (2003) was used to automatically analyse
clicks by extracting the maximum value from the cepstrum,
following Goold’s (1996) method. Only codas for which IPI
estimates were identical in 50% or more of the clicks were
included.
RESULTS
The sperm whales were found with their tails totally
immobilised by the net and one animal was completely
entangled (Fig. 1). All the whales showed numerous lesions
on their bodies; their flukes in particular appeared to have
been seriously injured. The estimated lengths of the two
adult females were 10-12m, while the younger animals
(gender unknown) was 5-7m long.
On the first day the divers managed to free two of the
trapped whales; an adult and a younger individual by
severing the nylon netting with cutters. It took 65 minutes to
free the entangled adult whale, which remained calm
throughout the rescue procedures and subsequently stayed
close by, frequently touching the other entangled members
of the group on their heads, flanks and flukes and observing
the divers whilst they released a juvenile animal. The
younger whale, freed 45min later, was relatively agitated
compared to the adult during the net cutting operations,
vigorously moving its fluke and frequently opening its
mouth. Both the freed whales remained nearby surveying
the divers at work and repeatedly rubbing on their trapped
schoolmates’ flanks with their heads and stroking their
entangled flukes. This physical interaction hindered the
liberation of the other animals by effectively stopping the
divers’ work. At 18:50hr, the young whale gradually
abandoned the rescue site and disappeared from the divers’
view. At 20:00hr, the rescue operations were suspended,
scheduled to recommence the next day at daybreak.
During the night, the whales were constantly monitored
from the Coast Guard vessel by radar and night-vision
equipment. The freed adult whale remained near the group
almost all night, moving away just before sunrise. On 10
August, at 06:30hr, the divers resumed cutting through the
netting, managing to liberate a second young whale at
08:15hr and at 08:30hr the remaining juvenile was cut free.
Finally, the largest animal was disentangled at 08:50hr.
Following release, this female lingered close to the divers
for over an hour, moving slowly and accepting hand contact
on her side.
The behaviour of the sperm whales during the rescue
operations included several specific actions: open mouthed
(29%); sideways roll (17%); agitation of fluke (21%) and
pectoral fins (12%); head rubbing (9%; Fig. 2), fluke contact
with head, flippers and back by the released animals (9%;
Fig. 3); and defecation (3%). No threatening or other
aggressive behaviour toward the divers was recorded.
As expected, the entangled individuals produced different
patterns of clicks, identified as ‘usual clicks’, ‘codas’ and
‘creaks’. Almost all of the sounds provided an estimated
length of between 9.30 and 9.35m (Fig. 4), suggesting that
they were produced by the same whale or by the two adults
that were similar in size.
A coda frequency rate of 1.13 codas per minute was
recorded, with a total number of 51 codas detected. About
88% lasted between 200 and 600ms. Only 10% exceeded
600ms in total duration and just 2% were shorter than 200ms
(Fig. 5). The overall mean duration of the codas was
398ms (N=51, SD=133.4, range 195-813, mode 284,
median 377).
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Fig. 1. The entangled whales. Fig. 2. Head rub.
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Six coda types, containing 3-6 clicks, were catalogued
and classified in accordance with Weilgart and Whitehead
(1993). Two patterns of 3-click and 3+1-click codas were
found to be the most common, constituting 80% of all codas
recorded (Table 1). Codas were principally heard when the
animals swished their flukes and during contact with other
entangled tails.
Codacreaks analysis indicated that the frequency rate was
10.51min
–1
, with a total number of 473 codacreaks detected.
The number of clicks in codacreaks ranged from 3 to 45,
with the modal value within the 20-30 clicks class (Fig. 6).
This type constituted about 36% of the total, followed by the
10-20 type (30%). The overall mean duration of the
codacreaks was 614ms (N=473, SD=302.8, range 43-1,706,
median 663, mode 765), with 61% lasting between 400 and
900ms. Only 14% of the codacreaks had an overall duration
of more than 900ms and 26% were shorter than 400ms (Fig.
7). From the behavioural point of view, codacreaks were
associated with ‘open mouthed’ and ‘sideways roll’
displays, even if, like codas, they were heard when the
animals swished their flukes.
DISCUSSION
‘Head rubbing’ and ‘fluke contact’ were the most obvious
demonstrations of social behaviour showed by these
animals. Female sperm whales are reported to cooperatively
assist their offspring and other whales in dangerous
situations, (Caldwell et al., 1966) and the members of a
school seem to safeguard injured calves. This tendency was
evident during this event, where the first freed adult female
whale showed a higher number of ‘fluke contact’ displays
than the younger individuals (even when liberated), clearly
trying to comfort the animals still entangled in the net.
Females and immature sperm whales socialising near the
J. CETACEAN RES. MANAGE. 10(2):131–135, 2008
133
Fig. 3. Fluke touch.
Fig. 4. Body length estimates from IPI.
Fig. 5. Frequency distribution of codas duration.
Fig. 6. Frequency distribution of clicks in codacreaks.
Fig. 7. Frequency distribution of codacreaks duration.
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surface often touch and stroke one another with the jaw or
flippers, actively maintaining physical contact with each
other (Whitehead and Weilgart, 2000). In this case, physical
contact appeared to be central to reassuring the entangled
animals. An intense ‘head rubbing’ movement, performed
primarily on the whales’ sides without emitting any form of
vocalisation, was also observed mutually between adults
and juveniles. The lack of acoustic patterns associated with
such contact seems to indicate the importance of touch to
reinforce bonds between group members and demonstrates
the significance of tactile signals as a direct form of support.
As reported, the first disentangled female remained with the
group for many hours after liberation, demonstrating this
supportive behaviour towards individuals who may or may
not have been related. Female sperm whales live in a social,
ecological and physical environment where supportive
behaviour may develop and be commonly practiced
(Mesnick et al., 2003). Reinforcing social bonds through
cooperation and association is adaptive behaviour and calf
protection would seem to be the most likely primary
functions of sociality among females (Whitehead and
Weilgart, 1991).
The other notable behaviour observed, both in adult and
immature individuals, was ‘open mouthed’. This action was
performed exclusively when the animals were trapped and
was repeatedly associated with vocalisations. ‘Open
mouthed’ displays are frequently reported in other
odontocetes species during aggressive/agonistic bouts
(Samuels and Gifford, 1997) or in stressful situations. It
seems likely that this type of behaviour was accentuated
during this traumatic experience; it may occur more
frequently in stressful circumstances than under normal
conditions.
‘Sideways roll’ and ‘agitation of flukes and pectoral fins’,
correlated with vocalisations, were principally observed in,
but not limited to, immature individuals. As sperm whales
often roll along each others’ bodies during interactive social
sessions or roll onto a flank, with one of its fluke lobes out
of the water, during prolonged periods of surface swimming
(Whitehead, 2003). It is possible they behaved in this way,
not only to try to free themselves from the net but also to
facilitate visual observation (in this case, the divers cutting
the net). Furthermore, the movements of flukes and flippers
were often associated with the cutting procedures near the
peduncle; this was possibly a reaction to pain caused by the
net cutting into the numerous wounds and vocalisations
were often heard. In female sperm whales there is a strong
correlation between categories of visually observable
behaviour and vocalisation types (Whitehead and Weilgart,
2000).
The recorded coda repertoire, i.e. the set of codas emitted
by a set of whales in a particular circumstance (Whitehead,
2003), dominated by 3R and 3+1, is analogous to that
recorded off the Balearic Islands (Nutthila, 2004) and in the
Tyrrhenian Sea (Drouot, 2003), suggesting that these whales
may belong to the same clan (sensu Whitehead, 2003).
However, the complete recorded repertoire of the entangled
whales, consisting of six different coda types, slightly
differs from data reported by Drouot (2003) for the
Tyrrhenian Sea and is in contrast with previous studies
(Borsani and Pavan, 1994; Pavan et al., 2000).
Codas were principally heard when the animals swished
their tails and during contact with entangled tails. This last
finding is consistent with other observations, which report
that an extensive coda repertoire is generally associated with
cohesive groups near the surface and during exchanges with
other whales (Whitehead and Weilgart, 1991). The acoustic
results strongly suggest an interactive function of codas and
codacreaks within the social group, underlining the link
between their production, communication and sociality
(Whitehead, 2003). It has been hypothesised that
vocalisations may not only reflect the general ‘disposition’
of an animal but may also be indicative of moods and
emotions. It cannot be ruled out that in such traumatic
conditions, vocalisations serve to communicate emotions,
since it has been suggested they may play a role in social
interaction (Aureli, 1997) and could be adaptive, evolving in
species where social bonding, group cohesion and mutual
interactions favour the species’ survival. Codas and
codacreaks production may be related to levels of anxiety
and possibly apparent dangers (they were higher during the
initial phases of the rescue procedures) and seem to be
emulative between animals. However, the echolocation
function for close objects cannot be excluded.
In the Mediterranean Sea, driftnets are still a major threat
to certain vulnerable pelagic species and, despite
international and national regulation banning them from the
region, numerous sperm whales have been found dead
following entanglement in driftnets illegally set for
swordfish. In the last three decades (from 1971 to 2004) the
documented number of sperm whales found dead or
entangled for Spain, France and Italy was collectively 229
and the true number is probably much higher (Reeves and
Notarbartolo di Sciara, 2006).
The majority of whale strandings (recognisable from the
characteristic wounds on the whales’ bodies or the presence
of net fragments) in Italy and Mediterranean Spain were
caused by entanglement in high seas driftnets (Lazaro and
Martin, 1999; Podestà and Magnaghi, 1989); deaths from
this illegal activity persist today (ACCOBAMS, 2003;
Tudela et al., 2003). While the true abundance of sperm
whales in the Mediterranean Sea is unknown, most
estimates suggest stock sizes in the hundreds rather than in
the thousands (Reeves and Notarbartolo di Sciara, 2006).
Given such low population numbers, there are major
concerns over the impact of this type illegal fishing on this
isolated population. Urgent management measures are
clearly needed to monitor illegal fisheries and to protect
cetaceans and other marine species, from the devastating
effects of driftnet bycatch.
ACKNOWLEDGEMENTS
We would like to thank the Italian Coast Guards for their
invaluable work and for providing us with professional
video images of the rescue. In particular, we are grateful to
Ammiraglio Pierluigi Cacioppo, ‘II Nucleo Operatori
Subacquei – Guardia Costiera Napoli’ and to CP 280.
Special thanks to TV(CP) Roberto Pagnanini for the
illustrations, Luke Rendell and Cormac Booth who helped
in IPI analysis, Giancarlo Giacomini for his useful
suggestions, Giuseppe Notarbartolo di Sciara and an
anonymous reviewer for their constructive comments on the
first version of this paper. Finally, we are very grateful to
Suzanne Bennett for her English advice.
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