KILLER WHALES (ORCINUS ORCA) FEEDING ON SCHOOLING HERRING (CLUPEA HARENGUS) USING UNDERWATER TAIL-SLAPS: KINEMATIC ANALYSES OF FEILD OBSERVATIONS
Review of research paper:
The Journal of Experimental Biology 203, 283-294 (2000) free
article
by Eirik Hagtvet, 4th year.
The killer whales hunt for herring is a well organized and disiplinated
event. Several individuals work together in a specific manner to make
the capture of fish as effective as possible.
The article presents a fieldwork on how the killer whales feed on herring.
The observations were carried out in November 1992 in Tysfjord, northern
Norway, which is the main wintering area for the Norwegian spring-spawning
Herring.
Materials and methods used for the observations included a hydrophone
for acustic analysis and an underwater video camera. Recordigs were done
in the upper 15 meters of the water. These instruments were on a 3.8-meter
small boat together with the researchers. Events were recorded during
four days observation of the whales. During this period the feeding behavior
of the whales were closely monitored.
The investigations showed that the whales used several steps in their
hunt for herring. To start with, groups of whales (10-20 individuals)
cooperatively herded herring into schools close to the surface. Secondly,
the whales then swimmed around and under the fish so that all the herring
were kept in one school. The killer whales did not try to attack the herring
during these first two phases. With periodically lunging into the school
the killer whale stunned the fish by a tail-slap with the ventral side
of their fluke. The whales could then easily eat the disoriented herring.
The lunging never resulted in whole body attacks. Consumption of herring
was always a result of a tail-slap with the fluke. It was registeded that
the tail-slap could be divided into two phases, the preparatory phase
and the slap phase. In the preparatory phase the tail was bended in a
ventro-dorsal direction forming a concav side dorsally. During the slap-phase
the concave side was ventrally. The preparatory phase starts with small
amplitude (m/s) followed by the slap phase of greater amplitude (m/s).
All these phases happened completely submerged.
The hydrophone registered a loud noise every time the whales hit the fish.
No noise was heared if the tail-slap missed the target. The recovery of
some of the herring floating on the surface showed broken backs and large
scratches. These observations suggested that the herring was made uncounsous
by physical contact.
It became clear that killer whales never tried to chase and capture the
herring by their mouth. They always stunned the herring with a tail-slap
and then ate the fish.
The article sugested two main resons why tail-slaping was the best way
to capture herring. First, tail-slap velocity could reach 13.6 m/s in
0,5 seconds. This is much faster than the maximum speed the herring can
produce. Due to the low acceleration with large sized aquatic vertebrates,
it is unlikely that the whales are able to produce such a high speed by
swimming. The herring would therfore easily escape any attempt from the
whale. Second, the stunning of several herrings (presumably between 10-47)
by one tail-slap is more efficient for the killer whale.
The results from this field trial are based on a small number of observations
within a small time interval. Acording to the autors, further investigation
is neccessary to clerify this physical-contact-hypothesis between the
killer whales and the herring.
Source: The Journal of Experimental Biology 203, 283-294 (2000)
Printed in Great Britain © The Company of Biologists Limited 2000
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