Feeding by humpback whales in Tasmanian waters

5 Pages·1998·2.2 MB·

by  P C Gill

Feeding by humpback whales in Tasmanian waters 1
Feeding by humpback whales
in Tasmanian waters
Peter C. Gill' and Karen J. Evans2
Australocctus Research
'¥Rackham', Gap Rd. Hartley Vale NSW 2790, Australia
2 146 Forest Rd, West Hobart Tas 7000, Australia
and
Hans Wapstra
Tasmanian Parks and Wildlife Service
GPO Box 44A
Hobart Tas 7001 Australia
Abstract
Humpback whales were observed feeding on their southward migration past the east and south
coasts of Tasmania. Feeding by this species has not been confirmed previously for Tasmanian
waters. Feeding behaviours were similar to those observed in Antarctic waters.
In one observation, humpbacks fed in an area where a variety of zooplankton was
simultaneously netted, including neritic euphausiids (coastal krill) Nyctiphanes australis, prey of
humpback whales elsewhere. The importance of this food resource to migrating humpback whales
is not known, but should be assessed before it is commercially exploited.
Introduction
Humpback whales (Megaptera novaeangliae) from the More recently. North Atlantic and North Pacific hump¬
Antarctic Area V population migrate from Antarctic backs have been sighted feeding in mid- and low-latitude
summer feeding grounds, and pass along the east waters (Baraff et al. 1991; Gendron & Urban 1993;
Australian coast (Paterson & Paterson 1984), traversing Swingle et al. 1993). Humpbacks have also been reported
Tasmanian waters on both northward and southward feeding during the period October-November oif Eden
migrations (Davies I960). Little feeding is thought to (37°S, 150"E), New South Wales (R. & G. Butt, pers.
occur away from Antarctic waters (Brown & Lockyer comm.). In December 1995 humpbacks were reported to
1984). However, Dawbin (1956) reported that humpbacks be feeding on Tasmania’s east coast (I. Marmion, pers.
killed by whalers on northward migration past New comm.). Here we present the first confirmation that
Zealand had been feeding on euphausiids Nyctiphanes humpback whales feed in mid-latitude Tasmanian waters,
australis. Also, during the southward migration past New with an indication of their prey species.
Zealand in the 1950s, humpback whales were frequently
seen feeding in surface plankton patches (identified as N.
australis) in November, near 46°20’S in Foveaux Strait
(Dawbin 1956). The southward migration past New
Zealand coincided with swarming of the galatheid decapod
Munichi gregaria (ibid.).
2 Records of the Queen Victoria Museum 107 (1998)
Methods
Sighting a) One of us (HW) responded to a report that
whales were ‘feeding’ in the Derwent River near Hobart,
and drove to the scene, where his observations were
conducted from shore.
Sighting b) Humpback whales were sought from the 12m
yacht Domino while sailing along the south-east
Tasmanian coast, during the period of the annual south¬
ward humpback migration. Searching was conducted both
visually and acoustically (using stationary hydrophones
when sea conditions allowed), with at least two observers
on watch in the yacht’s cockpit or on deck.
During this sighting, behavioural observations were
recorded on a hand-held cassette recorder, and prey
sampling was conducted immediately afterward by
towing a net (500 mm mesh plankton net. 500 mm codend Figure 1 Map of Tasmania showing locations of
mesh, 70 cm mouth, no flowmeter) for two minutes at 3.7 humpback whale feeding events described in the text.
km IT per tow (volume of water sampled approx 47.5 m5), The dotted line shows the approximate position of the
through the area where the whales had been observed. 200 m depth contour.
Codend contents were immediately preserved in 10%
formaldehyde in unbuffered seawater. Zooplankton were
Observed behaviour: The whales were about 400 m south
later identified in the laboratory using a variety of
of the vessel when sighted. As they were approached,
references. Underwater noise generated by breaking
there was considerable surface disturbance in their vicinity.
waves precluded acoustic monitoring.
A single fluke or pectoral fin frequently protruded from
the water, turning in a manner which indicated that the
Observations
whales were turning sharply while on their sides, just
below the surface; white ventral surfaces of the whales
Sighting a) On 29 October 1996. two humpback whales
were sometimes visible underwater. We call these
were sighted by HW at Blackmans Bay (43°00’S,
‘lateral turns’. This behaviour was typically followed by a
147°20'E) near the mouth of the Derwent River (fig. 1).
brief surfacing, blow and peduncle arch before the
They were observed to be in an area where the water
animals re-submerged. There were also fluke-up dives
surface was smooth, while the surrounding water was
of short duration (less than one minute) with the animals
‘rippled’. The pair acted in a coordinated manner, with
frequent angled head-rises, breaking the surface at an resurfacing nearby, and abbreviated head-lunges which
angle of between 45-60°. Their mouths were occasionally barely broke the surface. An open mouth was visible
observed to be open while surfacing, and distended above the surface at least once. There were also erratic
throats were also noted. Usually after these head-rises changes of direction, as well as frequent vigorous subsurface
they sank back into the water, but occasionally completed activity, as shown by swirling and splashing at the
a forward roll, showing the dorsal fin. Earlier reports to surface. The two whales were always observed within
HW on this day by other observers in the same vicinity 30-40 m of one another, and their overall speed throughout
had described whales as ‘feeding’, with the crew of a the area was low.
Water Police vessel in the area reporting behaviour similar
to that described above. When its engine overheated, its This activity ceased at about 1600 h, when the vessel
water intake was found to be clogged with ‘krill’. Whales approached within about 200 m, and the whales moved
were reported in this area between 29 October and 11 eastward, passing about 75 m to the north of the vessel. At
November 1996. 1602 h a tow net was deployed less than one meter below
the surface through the area where the whales had been
Sighting b) Two humpback whales were sighted at observed in vigorous activity. At about 1620 h blows were
42°30.2'S, 148°02.3’E, at 1540 h (Eastern Australian sighted about 800 m to the north-west of the earlier sighting,
Summer Time) on 22 November 1996, near Cape and at 1625 h a pair of humpbacks was approached,
Bougainville at the northern end of Mercury Passage, the engaging in all the vigorous behaviours described
strait lying between Maria Island and the east Tasmanian above. Although photo-identification was not possible,
coast (fig. 1). The weather was fine and clear with a 37 km pigmentation patterns and dorsal fin shape indicated that
h1 southerly wind. Sea surface temperature was 13.4°C. these were the same animals observed earlier. Both
Water depth was about 30—40 m. appeared to be of young adult size, about 11-12 m. The
Feeding by humpback whales in Tasmanian waters 3
whales moved away to the east at approximately 1700 h, higher than in surrounding areas. Unfortunately no
and were not followed. No seabirds or seals were sighted further tows were done for comparison with other areas
during these encounters. on that day. While copepods were the most abundant
zooplankton species in Tow #1. humpbacks are thought
On 23 November 1996 the vessel passed south through not to prey deliberately on these crustaceans, due to their
Mercury Passage in calm, fine conditions. Surface trawls relatively small size (Kawamura 1980). The low numbers
were conducted at 1040 at 42°36.8’S, 148°00.3’E, and at of euphausiids (and other active swimmers such as
1200 at 42°40.6’S, I48°00.0’E. Codend contents were decapods) in Tow #1 may be the result of active net
preserved using the method described above. avoidance, as observed in euphausiids elsewhere
(Hovekamp 1989). This may have been exacerbated by
Results of net tows
the net being towed through water already disturbed by
the passage of the vessel’s hull. Actual densities of
Details of net tows are given in Table 1. Tow #1 was N. australis- in the area where the whales were feeding
characterised by high species diversity, and a relatively may thus have been considerably higher than our tows
high biomass compared to Tows #2 and #3. Zooplankton indicated.
biomass in Tow #1 was dominated by copepods, with the
most abundant species being Centropages australiensis N. australis is described as a major component of the
and C. orsoni. The only euphausiid species present in zooplankton community in south-eastern Tasmanian
any tow was N. australis. Although low numbers of coastal waters, where the species is the main prey item for
N. australis and decapods (the only other organisms of many coastal fish and bird species (Young et al. 1993).
comparable size) were present in all tows, they were However, N. australis may be virtually absent in years
noticeably more abundant in Tow #1 than in others. (e.g. 1989) when La Nina events bring an influx of warm
subtropical water to eastern Tasmania (Harris et al. 1991;
Young et al. 1993). They are confined to the continental
Discussion
shelf, particularly towards its inner edge, often occurring
in relatively sheltered waters such as Mercury Passage
Behaviours observed during both sightings were not similar (G. Hosie, pers. comm.). Surface swarming occurs in
to those commonly seen in breeding areas, or during Tasmanian coastal waters al any time of year (O'Brien
coastal migrations but were more typical of behaviours 1988).
associated with feeding. For example, behaviours such as
short-duration fluke-up dives, shallow ‘lateral turns’, We consider that the humpbacks observed on 22
subsurface disturbance and erratic movement were often November were feeding in a mixed assemblage of
seen in humpbacks during a survey in Antarctic Area IV zooplankton which included N. australis. Support for this
in early 1996, where they were feeding on euphausiids or includes:
other zooplankton near the surface (D. Thiele, Gill, Evans • behaviours observed were typical of those noted in
et al„ unpublished observations). feeding situations in Antarctic waters;
• feeding is known to occur during the southward
Pod size of feeding humpbacks we observed in Antarctic migration of Area V humpbacks, both off New Zealand
waters was usually two individuals, supporting extensive (where N. australis arc prey) and off Eden, NSW;
data from other Antarctic sightings surveys (Kasamatsu et • the vicinity of Mercury Passage is regarded as a focal
al. 1996), and from the Gulf of Maine (Weinrich & area for aggregations of N. australis (G. Hosie, pers.
Kuhlberg 1991). In Alaska, humpbacks feed on small, comm.), which are also abundant in Storm Bay (see
dense, scattered patches of euphausiids (Dolphin 1987b). Sighting 1) (Young et al. 1993).
Small cooperative groups of feeding humpback whales
may optimise the utilisation of such patchy prey resources Energetic demands on migrating humpback whales are
(Weinrich & Kuhlberg 1991; Claphant 1996). The pairs of high (Brown & Lockyer 1984). It is thus not surprising
animals in the two present observations appeared to act that humpbacks feed at this stage of their southward
cooperatively: they were always in close proximity to one migration, as by the lime they enter Tasmanian waters,
another, oriented in the same direction, frequently diving it may be seven months or more since they last fed. It is
and resurfacing together. Humpback whale feeding dive not yet known whether these incidences of feeding are
duration is correlated with depth (Dolphin 1987a, b). The isolated events, or whether feeding by humpbacks in
short dives (less than one minute) and the behaviours these waters was historically a regular occurrence, now
observed here are consistent with prey availability in or becoming more apparent as the Area V humpback
close to the surface layer. stock increases (Paterson et al. 1994). If zooplankton
stocks (particularly N. australis) in Tasmanian waters do
Relative abundance of zooplankton was highest in Tow #1 represent a significant nutritional resource for migrating
on 22 November (I able I), presumably because they were humpback whales, this must be considered in any future
taken in an area where whales had detected prey densities plans for direct exploitation of these stocks.
Records of the Queen Victoria Museum 107 (1998)
Tow # 1 2 3
Date 22/11/96 23/11/96 23/11/96
Latitude start 42 30.2 S 42 36.8 S 42 40.6 S
Longitude start 148 02.3 E 148 00.3 E 148 00.0 E
Time in 16.02 10.33 12.01
Time out 16.04 10.35 12.03
Speed 3.7 km/h 3.7 km/li 3.7 km/h
TAXON ABUNDANCE
Phylum Cnidaria
Hydromedusae - unidentified 6
Phylum Mollusca
unidentified 14 10
Phylum Crustacea
Subclass Copepoda
Pseudodioptomus comutus 6
Suheucalanus sp. 4
Centropages furcatus 1
Clausoccdanus sp. 7 4
Arcatia sp. 29 4
Neocalanus tonsus 6 1 1
Calculus australis 14 2
Centropages australiensis >1000 61 18
Centropages orsoni >1000 49 6
Subclass Malacostraca
Order Amphipoda
Unidentified 8
Order Ostracoda
Unidentified 1
Order Mysidacea
Unidentified 16
Order Euphausiacea
Nyctiplianes australis 7 2 1
Order Decapoda
Sub-Order Natantia 11 1
Sub-Order Caridea 3 i
Unidentified 2
Order Brachyura
Plagusia sp. 2
Unidentified 1
Nectocarcinus antarcticus 8 3
Cancer novaezelandiae I 1
Phylum Chaetognatha
Unidentified 1
Phylum Chordata
Thaliacea 3 71
Phylum Vertebrata
Superclass Pisces
Atherinidae 1
Misc. eggs 39 1 12
Table 1 Details of zooplankton net tows near Cape Bougainville and in Mercury Passage,
eastern Tasmania. Numbers indicate numerical abundance of plankton.
Feeding by humpback whales in Tasmanian waters 5
Acknowledgements Euphausiacea) off Tasmania, Australia’, Marine
Ecology Progress Series, vol. 12, pp. 219-33.
Thanks to Domino's crew — Greg Boyes, Ian Brokenshire Paterson, R. & Paterson, P. 1984, ‘A study of the past and
and Diane Brokenshire — for their assistance and enthusiasm present status of humpback whales in east Australian
during the cruise. Thanks also to David Pemberton of waters’, Biological Conservation, vol. 29, pp. 321—43.
Tasmanian Parks and Wildlife Service, and the Princess Paterson, R., Paterson, P. & Cato, D.H. 1994, ‘The status
Melicoff Trust for financial assistance. Graham Hosie of humpback whales Megaptera novaeangliae in east
provided many useful comments concerning Nyctiphanes. Australia thirty years after whaling’, Biological
The manuscript was improved by suggestions from Conservation, vol. 70, pp. 135-42.
Bill Dawbin. Thanks to the following for assistance in Swingle, W. M„ Barco, S. G., Pitchford. T. D.. McLellan,
identifying plankton: Graham Hosie, Aleks Terauds, W. A. & Pabst, D. A. 1993, ‘Appearance of juvenile
Craig Proctor. Russell Bradford and Barry Bruce. Lastly, humpback whales feeding in the nearshore waters of
special thanks to Rob Easther for the use of Domino. Virginia’, Marine Mammal Science, vol. 9, no. 3, pp.
309-15.
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