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What
brings a deep-diving whale to our shallow shores?
By
Laura Mandleberg (Sightings Officer)
The northern bottlenose whale
that swam up the Thames estuary and into Central London last week has
left the nation asking itself; how and why did a deep-diving oceanic
species, thousands of miles away from its natural habitat, end up swimming
past the London Eye and the Houses of Parliament? In an attempt to unravel
this apparent mystery, previous accounts of northern bottlenose whales
around the Scottish coast are described. Although the west coast of
Scotland is considerably closer to the species’ natural habitat
both in distance and character than the Thames estuary, a common theory
to explain these unusual events is discussed.
The northern bottlenose whale belongs to a family of deep-diving whales
called the beaked whales (Ziphiidae). These whales spend the majority
of their lives in deep-water habitats in the North Atlantic meaning
that scientists, let alone members of the public, are rarely lucky enough
to catch a glimpse of these elusive animals. Their oceanic lifestyle
has led to a great gap in our knowledge about general ecology of beaked
whales, with many aspects of their life history remaining a mystery.
Curiously, from time to time, the Northern bottlenose whale turns up
around our coasts, often extremely close to the shore. However, this
is by no means a common affair for over the last decade, northern bottlenose
whales have been reported to HWDT just four times:
19th August 1998: Kyle of Lochalsh (2 individuals)
17-22nd September 1998: Kyles of Bute (2 individuals
– stayed for about a week – lots of surface activity –
breaching and tail slapping)
21st July 2002: Loch Seaforth, Isle of Lewis (3 individuals
– Seen from a fish farm – not much surface activity –
appeared to be resting at surface)
6th September 2005: Loch Brachadale, Isle of Skye (2
individuals – feeding, tail slapping, breaching – lots of
surface activity)
These unusual, albeit rare accounts of a deep-diving, oceanic species
arriving in Scottish sea lochs certainly challenge what we thought we
knew about these animals, and potentially shed new light into to their
feeding habits. The most recent sighting in September last year described
the whales apparently chasing shoals of fish in the sea loch! This behaviour
seems unusual, given that these whales are specialised in feeding on
deep-sea fish and squid down as far as a kilometre below the surface.
Perhaps the observers, who believed that the whales were chasing schools
of mackerel, were mistaken? Or perhaps not, since this species has been
known to consume herring, which is also found in coastal waters. A sighting
of a northern bottlenose whale reported in 1998 also describes whales
feeding. On this occasion, the whales stayed for nearly a week in the
Kyles of Bute, which are narrow channels separating the Isle of Bute
from mainland Scotland. Throughout this entire period, the whales were
also noticed to be particularly active, enthralling observers on land
with their acrobatic displays.
Interestingly, these accounts do not seem to fit the description of
a lost, distressed or disorientated animal that has drifted into the
area by mistake, or through a weakened condition from illness. Instead
it seems more likely that they describe the arrival of healthy animals
into these sheltered areas. There is one theory that may explain the
presence of these unlikely visitors in our coastal areas. It is possible
that these sightings support the ‘migration error’ hypothesis,
which simply suggests that if healthy animals make errors in direction
of their migratory movements, they can end up in the wrong place. Dr.
Colin MacLeod, an expert in beaked whale ecology, has studied the seasonal
patterns in northern bottlenose whale strandings along the entire British
coastline (MacLeod 2004). His analysis shows that there is a significant
rise in strandings between July and October, with a peak in September,
and that this increase may represent a seasonal movement of the whales
in late summer and autumn from the whales’ usual habitat (deeper
waters off the continental shelf).
A southwards migration is thought to take place during this time, with
the whales primarily staying in deeper water beyond the shelf edge.
However some animals may also move into shallower shelf areas, which
may explain the occasional sighting, and the increase in strandings,
of northern bottlenose whales around our coasts. According to Dr. MacLeod,
there are at least three possible reasons for animals entering such
coastal waters as they move south. These are (1) navigational errors;
(2) animals following movements of prey which enter coastal waters at
the time northern bottlenose whales are heading south; and (3) that
such areas are used as ‘short-cuts’ between deep water areas.
But how do these theories explain a northern bottlenose whale swimming
up the Thames estuary, not just hundreds but, thousands of miles away
from its natural habitat? Is it possible that the whales can make such
drastic errors in navigation?
The answer appears to be yes! If we apply this same ‘migration
error’ hypothesis to the whale that swam up the Thames estuary
last week, we can come up with a rational and simple explanation to
how and why this happened. The ‘migration error’ hypothesis
actually originates from a study that looked at Sperm whales in North
Sea (Smeenk 1997). Sperm whales, like northern bottlenose whales are
deep-diving, oceanic species, that you wouldn’t expect to see
in the North Sea, which is like a big shallow basin. Nevertheless, information
from strandings records, and the occasional live sighting tell us that
from time to time sperm whales also enter the North Sea and this is
how it is thought to happen; imagine that a whale travelling south from
the northern North Atlantic makes a navigation error, for whatever reason,
and ends up coming down the wrong side of Shetland, it would inadvertently
enter the North Sea. Once in the North Sea, whales may try to continue
their usual, perhaps instinctive movement towards the southwest, which
would normally take them through deep water to the west of the UK and
Republic of Ireland. As a result the whales, effectively ‘trapped’
in the North Sea, strand in areas that appear to offer outlets to the
southwest, such as large estuaries like the Thames.
The ‘migration error’ theory may therefore provide a reasonable
explanation for the seemingly impossible scenario of a northern bottlenose
whale swimming through central London! The whale simply took a ‘wrong
turn’ on its southwards migration and became trapped in the North
Sea. However, whilst we may be able to explain how the whale got there,
the reasons why a creature with such a highly developed internal navigational
system should so grossly mis-judge its course may not be so easy to
explain. Do these navigational errors arise naturally during the lifetimes
of these creatures, or have more recent human influences tipped the
natural balance of their environment forcing a deep-water oceanic species
towards our coasts? One thing is certain; a northern bottlenose whale
turning up outside the houses of parliament is a rare occurrence and
one that has brought the whole nation to ask itself these questions.
References
MacLeod, C.D; Pierce,G.J and Santos, M.B. ‘Geographic and temporal
variations in strandings of beaked whales (Ziphiidae) on the coasts
of the UK and of the Republic of Ireland from 1800-2002. Journal of
Resource Management: 6 (1): 79-86, 2004
Smeenk, C. ‘Sperm whale deaths in the North Sea: Science and Management.
Strandings of sperm whales Physeter macrocephalus in the North Sea.
Bull. Inst. R. Sci. Nat. Belg. Biol. B67 (Suppl): 15-28. 1997
For
more facts about the Northern Bottlenose Whale look at our
Factsheets.
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