The IUCN/SSC Shark Specialist Group
Shark News 6: March 1996
|
Status of the basking shark
Cetorhinus maximus (Gunnerus)
Compiled by Sarah Fowler
Taxonomy
The basking shark is the only species of the family Cetorhinidae.
It is classified in the order Lamniformes, with the sand tiger,
thresher and mackerel sharks.
Basking shark Cetorhinus maximus.
© 1995 by Sid Cook. All rights reserved.
|
Distribution
Basking sharks occur in surface coastal waters of temperate and
boreal oceans in the North and South Atlantic, North Pacific, South
Pacific, and in southern Australian and New Zealand waters. They
have not been recorded from tropical areas, and are very rarely seen
in surface waters of the open ocean. In some areas records are made
only during the spring and summer months, suggesting a seasonal
migration, either from deep to shallow water or from lower to higher
latitudes in warmer weather. Exchange between northern and southern
populations could occur in deep cold waters below the thermocline,
but fisheries data suggest local stocks are highly isolated.
Description
The basking shark is named from its habit of 'basking' on the surface
in good weather conditions, when both the dorsal and upper lobe of
the tail fin, and sometimes the snout, may break the surface. It has
extremely long gill slits, very small teeth and modified gill rakers for
feeding on plankton and small fish. It is also huge (the second largest
fish after the whale shark), with maximum recorded lengths of 10 m
and unconfirmed records of over 13 m. The smallest free-swimming
young recorded have been 1.7-1.8 m. Coloration is variable, dark to
light grey and mottled on the back and lighter on the underside.
Distinctive patterns and scars have been used to re-identify individual
sharks. The liver is very large and may amount to 17%-25% of the
body weight, accounting for much of its commercial importance.
Ecology and reproduction
The basking shark is a filter feeder, and sightings are often associated
with surface aggregations of zooplankton. Stomach contents also
contain deep water zooplankton. Large aggregations (of 50 to 100
sharks) are sometimes reported, particularly along ocean fronts, or off
shoals, headlands and islands in areas of strong tidal flow. These may
be feeding or possibly breeding aggregations (pairing takes place in
early summer in the UK), and tend to occur in favoured areas. Surface
breaching of sharks is occasionally recorded here.
The presence of the large liver and high levels of squalene may
indicate a deep water habit for at least part of the life history. Winter
records are very uncommon in coastal areas at high latitudes. Some
specimens caught in the north-east Atlantic in winter had shed their
gill rakers, possibly indicating inactivity in deep water when low
zooplankton populations in winter make feeding activity inefficient.
Commercial summer surface catches in Scotland were dominated
by non-pregnant females (with a ratio of about 18:1). Incidental
catches in deeper water around Newfoundland were of just over two
males per female. There is only one record in literature of a pregnant
female, which gave birth to five large (1.5 to 2.0m) live young and
one still-born on board a fishing vessel. There appears to be
segregation during the life cycle and between sexes.
The reproductive biology of basking sharks is considered to be
similar to that of other lamnoid sharks. A single functional ovary
contains a very large number of small eggs. Ovoviviparity occurs,
with fertile eggs hatching within the uterus and the embryos gestating
for one or possibly up to three years, feeding on infertile eggs.
Pregnant females are so rarely caught in commercial fisheries that they
presumably segregate to an area where no fishery takes place (possibly
in deep water). Females may 'rest' for one year after giving birth
before mating again. The young, born at between 1.5 and 2 m, are rarely
encountered until they reach more than 3 m in length. Growth rates and
age at sexual maturation are unknown. Males become mature at 5-7 m,
possibly 12-16 years, and females at 8.1-9.8 m, possibly 20 years old.
Threats
Directed fisheries mainly utilise liver oil, fins and meat for food or
fishmeal. Small-scale active fisheries still occur in the north-east
Atlantic and north-west Pacific. Most localised fisheries, even where
very small scale, have provided initial high yields followed by serious
and long-lasting depletion of local stocks. This is considered to be one
of the species of sharks most vulnerable to overfishing.
Two of the best-documented fisheries occurred off the west coast
of Ireland. The Sunfish Bank fishery took large numbers of basking
sharks from small boats from 1770 to the 1830s, when sharks became
very scarce. Basking sharks were next recorded in abundance off this
coast in the 1940s, and a shore-based net fishery started at nearby
Achill Island in 1947. This fishery boomed for several years, then
declined steeply despite increasing shark oil prices and investment in
vessels with harpoon guns to enable exploitation of sharks further
away from the base (see Figure). This decline has been long-term;
very few basking sharks occur in the area today, 40 years later.
Number of basking sharks caught per year in the Achill Island fishery, Ireland.
|
The problems caused by entanglement in net gear have resulted
in some deliberate kills of basking sharks, for example in Barkley
Sound off Vancouver Island, where the kill of some hundreds of
basking sharks in the 1950s appeared to remove most of the population,
which has shown no significant recovery since then. Incidental
catches of basking sharks in other fisheries can also be significant. For
example, 77-120 basking sharks are taken annually in a bottom set
gill net fishery in the Celtic Sea. Certainly, sightings of basking
sharks around the Isle of Man in the Irish Sea are declining, despite
no known large-scale targeted fishery in the region.
IUCN threatened species assessment
With increasing emphasis being placed on shark fisheries, it seems
very likely that an overall, world-wide population reduction of at
least 20% will occur within the next 50-60 years, even if only the
resultof a somewhatdesultory and opportunistic rise in landings. The
species is therefore assessed as Vulnerable (A2d) throughout its range.
Some local or regional populations where targeted fisheries are in
progress, are likely to occur, or have resulted in a past population
decline of more than 80% with no recovery apparent after 40 years
(e.g. west coast of Ireland) are considered to be Endangered (A1 d, 2d,
D), or even Critically Endangered (Aid, 2d, and possibly C1 in the
Barkley Sound area).
Editor's note. The above is a greatly abbreviated version of the
draft account supplied by the author for the Shark Action Plan. The
original, including many references, is available from the Editor.
|
|
|
|
|
