Shag Phalacrocorax aristotelis desmarestii
The Mediterranean Shag Phalacrocorax aristotelis desmarestii is a subspecies
endemic to the Mediterranean and Black Seas. Despite their numbers are
not well known experts agree that a reduction
of the population has taken place. It is a seabird species linked to the
coast where it breeds in rocky areas.
The Mediterranean population of the species is included in Annex II of
the Bern Convention, and thesubspecies is listed in Annex I of the Birds
The species experienced catastrophic declines in many parts of its former
range due mainly to agricultural intensification (e.g. Schulz, 1985, del
Hoyo et al., 1996). Its most important populations are now located in the
EU, mainly within the
Iberian Peninsula. The available but incomplete figures suggest that 80-90%
of the world population may be concentrated in Spain and Portugal.
This Action Plan relies largely on the results of a Workshop on the subspecies
held in the Balearic Islands, on September 1999 and on replies to a specific
questionnaire sent to all non-EU range states in
2001. The species is sedentary and partially dispersive, but generally
philopatric. Only through adequate management at local level it would be
possible to ensure the survival of all populations. Numbers and
trends of the main colonies of the species are still unknown.
The species nests primarily in winter, however from year to year there
seems to be a great difference in nesting periods (Guyot 1984). The laying
dates range from November to March depending upon the region, and younger
birds breed later, occupying sub-optimal nest sites and having lower breeding
success. The commonest clutch size is three eggs and incubation lasts 30
days. The fledging of the chicks lasts for about 53 days.
The subspecies feeds mainly on coastal fishes, from bottom or mid water
over rocky or sandy seabeds. The Mediterranean Shag feeds mainly by pursuit-diving,
and normally alone. Economically important fish seems to form a very small
part of the diet.
Habitat requirements The species has a strong preference for rocky coasts
and islands. It is not normally found far from land. Roost always in the
seashore on rocks and stacks. During breeding period it forms sparse colonies,
nesting in crevices or caves, on ledges or amongst boulders, often a few
meters above the sea level. The
nest is built with a variety of vegetal materials, and is frequently reused
in successive seasons.
Threats and limiting factors
Mediterranean Shag is a shy bird which is severely
affected by frequent visit to the colonies. (Guyot 1993). The increase
of length of the tourist season and their activities close to the breeding
sites, the development and the lack of effective protection of some important
colonies can represent a critical threat. Birdwatching and research activities
can also cause serious disturbance. These threats are not only limited
at the colonies but also at roosting places.
Incidental oil spills or illegal washing of tanks are a proved threat (Lambertini & Leonzio
1986) that can have lethal and sub-lethal effects on adults and eggs through
Some fishing methods such as gill nets and fish traps, particularly when
located permanently close to the sea shore, are responsible of the killing
of a significant numbers of shags, as has been reported in
Balearic islands (Aguilar 1991).
Importance: unknown, locally high
Favourable habitat (for breeding, roosting and feeding) is most often unaffected,
but habitat availability for shags can locally be reduced by developments,
illegal trawling, construction of ports,
marinas and sea walls, uncontrolled anchoring of yachts and sand extractions
for beach regeneration. The latter can strongly affect the Posidonia beds
and other benthonic communities where Mediterranean Shag
Importance: medium - locallyhigh
Despite legal protection, illegal prosecution of the species is still frequent
in some areas.
Importance: lowPage 7
Predation by introduced mammals
It is possible that predation by rats in islets with high rodents’density
would be important, but there are not enough data on the effects of predation
by rats. Temporary presence of dogs may cause
serious disturbances and mortality of chicks and adults.
Levels of Hg, Se, Pb and PCBs are high in 3 studied animals, as well as
contents of PCBs in two eggs reported by Lambertini & Leonzio (1986).
There is no direct evidence of the impact of high
concentration of mercury and other heavy metals on the species’biology
in the Mediterranean. More field data are required.
Competition with other species.
Predation and competition for nesting sites by Yellow-legged Gull is potential
a problem since gull populations are increasing and many of their colonies
are located close to the Shag breeding sites.
Depletion of fish stocks may cause the declining of entire populations.
Conservation status and recent conservation measures
The species is protected in all the member states of the European Union.
Estimates of the total breeding population are incomplete and not globally
updated since the 1980s