Náttúrufræðingurinn - 2008, Blaðsíða 35

Tímarit Hins íslenska náttúrufræðifélags
komið í veg fyrir varp eða álegu eða
ungar drepnir litlir, að ekki sé talað
um þegar eitthvað af þessu gerist ár-
lega, er ólíklegt að varp haldist á við-
komandi stað til frambúðar.
Hvers vegna leggjast sum vörp af
frekar en önnur? Aðeins eru nú eftir
örfá af mörgum teistuvörpum á
Ströndum og eru þau öll á nær sam-
felldu svæði frá Skeljavík að
Broddanesi (1. mynd). Athyglisvert
er að þessi vörp eru á svæði þar sem
mannabyggð og umferð er hvað
mest og sum nærri bæjum og veg-
um. Svo virðist sem teistan njóti
einhvers konar verndar af nábýli
við menn. Til dæmis mætti ætla að
fremur verði vart við minka, og að
tilkynnt sé um þá, þar sem byggð er
meiri og greni fái þar síður að vera í
friði. Þá má spyrja hvernig varpi á
vöktuðu svæðunum hefði reitt af ef
minkar hefðu ekki verið unnir þar.
Framtíð þeirra teistuvarpa sem
eftir eru á Ströndum virðist ótrygg.
Til þurfa að koma markvissar að-
gerðir ef takast á að koma í veg fyrir
að þau líði undir lok eins og svo
mörg hafa gert.1 Sem verndarað-
gerðir mætti hugsa sér öfluga
minkaleit sem miðaði sérstaklega
að því að vernda teistubyggðirnar
og samhliða þyrfti að friða teistu
fyrir skotveiðum, svo tvennt það
mikilvægasta sé nefnt. Að öðrum
kosti er líklegt að teista hverfi að
mestu eða öllu sem varpfugl á
Ströndum áður en langt um líður.
SUMMARY
The effect of mink-predation on six
monitored Black Guillemot colon-
ies in Strandasýsla, NW-Iceland.
In the former paper we gave an account
of our findings of a dramatic decrease of
the Black Guillemot population in a re-
search area in Strandasýsla since the mid-
dle of the 20th century. This paper
reports on a 10-year (1996-2005) moni-
toring of six Black Guillemot colonies in
the ca. 42 km long Skeljavík-Broddanes
coastal area (Fig. 1). That stretch of coast-
line holds the greatest proportion (98%)
of the breeding population of Black
Guillemot in the research area in
Strandasýsla.1 Feral mink (Mustela vison)
colonized the area in the mid-1950s and
tumed out to be the only significant
predator in the Black Guillemot colonies.
Although mink-control is considered to
have been most effective in the Skeljavík-
Broddanes area, all of the colonies suffer-
ed considerable predation by mink dur-
ing the monitoring period, and all chicks
were killed in some of the subcolonies
almost every year. In some other years
incubation failed totally in certain colo-
nies, probably due to the presence of
mink. Fewer pairs (in one case none)
bred in years following heavy predation.
Data was gathered on occupied nest-
sites, breeding attempts, clutch size,
hatching success, fledging survival and
predation by mink. Mean clutch size was
1.87 (s = 0.351, n = 1297), hatched chicks
1,44 (s = 0.792, n = 1263) and mean num-
ber of fledglings compared with laid
eggs 1.15 (s = 0.893, n = 1274) per nest.
There was a statistically significant effect
at all stages in the breeding cycle regard-
ing whether mink visited the colonies or
not, shown most significantly in lowered
production of fledglings. Reduction in
clutch size was 3%, hatched chicks 30%
and fledglings 60% in colonies visited by
mink compared with colonies not visited
(Fig. 4).
4be Black Guillemots are relatively
long-lived like many sea birds and the
population should therefore tolerate
breeding failure in individual years. It
presumably makes difference at which
stage in the breeding-cycle the predation
occurs. If the chicks are killed shortly
before fledging the parent-bonds might
have slackened and this would presuma-
bly not affect the next year breeding. On
the other hand, if heavy predation occurs
at other stages during the breeding sea-
son, i.e. chicks removed when young and
especially if laying and incubation is
hindered by the presence of mink, the
colonies might vanish in time. The mink
population seems to be stable in the area
and present control having only tempo-
rary and local effects.
The monitoring showed that mink can
do a considerable damage in individual
colonies in individual years. Although
there has been a dramatic decrease of the
Black Guillemot population in the
Strandasýsla area as a whole since mid-
dle of the 20th century,1 the breeding
population in the monitored colonies
seems to tolerate the present level of pre-
dation. The total breeding population
was relatively stable throughout the
monitoring period, 135-195 pairs. Tl-iere
was a decrease in two of the colonies
(Broddanes and Kollafjarðames), where
the predation was heaviest, and a slight
increase in one (Kirkjuból). The monitor-
ed colonies might therefore be on the
brink of what they can tolerate of preda-
tion or other casualties. Effective mink-
control, which is aimed specially at pro-
tecting the Black Guillemot colonies, and
general protection from shooting seem
urgent. At present level it might be ex-
pected that the Black Guillemot will van-
ish as a breeding species in the Stranda-
sýsla area within long.
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