Náttúrufræðingurinn - 2006, Síða 18

Náttúrufræðingurinn
Barðaströnd/ en henni var tekið að
fækka þar aftur fyrir 1990 og hefur
stöðugt fækkað síðan þrátt fyrir að
enginn minkur sé í Flatey.20 Upplýs-
ingar skortir að mestu frá öðrum
landshlutum enda fátt um skipuleg-
ar athuganir utan áðurnefndra
tveggja svæða, Breiðafjarðareyja og
Stranda.
Oft heyrist því haldið fram að
fuglalífi hafi almennt hrakað við til-
komu minks í íslenskt lífríki, en
engar beinar rannsóknir þar að lút-
andi hafa verið tiltækar hingað til.
Þótt tegundum varpfugla á Strönd-
um hafi fjölgað á undanförnum ára-
tugum6 hefur teistustofninn látið
verulega á sjá og benda niðurstöður
okkar til þess að minkur eigi þar
meðal annars hlut að máli, en sjald-
an er um eina einhlíta skýringu að
ræða á breytingum sem þessari. Af-
rán minks virðist að stórum hluta
skýra þær miklu viðkomusveiflur
sem fram komu í þeim vörpum sem
vöktuð voru (Jón Hallur Jóhanns-
son og Björk Guðjónsdóttir, í undir-
búningi). Erfiðara er að fullyrða um
orsakir langtímabreytinga, vegna
skorts á gögnum, þótt minkur hafi
vafalítið einnig átt sinn þátt í þeim.
Ahrif minks gætu hafa komið hart
niður á teistunni sem viðbót við
hrognkelsanet, skotveiðar, sam-
keppni við lunda, hugsanlegar
breytingar á fæðuframboði og aðra
þá þætti sem nefndir hafa verið, þ.e.
samlegðaráhrif gætu verið að verki.
SUMMARY
Changes in the breeding distri-
bution and population size of the
Black Guillemot Cepphus grylle in
Strandasýsla, NW-Iceland
In this paper we give an account of our
study on a 290-km coastline of Stranda-
sýsla (Fig. 3), during 1995-2005. Along
with the mapping of the breeding distri-
bution of the Black Guillemot, six colon-
ies were monitored during a ten year
period. The results from the monitoring
will be published in an other paper (Jón
Hallur Jóhannsson & Björk Guðjóns-
dóttir in prep.).
On gathering information on the for-
mer population status the focus was on
the period ~1950-1960 when then
American mink Mnstela vison first app-
eared in the region. Information
obtained from local informants was
used to map the breeding distribution
and estimate the size of colonies (num-
ber of breeding pairs) in that period
(Appendix 1). All colony areas were
controlled for present use. A total of 37
colonies were recorded in the study
area with an estimated population size
of 890-1750 breeding pairs (3-6
pairs/km) in the mid 1950s (Table 1). In
1995-1999 a total of 19 colonies were in
use with estimated 234 breeding pairs.
Survey in 2005 resulted in only 10
colonies with a total of 207 breeding
pairs (0.7 pairs/km). Accordingly, a
76-87% decrease in the size of the
breeding population and a 73% decrea-
se in the number of occupied colonies
has taken place over a c. 50-year period.
This decline seems to have continued
during the previous decade. One colony
(Kirkjuból-Heydalsá) was established c.
1960 in the centre of the study area with
a steady growth and 111 nests in 2005.
This correlates with the fact that mink
control has been more intensive in this
part of the area, which currently holds
98% of the breeding pairs in the study
area. Predation by mink is presumed to
have affected the Black Guillemot pop-
ulation severely since its arrival, alt-
hough direct scientific evidence is
scarce. As a part of our study a total of
1688 birds were ringed, mostly nest-
lings. By the end of 2005 we had
recovered 60 birds (3,6%), of which 30
(65%) were immature birds. Of all the
recoveries, 36% were birds that drow-
ned in Lumpsucker Cyclopterus lumpus
nets, mostly in the vicinity of the colon-
ies. Fishing gear and shooting made up
61% of the recoveries (Table 2.). Accor-
ding to our study, supported by the
results of the monitoring, predation by
mink is probably an important cause of
the dramatic decline of the Black
Guillemot population in the study area
since the mid 20,h century. Other factors
could collectively have contributed to
the long-term decline in the Black
Guillemot population, increased
Lumpsucker fisheries since c. 1950 and
considerable shooting being the two
most important. Changes in the distri-
bution of the puffin Fratercula arctica
population (due to competition for nest-
ing burrows) can be a local factor and
expansion in the population size and
breeding distribution of Fulmar
Fulmarus glacialis is speculative. Long-
term changes in general food availa-
bility in coastal waters are of import-
ance but are largely unknown.
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