Náttúrufræðingurinn

Volume

Náttúrufræðingurinn - 2006, Page 18

Náttúrufræðingurinn - 2006, Page 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. 78

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