Áhrif minks á teistuvarp á Ströndum
Jón Hallur Jóhannsson og Björk Guðjónsdóttir
Bls: 29–36 1.–2. hefti 76. árg. 2007
Í fyrri grein var fjallað um rannsókn sem höfundar gerðu á varpútbreiðslu og stofnstærð teistu (Cepphus grylle) í Strandasýslu sunnan Ingólfsfjarðar. Sýnt var fram á mikla fækkun í varpstofni teistu á rannsóknarsvæðinu frá því um miðbik 20. aldar og að mörg teistuvörp hafa liðið undir lok. Einnig var getið nokkurra umhverfisþátta sem hver um sig eða sameiginlega kunna að hafa stuðlað að hnignun teistustofnsins, þar á meðal afrán minks (Mustela vison).
Hér verður fjallað um sex teistuvörp sem vöktuð voru 1996–2005 og þau áhrif sem afrán minks hafði á þau. Beinar rannsóknir á áhrifum minks á fuglastofna hér á landi hafa ekki verið gerðar til þessa svo vitað sé. Ekki er okkur heldur kunnugt um að áhrif minks á ferskvatnsfiska, t.d. lax og silung, hafi verið rannsökuð hér á landi. Rannsókn sem gerð var í Noregi bendir til að þau kunni að vera nokkur við tilteknar aðstæður, þ.e. á laxa- og urriðaseiði í litlum vatnakerfum þar sem þéttleiki er mikill. Engu að síður hefur oft verið gefið í skyn að minkur hafi áhrif á íslenskt lífríki. Vöktun teistubyggða hefur hvergi farið fram nema í Flatey og nálægum eyjum á Breiðafirði en þar er ekki minkur.
The effect of mink-predation on six monitored Black Guillemot colonies 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 research area in Strandasýsla since the middle of the 20th century. This paper reports on a 10-year (1996–2005) monitoring of six Black Guillemot colonies in the ca. 42 km long Skeljavík-Broddanes coastal area (Fig. 1). That stretch of coastline holds the greatest proportion (98%) of the breeding population of Black Guillemot in the research area in Strandasýsla. Feral mink (Mustela vison) colonized the area in the mid-1950s and turned 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 suffered considerable predation by mink during 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 colonies, probably due to the presence of mink. Fewer pairs (in one case none) bred in years following heavy predation. Data was gathered on occupied nestsites, 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 number 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 regarding 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).
The 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 presumably not affect the next year breeding. On the other hand, if heavy predation occurs at other stages during the breeding season, 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 temporary 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 middle of the 20th century, the breeding population in the monitored colonies seems to tolerate the present level of predation. The total breeding population was relatively stable throughout the monitoring period, 135–195 pairs. There was a decrease in two of the colonies (Broddanes and Kollafjarðarnes), where the predation was heaviest, and a slight increase in one (Kirkjuból). The monitored colonies might therefore be on the brink of what they can tolerate of predation or other casualties. Effective minkcontrol, which is aimed specially at protecting the Black Guillemot colonies, and general protection from shooting seem urgent. At present level it might be expected that the Black Guillemot will vanish as a breeding species in the Strandasýsla area within long.