Mosbech, A., Merkel, F., Boertmann, D., Falk, K., Frederiksen, M., Johansen, K. & Sonne, C. 2009: Thick-billed Murre studies in Disko Bay (Ritenbenk), West Greenland. National Environmental Research Institute, Aarhus University. 60 pp. – NERI Technical Report No. 749.
This report describes studies carried out in 2005-07 in the thick-billed murre colony at Ritenbenk/Innaq in NE Disko Bay. This is the only remaining murre colony in Disko Bay, and breeding numbers have probably declined more than tenfold in the last 50 years. The purpose of the study programme was to improve understanding of the population decline and its causes, as well as to map the swimming migration and identify areas critical to the well-being of the colony. The aim has been to create an improved background knowledge which can be used to limit the negative effects on colony growth from oil exploration, hunting and human disturbance.
The studies have been carried out as part of a background survey programme for the Strategic Environmental Impact Assessment of oil activities in the Disko West area, the preliminary results of which are published in Mosbech et al. (2007). An updated version of the SEIA, including results of further biological surveys of the Disko West area, is planned for publication in 2010.
The entire colony was counted both directly and from digital photos (chapter 2). In order to correct the total counts for diel as well as day-to-day variation in murre numbers, repeated counts of study plots were carried out. The most reliable count was 2,447 individuals, based on a total direct count 19 July 2006 and corrected for diel and day-to-day variation. Taking colony attendance into account, this is estimated to correspond to 1,835 breeding pairs.
The most recent previous count of the Ritenbenk colony was in 1998, when a comparable survey method provided an estimate of 3,415 individuals. This corresponds to a decline of 28%, or 4% per year.
Both the total counts and the repeat counts of study plots indicate an increase in the number of birds present from 2005 to 2006. However, this positive result should not be overemphasized, since considerable year-to-year variation in attendance may mean that the negative trend in colony size has not been reversed.
In order to investigate the mechanisms behind the population decline a simple matrix model of the thick-billed murre population at Ritenbenk was constructed. The model estimates the maximum sustainable harvest based on a number of assumptions (chapter 4). Model results were compared to the reported numbers shot in the official harvest statistics (Piniarneq). Harvest statistics show that the annual hunting bag in Ilulissat municipality in 1993-2001 was between 100 and 206 birds (except 40 in 1998). These birds were presumably mainly adult breeders, since few immatures occur near the colonies at this time of year. A comparison with results of the harvest model indicates that shooting of adult breeders is the most likely cause for the decline in colony size up to 2001, when summer hunting was banned. The model indicates that if this ban is respected and winter hunting pressure remains constant, there is a chance that the population will stabilise.
Migration routes and geographical distribution outside the breeding season of murres from Ritenbenk were studied using both ring recoveries and satellite tracking (chapter 5). During the last 60 years, approx. 2,800 murres have been ringed and 372 ringed birds have been recovered, 89% of these as shot in Disko Bay. The recoveries also show that Ritenbenk murres winter both in western Greenland and off Newfoundland.
Twenty-seven murres were equipped with satellite transmitters and tracked for up to 112 days. After leaving the colony in late July, the murres perform a swimming migration, during which the adults moult their flight feathers and the male parent accompanies the chick. The obtained tracks showed that 15 out of 16 males left Disko Bay through Vaigat, whereas females used routes N and S of Disko Island equally. Later in August and September most tracked murres occurred dispersed in SE Baffin Bay. While most birds thus moult their flight feathers in Baffin Bay, two of the birds migrated SW towards Labrador and Newfoundland. It is concluded that a large part of the population migrates through Vaigat and past Hareøen around 1 August, and that they will be very sensitive to an oil spill in this area. Similarly, the population will be very sensitive to oil spills in Disko Bay when they arrive in May.
Foraging behaviour (dive activity and chick feeding) was studied in 2006 to investigate whether food limitation during the breeding season might affect chick survival and thus population growth. Dive activity was recorded using miniature leg-mounted data loggers, whereas chick feeding frequencies were observed directly. Capelin was an important food item in 2006, feeding trips were relatively brief, and the proportion of time birds spent diving was relatively low (< 10%). The overall impression was thus that food availability was sufficient in 2006.
The kittiwake population was counted both in 2005 and 2006 and had 2783 and 1811 ’apparently occupied nests’ respectively. In 1984 the population was counted to c. 22000 nests, and in 1994-98 to c. 6000 nests, indicating a severely population decline of 14% per year. Breeding success was rather high in both 2005 and 2006 with 1.8-1.7 chicks per successful nest and about 10% of the nests had three chicks, indicating that foraging conditions during breeding were favourable in both years despite the decline in population size.
The study programme also included a survey of tourist activities around the colony. Tour operators in Ilulissat were interviewed, and during fieldwork two tourist boats were observed near the colony. None of the Ilulissat tour operators made regular trips to the bird cliff at Ritenbenk, and the main reasons cited for this were distance (too far for a day trip) and lack of specific interest among tourists mainly focussed on icebergs. However, the declining population at the colony and the risk of encountering illegal hunting activities were also mentioned as contributing causes for the limited use of the colony in tourism contexts.
Overall, the evaluation is that the thick-billed murre colony at Ritenbenk has a chance of recovering if spring/summer hunting in Disko Bay has ceased, and if the high food availability inferred in 2006 is representative. The decrease in the kittiwake population is of concern and needs further studies.
We recommend at the local scale:
Frequent counts in order to monitor population growth of thick-billed murre and kittiwake
Communicating knowledge locally in relation to changes in colony size and the importance of the ban on spring/summer hunting
Repeated surveys of foraging behaviour using data loggers, in order to improve the knowledge base and aid generalisation
That the important areas for the thick-billed murres are included in the Oil Spill Sensistivity Map for the region.
The study has supported the national Greenland implementation of Arctic Council’s CAFF International Murre Strategy and Action Plan. The joint analysis of studies like this from murre colonies in the CAFF area increases the understanding of the dynamics of the populations and greatly improves the knowledge available for sustainable management of the populations. We know from ringing recoveries that murres from colonies all of over the north Atlantic mix during migration and wintering especially in Canada and West Greenland. In CAFF CBird we are trying to model what hunting in the different areas and the chronic oil spill induced mortality off the coast of eastern Canada means for the different murre populations (colonies). However, the data available has too many gaps to link winter mortility back to the colonies. The use of satellite tracking has contributed with new detailed knowledge about some of the migration routes. However, a cheaper new tool for tracking, the geodatalogger, based on recapture on the breeding ledge after one year offers new possibilities. The geodatalogger weighs only few grams and can be attached with a conventional leg band. It will give sufficient data to identify wintering areas and main migration routes. Some models can also collect diving information reflecting how hard the murres work to feed themselves during the year in different areas, which is important information for assessing impacts of climate change.
Greenland takes the initiative in CAFF CBird to start a CAFF coordinated North Atlantic thick-billed murre geolocator banding program. The results from such a program would provide information to bring management of murre populations to a higher level where effects of climate change, chronic oil pollution and hunting could be linked to the population development in colonies in different areas.
Full report in PDF format (4,4 MB)
