A comparison of image and observer based aerial surveys of narwhal Author Bröker, Koen C. A. Shell Global Solutions International B. V., Kesslerpark 1, 2288 GS, Rijswijk, the Netherlands and Groningen Institute for Evolutionary Life Sciences, University of Groningen, PO Box 11103, 9700 CC Groningen, the Netherlands; koen.broker@shell.com Author Hansen, Rikke G. Greenland Institute of Natural Resources, ℅ Greenland Representation, Strandgade 91, 2, DK- 1401 KØbenhavn K, Denmark; Author Leonard, Kathleen E. LGL Alaska Research Associates, Inc., 2000 West International Airport Road, STE C- 1, Anchorage, Alaska 99502, U. S. A.; Author Koski, William R. LGL Limited environmental research associates, 22 Fisher Street, PO Box 280, King City, Ontario L 7 B 1 A 6, Canada; Author Heide-Jørgensen, Mads Peter Greenland Institute of Natural Resources, ℅ Greenland Representation, Strandgade 91, 2, DK- 1401 KØbenhavn K, Denmark. text Marine Mammal Science 2019 2019-01-31 35 4 1253 1279 http://dx.doi.org/10.1111/mms.12586 journal article 10.1111/mms.12586 1748-7692 13221822

Table 2. Summary of sightings by image analysts and/or aerial observers, with and without truncation at 500 m. Where both aerial observer pairs had the same sighting with different group size or perpendicular distance estimates, an averaged value was used.

			Only		Sighted by	Sighted by
	Sighted by		sight by	Only	image	image analyst
	image	Sighted by	image	sighted by	analyst	and/or
	analyst	observer	analyst	observer	and observer	observer
No truncation
No. of groups	62	63	34	35	28	97
No. of animals	135	227	62	124	88	274
Average group size	2.2	3.6	1.8	3.5	3.1	2.8
Average distance	227	580	237	839	329	454
Truncation = 500 m
No. of groups	62	36	34	8	28	70
No. of animals	135	126	62	24	88	173
Average group size	2.2	3.5	1.8	3.0	3.1	2.5
Average distance	227	269	237	329	234	247