348 



Abstract — The importance of glacial 

 ice habitats to harbor seals (Phoca vit- 

 ulina) in Alaska has become increas- 

 ingly apparent. However, enumerating 

 harbor seals hauled out on ice in 

 glacial fjords has been difficult. At 

 Johns Hopkins Inlet in Glacier Bay, 

 Alaska, we compared a shore-based 

 counting method to a large-format 

 aerial photography method to esti- 

 mate seal abundance. During each 

 aerial survey, shore-based observers 

 simultaneously counted seals from an 

 observation post. Both survey meth- 

 ods incurred errors in double-counting 

 and missing seals, especially when 

 ice movements caused seals to drift 

 between survey zones. Advantages 

 of shore-based counts included the 

 ability to obtain multiple counts for 

 relatively little cost, distinguish pups 

 from adults, and to distinguish mobile 

 seals from shadows or glacial debris 

 of similar size. Aerial photography 

 provided a permanent record of each 

 survey, allowing both a reconcilia- 

 tion of counts in overlapping zones 

 and the documentation of the spatial 

 distribution of seals and ice within 

 the fjord. 



Comparison of survey methods 



for estimating abundance of harbor seals 



{Phoca vitulina) in glacial fjords 



John L. Bengtson (contact author)' 

 Alana V. Phillips' 

 Elizabeth A. Mathews^ 

 Michael A. Simpklns'-^ 



E-mail: John. Bengtsonm'noaa. gov 



' National Marine Mammal Laboratory 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way NE 



Seattle, Washington 98115 

 ^ University ol Alaska Southeast 



Department of Natural Sciences 



11120 Glacier Highway 



Juneau, Alaska 99801 

 ^ Marine Mammal Commission 



4340 East-West Highway, Rm. 905 



Bethesda, Maryland 20814 



Manuscript submitted 2 December 2005 to 

 the Scientific Editor's Office. 



Manuscript approved for publication 



20 December 2006 by the Scientific Editor. 



Fish. Bull. 105:348-355 (2007). 



Harbor seals (Phoca vitulina) in 

 Alaska occupy a geographically exten- 

 sive range and topographically diverse 

 haul-out habitats. They are present in 

 U.S. waters from approximately 172°E 

 to 130°W (over 3500 km east to west) 

 and from SrN to 6rN (over 1000 km 

 north to south), hauling out on a vari- 

 ety of substrates including sand, rock, 

 and, in Alaska, glacial ice. Alaska 

 harbor seal populations have declined 

 at several locations since the late 

 1970s. For example, counts of harbor 

 seals at Tugidak Island (southwest of 

 Kodiak Island, Alaska) declined 85% 

 between 1976 and 1988 (Pitcher, 1990), 

 and counts in Prince William Sound 

 indicate population declines of approxi- 

 mately 63% between 1984 and 1997 

 (Frost et al., 1999). Additional evidence 

 indicates that harbor seal numbers 

 near Kodiak Island, including those 

 at Tugidak Island, increased 6.6%/yr 

 during 1993-2001 (Small et al., 2003), 

 but that seals in Prince William Sound 

 have continued to decline 3.3%/yr 

 during 1988-99 (Ver Hoef and Frost, 

 2003). In Glacier Bay, Alaska, harbor 

 seal numbers declined by 75% (-14.5%/ 

 yr) during 1992-2002 at terrestrial 

 resting sites and by 64% (-9.6%/yr) 

 from 1992 to 2001 in Johns Hopkins 

 Inlet, the primary breeding site, which 



is a glacial fjord where seals haul out 

 on floating ice (Mathews and Pendle- 

 ton, 2006). Because it is estimated that 

 10% or more of harbor seals in Alaska 

 use glacial ice habitats during the 

 molting season (August-September) 

 (J. L. Bengtson, unpubl. data), there 

 is a pressing need to develop reliable 

 survey methods to assess harbor seal 

 abundance in such areas. Here we 

 evaluate two such survey methods: 

 counts from shore-based observations 

 and counts from large-format aerial 

 photography. 



Shore-based surveys of harbor seals 

 in two glacial fjords in Glacier Bay 

 National Park, Alaska, have been 

 made from elevated shore sites over 

 the past three decades (Calambokidis 

 et al., 1987; Mathews, 1995; Mathews 

 and Pendleton, 2006) and in Aia- 

 lik Bay, a glacial fjord in the Gulf 

 of Alaska (Hoover, 1983). In 1997, 

 Mathews et al.' conducted a pilot 



^ Mathews, E. A., W. L. Ferryman, and L. 

 B. Dzinich. 1997. Use of high-resolu- 

 tion, medium-format aerial photography 

 for monitoring harbor seal abundance 

 at glacial ice haulouts, 15 p. Unpubl. 

 report to Glacier Bay National Park and 

 Preserve, P. O. Box 140, Gustavus, AK 

 99826. Website: http://www.uas. alaska. 

 edu/biology /faculty /ma thews/publica- 

 tions. html (accessed 1 December 2006). 



