Dahlheim et al.: Temporal changes in abundance of Phocoena phocoena inhabiting the inland waters of Southeast Alaska 
249 
Maps with locations of sightings of harbor porpoise ( Phocoena phocoena) and completed tracklines for line-transect surveys 
conducted in the inland waters of Southeast Alaska during summer in (A) 2010, (B) 2011, and (C) 2012. 
crease in the early 2010s, abundance in region 1, with 
Glacier Bay and Icy Strait, remained relatively stable 
during the 22-year period of this study (Table 6, Fig. 6). 
Discussion 
Estimation of abundance 
Overall abundance was found to vary among survey pe- 
riods in this 22-year study (1991-2012). The abundance 
UV=1076) in the early 1990s in the first survey period 
was relatively high, lower for the period 2006-2007 
(1V=604) and higher again for the period 2010-2012 
( N=975 ). Because the surveys conducted in this study 
covered a long period of time, they were subject to some 
changes in methods that could affect the abundance es- 
timates. In addition, multiple factors could have affect- 
ed the sightability and identification of harbor porpoise 
groups, and these factors are discussed below. 
To the best of our ability, we kept survey effort com- 
parable throughout the years. With the exception of the 
2007 survey, during which effort was reduced because 
of a 3-day mechanical breakdown and fog conditions 
encountered throughout various regions of the survey 
area, effort remained fairly consistent and only minor 
changes were made in trackline and coverage because 
of adverse weather conditions, ship mechanical break- 
downs, or cruise duration. However, the number of bi- 
ologists who participated in the survey varied over the 
22-year study period from 4 to 6 due to either the lack 
of vessel accommodations or NOAA restrictions. Both 
experienced and inexperienced observers were used 
Table 3 
Most supported models of detection probability and 
estimates of detection probability of harbor porpoise 
(Phocoena phocoena) in Southeast Alaska during the 
periods 1991-1993, 2006-2007, and 2010-2012. hn=half 
normal; hr=hazard rate; Beaufort=Beaufort sea state; 
CV=coefficient of variation; P=detection probability. 
Detection 
Year 
probability model 
P 
CV (P) 
1991-1993 
hn + Beaufort 
0.51 
0.04 
2006-2007 
hr 
0.61 
0.08 
2010-2012 
hn + Beaufort 
0.47 
0.04 
and some individuals participated in several different 
surveys and one observer participated in all surveys. 
Laake et al. (1997) showed that observer experi- 
ence affects their ability to detect harbor porpoise in 
aerial surveys; inexperienced observers miss porpoise 
groups 2-3 times more than experienced observers. 
We assumed when comparing experienced with expe- 
rienced observers that the same pattern would occur 
during vessel surveys. However, inexperienced observ- 
ers can also have difficulty both with correctly estimat- 
ing group size and with accurately identifying a species 
(NMML, unpubl. data 6 ). Average group sizes did not 
6 NMML (National Marine Mammal Laboratory). 2010. Un- 
publ. data. Alaska Fish. Sci. Cent., Natl. Mar. Fish. Serv., 
NOAA, 7600 Sand Point Way NE, Seattle, WA. 98115-6349. 
