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Fishery Bulletin 1 13(3) 
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Figure 6 
Estimates for overall and regional abundance of harbor por- 
poise (Phocoena phocoena) from surveys conducted in South- 
east Alaska during the periods 1991-1993, 2006-2007, and 
2010-2012. Note the significant declines (nonoverlapping 
confidence intervals) in the estimates provided for overall 
Southeast Alaska and for the region that included Wrangell 
and Zarembo Island in the mid-2000s and the relatively sta- 
ble trend over the 22-year study period (1991-2012) for the 
region that included Glacier Bay and Icy Strait. Error bars 
indicate 95% confidence intervals. 
iar with these waters also revealed no other areas of 
major porpoise concentrations. 
Because of the patterns of clumped distribution ob- 
served for harbor porpoise, if we did miss an entire re- 
gion where animals were concentrated both temporally 
and spatially, that omission would significantly affect 
our abundance estimate for that particular year. How- 
ever, by pooling data across sequential years, we re- 
duced the variance resulting from the naturally patchy 
distribution of harbor porpoise. 
The abundance estimates we derived for harbor por- 
poise in Southeast Alaska are likely biased low for 2 
major reasons. We did not sample all areas used by 
harbor porpoise in inlands waters of Southeast Alaska. 
This region encompasses an area of 27,808 km 2 , but 
only 17,665 km 2 were actually surveyed. Although 
there is limited evidence that any of the regions that 
were not surveyed corresponded with high-density por- 
poise habitats, the occurrence of a small number of 
animals in these regions would lead to an underesti- 
mated abundance. Another important source of nega- 
tive bias in results of harbor porpoise studies comes 
from animals that are missed along the trackline, that 
is from the violation of the assumption ofg(0)=l. Previ- 
ous studies have documented the importance of obtain- 
ing an estimation of the proportion of animals missed 
along the trackline (i.e. , the proportion derived 
from g[0] experiments) to compute absolute esti- 
mates of abundance (Barlow, 1988; Palka, 1995, 
1996). These studies have shown that approxi- 
mately 20-50% of harbor porpoise groups are 
missed. 
If g(0) correction factors obtained from other 
vessel studies (Barlow, 1988; Palka, 1995) are 
used to adjust for animals missed by observers, 
the total number of harbor porpoise in South- 
east Alaska may be 1. 5-2.0 times greater than 
the numbers reported here. However, the actual 
value of the correction factor for our study is un- 
known and may vary considerably on the basis 
of many aspects and circumstances that affect 
porpoise sightability, including visual search pro- 
tocols of observers, weather and visibility condi- 
tions, survey platform, behavior of porpoise, and 
density of animals in different regions (Laake 
and Borchers, 2004; Laake et al., 1997; Palka, 
1995). Clearly, g(0) experiments are needed to 
obtain absolute abundance of harbor porpoise in 
Southeast Alaska. Ideally, these estimates should 
be survey-specific in order to assess whether and 
how very different survey teams and conditions 
affect the estimation of animals missed along the 
tracklines. 
Before the surveys conducted in 2011 and 
2012, a trend analysis, completed to include the 
1991-1993, 2006-2007, and 2010 surveys, indi- 
cated a high probability of a decline in porpoise 
numbers ranging from 2% to 4% per year for the 
whole study area (Zerbini et al.7). However, when 
data from 2011 and 2012 were added to this anal- 
ysis, the rate of decline over the entire study period 
decreased substantially and was no longer significant. 
The increase in abundance observed for the waters sur- 
rounding Wrangell and Zarembo Island represented a 
three-fold increase that is not biologically plausible 
given the rate at which this species reproduces. There- 
fore, it is likely that this increase reflected a combina- 
tion of factors, including possible population growth, 
a shift in distribution, or the influx of porpoise from 
other regions (e.g., offshore waters). 
It is unclear why the decline in porpoise numbers oc- 
curred between the early 1990s and the mid-2000s (Fig. 
6) or, in fact, if this decline is real. The low abundance 
in 2006 and 2007 cannot be explained by reduced sur- 
vey effort (i.e., 2 survey years versus 3 survey years for 
the periods 1991-1993 and 2010-2012) because similar 
effort per unit of area took place where high-densities 
of porpoise occur for all years. Given the clumped 
distribution of harbor porpoise in the study area, it 
7 Zerbini, A., M. E. Dahlheim, J. Waite, A. Kennedy, P. R. 
Wade, and P. J. Clapham. 2011. Evaluation of population 
declines of harbor porpoise (Phocoena phocoena) in Southeast 
Alaska inland waters. In Book of abstracts: 19th biennial 
conference on the biology of marine mammals; Tampa, FL., 
28 November-2 December. Society for Marine Mammalogy, 
Moss Landing, CA. 
