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Fishery Bulletin 96(4), 1998 
In this study, SST was measured along tracklines 
and may not have represented water temperatures 
where harbor porpoise were sighted. Tide rips mix- 
ing water, or currents moving through the study 
area could have altered water temperatures by a few 
degrees between trackline and harbor porpoise 
locations. Sea surface temperature varied by 5°C 
from beginning to end of the 8-km tracklines. We 
assumed, however, less bias was introduced by 
collecting SST along the trackline than by continu- 
ously going off transect and potentially disturbing 
harbor porpoise ahead of the vessel. Because our 
methods were consistent over the study period, the 
comparison in use versus availability of SST is likely 
representative. 
It is doubtful that time of day had a significant 
effect on the ability to sight harbor porpoise in our 
study; therefore, other environmental factors must 
have affected harbor porpoise distribution in rela- 
tion to time of day. Occurrence of harbor porpoise 
appears closely associated with the strength of tidal 
currents. From shore-based surveys within our study 
area (Raum-Suryan, 1995), mean number of porpoise 
observed per minute was greatest two hours before 
each peak in the maximum flood tide, and signifi- 
cantly more (P<0.05) porpoise were observed per 
minute during flood than ebb tides. From June to 
October 1992, the majority of low tides in the north- 
ern San Juan Islands occurred in the early morning 
hours. The relation between the occurrence of har- 
bor porpoise with tide and time of day indicates that 
porpoise movements may have been associated with 
concentrations of prey in flood currents and tide rips. 
It is possible that harbor porpoise range throughout 
Washington Sound but continue to return to north- 
ern San Juan Island waters as a primary foraging 
area. 
We found a large proportion of the harbor porpoise 
population of Washington Sound located within our 
study area. Calambokidis et al. 2 estimated the popu- 
lation size of harbor porpoise for the San Juan Is- 
lands (2291 km 2 ) at 960 animals (corrected as in den- 
sity estimate). In approximately 10% (237 km 2 ) of 
the area that Calambokidis et al. 2 surveyed, we esti- 
mated 30% (299 porpoise) of the harbor porpoise 
population. Given that ourg(0) was assumed to be 
one (thus underestimating the population size), the 
proportion of harbor porpoise within our study area 
is likely greater than 30% of the total population 
within the San Juan Island region. On the basis of 
pollutants detected in harbor porpoise tissues, por- 
poise along the west coast do not mix freely between 
California, Oregon, and Washington (Calambokidis 
and Barlow, 1991). In addition, Washington and Cali- 
fornia are considered repositories of genetic diver- 
sity for harbor porpoise of the Northeast Pacific 
(Rosel et al., 1995) and also indicate that harbor por- 
poise ranges may be restricted. In addition to aerial 
surveys conducted over Washington waters (Calam- 
bokidis et al. 2 ), our study area appears to be an im- 
portant site for monitoring trends in distribution and 
abundance of harbor porpoise in inland water of 
Washington. 
It is not clear why harbor porpoise are not as abun- 
dant in other areas of Washington Sound as they are 
in our study area. The relatively low abundance out- 
side our study area may be due to factors other than 
food availability, such as pollution, fishing pressure, 
increased boat traffic, or other environmental 
changes. We believe that harbor porpoise are more 
abundant in our study area than in other parts of 
Washington Sound because certain environmental 
conditions (deep, cool water, and strong tidal mix- 
ing) influence the distribution of harbor porpoise 
prey. Future monitoring studies on oceanographic 
conditions and prey availability associated with har- 
bor porpoise sightings would greatly assist in deter- 
mining mechanisms affecting harbor porpoise abun- 
dance and distribution in this and other areas and 
help in managing this genetically important stock. 
Acknowledgments 
This study would not have been possible without the 
help of many people. We would especially like to 
thank Birgit Kriete, Robert DeLong, Steve Jeffries, 
Steve Osmek, Harriet Huber, and Dave Rugh for lo- 
gistical support and assistance in obtaining funding 
in 1991 and 1992. We sincerely thank Jay Barlow 
and William Broenkow for critical review of an ear- 
lier version of this manuscript and three anonymous 
reviewers for helpful comments on the final manu- 
script. We are indebted to those who assisted in the 
field, Rob Suryan.Tomo Eguchi, Karen Russel, Doug 
Huddle, John Raum, and Marilyn Raum. Funding 
and support for this project was provided by the 
National Marine Fisheries Service (National Marine 
Mammal Laboratory), The Washington Department 
of Wildlife,. Earl H. and Ethel M. Myers/Oceano- 
graphic and Marine Biology Trust, Lerner-Gray Fund 
for Marine Research, Packard Foundation, The 
Whale Museum, and Save The Whales, Inc. 
Literature cited 
Amudin, M., and B. Amudin. 
1974. On the behaviour and study of the harbour porpoise, 
Phocoena phocoena, in the wild. In G. Pilleri (ed.), Inves- 
