Raum-Suryan and Harvey: Distribution and abundance of and habitat use by Phocoena phocoena 
819 
Islands but observed more harbor porpoise in sum- 
mer months (June to August) than other times of the 
year. Surveys of harbor porpoise throughout the year 
along the east and west coasts of the United States 
have indicated a seasonal pattern among various 
regions (Neave and Wright, 1968; Gaskin and 
Watson, 1985; Barlow, 1988; Green et al. 5 ). Results 
of fixed transect surveys conducted in our study in- 
dicated no change in distributions of harbor porpoise 
between July and August 1991 and 1992. Clumped 
distribution of harbor porpoise along tracklines was 
likely associated with habitat features (harbor por- 
poise were sighted most often over deep water). 
Among island regions, such as the Bay of Fundy, 
Glacier Bay, Alaska (Taylor and Dawson, 1984), and 
off the San Juan Islands, harbor porpoise are more 
often associated with deeper waters than along 
coastal regions of North America. Most harbor por- 
poise observed off the coast of California, Oregon, 
and Washington occurred at shallow water depths, 
and sightings decreased with increasing depth 
(Barlow, 1988; Dorfman, 1990; Calambokidis 3 ; 
LaBarr and Ainley 6 ). Incidental net entanglement 
of harbor porpoise within Washington waters oc- 
curred at the bottom of nets, at depths of 73 to 81 m 
(Scheffer and Slipp, 1948), and near the bottom or in 
the lower one-half of nets set from 11 to 18 m deep, 
indicating porpoise were foraging along the bottom 
or in deeper areas of the net (Gearin et al. 7 ). The 
depth of water where harbor porpoise were sighted 
in this study may have been due to occurrence of prey 
within these areas. 
The Pacific herring (Clupea pallasi) population in 
the Strait of Georgia is the largest known in Wash- 
ington state, and herring are quite abundant in sec- 
tions of the eastern Strait during summer, fall, and 
winter (Lemberg, 1978). During this study, harbor por- 
poise, harbor seals, and a minke whale were observed 
feeding on a school of Pacific herring. The dominant 
prey items in stomachs of harbor porpoise taken in a 
setnet fishery in summer off northern Washington were 
Pacific herring, market squid (Loligo opalescens), 
gadids, and osmerids (Gearin and Johnson 8 ). Pacific 
herring and market squid migrate vertically within the 
water column, remaining close to the seafloor during 
6 LaBarr, M. S., and D. G. Ainley. 1985. Depth distribution of 
harbor porpoise off central California: a report of cruises in April 
and May-June 1985. NMFS Contract No. 41 USC 252, 23 p. 
7 Gearin, P. J., M. A. Johnson, and S. Joner. 1991. Harbor por- 
poise interactions with the Makah Chinook Salmon Set-Net 
Fishery, 1988-89. In H. Kajimura (ed.), Harbor porpoise in- 
teractions with Makah salmon set net fishery in coastal Wash- 
ington waters, 1988-89. National Marine Mammal Laboratory 
Processed Report. National Marine Mammal Laboratory, Natl. 
Mar. Fish. Serv., NOAA, 7600 Sand Point Way NE, Seattle, WA 
98115-0070. 
the day, and approach the surface at night (Hart, 1973; 
Blaxter, 1985; Flaherty and Stark 1 ). Characteristics of 
these prey items and the greater occurrence of porpoise 
over deep waters may indicate that harbor porpoise 
feed in deep water during the day. Aggregations of sur- 
face schooling fish and associated harbor porpoise were 
rarely ( 1% of surveys) observed within our study area, 
further indicating that harbor porpoise were likely feed- 
ing on prey in deep water. 
In our study, harbor porpoise were sighted most 
often in shallow sloping areas with little bathymet- 
ric relief. These results contrast with those of 
Flaherty and Stark, 1 in which 70% of harbor por- 
poise sighted were found in areas with seafloor re- 
lief greater than 40%, and with those of Calam- 
bokidis, 3 who observed significantly more harbor 
porpoise than expected within areas of uneven bot- 
tom topography off the outer Washington coast. It is 
likely that slope of the seafloor does not significantly 
affect the distribution of harbor porpoise or their prey 
in our study area. We believe that harbor porpoise 
and their prey are associated with deeper waters in 
this region which, in general, has shallow slopes. 
Water temperature may influence the distribution 
of harbor porpoise. Calambokidis 3 reported harbor 
porpoise sightings in water temperatures ranging 
from 9° to 16°C off Washington. In the Bay of Fundy, 
Watts and Gaskin (1985) found a negative correla- 
tion between harbor porpoise abundance and mean 
August SST, and Watson (1981) reported that har- 
bor porpoise occurred in water temperatures less 
than 15°C in the Bay of Fundy. It is unlikely, how- 
ever, that SST alone would influence harbor porpoise 
distribution. Most harbor porpoise entered Fish Har- 
bor, New Brunswick, Canada, when SST was between 
9° and 10°C, a period when large numbers of juve- 
nile herring were also entering the region (Gaskin 
and Watson, 1985). Within the Bay of Fundy, Watts 
and Gaskin (1985) found herring associated with 
vertically mixed waters and reduced surface tempera- 
tures. This association was possibly due to increased 
concentrations of zooplankton, which also occurred 
along convergent zones (Watts and Gaskin 1985). Sea 
surface temperatures off the northern San Juan Is- 
lands, therefore, were possibly related to tidal cur- 
rents that may be associated with concentrations of 
harbor porpoise prey. 
8 Gearin, P. J., and M. A. Johnson. 1991. Prey identified from 
stomachs of harbor porpoise and Chinook salmon from the 1988- 
89 Makah Salmon Set-Net Fishery. In H. Kajimura (ed ), Har- 
bor porpoise interactions with Makah Salmon set net fishery in 
coastal Washington waters, 1988-89. National Marine Mam- 
mal Laboratory Processed Report. National Marine Mammal 
Laboratory, Natl. Mar. Fish. Serv., NOAA, 7600 Sand Point Way 
NE, Seattle, WA 98115-0070. 
