371 
Differentia! parasitism by Naobranchia occidentals 
(Copepoda: Naobranchiidae) and Nectobrachia 
indivisa (Copepoda: lernaeopodidae) on northern 
rock sole ( Lepidopsetta polyxystra Orr and 
Matarese, 2000) and southern rock sole 
(£. bilineata Ayres, 1855) in Alaskan waters 
Abstract— Northern rock sole ( Lepi- 
dopsetta polyxystra Orr and Matarese, 
2000) and southern rock sole (L. bilin- 
eata Ayres, 1855) from the Gulf of 
Alaska and northern rock sole from 
the Aleutian Islands were examined for 
gill parasites. Four species of copepod 
parasites were identified: Naobranchia 
occidentalis and Nectobrachia indivisa 
were the most common. Both parasites 
were more prevalent on northern rock 
sole (22% and 15%, respectively) than 
on southern rock sole (5% and 1%, 
respectively) in the Gulf of Alaska sam- 
ples. Northern rock sole tended to have 
a greater mean intensity of Naobran- 
chia occidentalis than southern rock 
sole but there was not a significant dif- 
ference because of the high variance 
about the means; too few southern rock 
sole were infested by Nectobrachia indi- 
visa for comparison. Northern rock sole 
from the Aleutian Islands region had a 
significantly greater prevalence (36%>) 
and mean intensity (10.2/infested fish) 
of Naobranchia occidentalis than north- 
ern rock sole from the Gulf of Alaska 
(22%, and 4.4, respectively) but did not 
differ significantly in prevalence and 
mean intensity of Nectobrachia indi- 
visa. Parasitized male northern rock 
sole from the Gulf of Alaska had a 
significantly reduced weight at length, 
indicating a possible effect of para- 
sitism. Naobranchia occidentalis selec- 
tively infested larger northern rock 
sole and only the largest southern rock 
sole. Nectobrachia indivisa also were 
found on larger northern rock sole but 
did not infest enough southern rock 
sole to describe a trend. Southern rock 
sole males were not infested by either 
parasit e. Naobranchia occidentalis pre- 
ferred to infest the middle gill arches 
of hosts and Nectobrachia indivisa pre- 
ferred to infest the exterior gill arches 
of hosts. 
Manuscript accepted 14 November 2000. 
Fish. Bull. 99:371-380 (2001). 
Mark Zimmermann 
Robin C. Harrison 
Anthony F. Jones 
Alaska Fisheries Science Center 
National Marine Fisheries Semce, NOAA 
7600 Sand Point Way NE 
Seattle, Washington 98115-0070 
Email: Mark.Zimmermann@noaa.gov 
The rock soles (Lepidopsetta spp.) are 
important commercial flatfish species 
that inhabit the continental shelf of the 
north Pacific Ocean (Hart, 1973). Stock 
assessment bottom trawl surveys con- 
ducted by the Alaska Fisheries Science 
Center (AFSC) historically recognized 
only one species of rock sole occurring in 
four survey areas: eastern Bering Sea, 
Aleutian Islands, Gulf of Alaska, and 
western U.S. coast. Recent larval mor- 
phometries by Matarese et al. ( 1989 ) and 
a generic revision by Orr and Matarese 
(2000) have demonstrated that there 
are actually two species of rock sole that 
overlap in the AFSC survey areas. The 
northern rock sole (L. polyxystra Orr 
and Matarese, 2000) ranges throughout 
the Bering Sea and Aleutian Islands 
to Puget Sound, whereas the southern 
rock sole (L. bilineata Ayres, 1855) 
ranges from the Islands of Four Moun- 
tains in the eastern Aleutian Islands to 
Mexico (Orr and Matarese, 2000). With 
the knowledge of this recent research, 
field biologists began separating the two 
rock sole species in AFSC bottom trawl 
surveys starting in 1996. The northern 
rock sole is distinguished by higher gill- 
raker counts and a whiter blind side 
than southern rock sole occurring in the 
same trawl hauls (Orr and Matarese, 
2000). 
While examining gill rakers to iden- 
tify specimens of rock soles captured 
near the Shumagin Islands during the 
1996 Gulf of Alaska survey, we noticed 
that several live, apparently healthy, 
northern rock soles had pink or white 
gill filaments, a condition normally as- 
sociated with dead fish. On close exam- 
ination, we found small copepod para- 
sites attached to the gill filaments of 
these fish. Southern rock soles from 
the same trawl hauls usually had red, 
apparently healthy gill filaments and 
were less frequently parasitized. There- 
fore we surveyed northern and south- 
ern rock soles in selected hauls from 
the Gulf of Alaska in 1996 for the pres- 
ence of these parasites and saved infest- 
ed heads for parasite identification and 
enumeration in the laboratory. In 1997, 
we conducted the same parasite inves- 
tigation in several hauls in the Aleu- 
tian Islands region. Our main objective 
was to quantify differences in the prev- 
alence (percent fish infested) and mean 
intensity (average number of parasites 
per infested fish) of gill parasites in- 
festing the northern and southern rock 
soles (see Margolis et al., 1982; Bush et 
al., 1997, for complete definitions of eco- 
logical terms in parasitology). Second- 
ary objectives were to test for regional 
differences in prevalence and mean in- 
tensity, to describe and compare site 
preferences of the parasites, and to de- 
termine if parasites influenced weight 
at length for parasitized fish. 
