Zimmermann et al.: Parasitism by Naobranchia occidentalis 
375 
at sea (Table 2). None of the fish had abnormally pale gill 
filaments. Naobranchia occidentalis or Nectobrachia indi- 
visa (or both) were found on all but one of the northern rock 
soles examined in the laboratory. No other parasite spe- 
cies were observed. Approximately 17% of the Naobranchia 
occidentalis were juvenile females. Northern rock soles 
infested by one species of parasite had an increased likeli- 
hood of also being infested by the other species of parasite 
(chi square test, a=0.05, P<0.001), similar to our observa- 
tions for northern rock soles from the Gulf of Alaska. 
Area and species infestation comparisons 
Naobranchia occidentalis was more prevalent (chi square 
test, a=0.05, PcO.Ol) on the northern rock soles in the 
Aleutian Islands (36%) than on fish from the Gulf of Alaska 
(22%) (Fig. 2C). The northern rock soles from the Aleutian 
Islands also had a significantly higher mean intensity of 
N. occidentalis (Welch’s approximate f-test, a=0.05, df=127, 
P<0.001; 10.2/fish) than northern rock soles from the Gulf 
of Alaska (4.4/fish, Fig. 2D). There were no differences 
in either prevalence (chi-square test, a=0.05, P >0.05) or 
mean intensity (Welch’s approximate f-test, a=0.05, df=50, 
P>0.05) of Nectobrachia i?idivisa on the northern rock soles 
from the Aleutian Islands and Gulf of Alaska. 
Trends in parasite infestations— arches and sides 
Heads of fish from the 1996 Gulf of Alaska survey had 
been frozen for a year before examination and the gill fila- 
ments were in poor condition. As a result, only a small per- 
centage of Naobranchia occidentalis were still attached to 
a gill arch, and it was not possible to detect preferences of 
N. occidentalis for individual gill arches, or for eyed-side or 
blind-side arches, on northern rock sole (Table 3). Ninety- 
one percent of the observed Nectobrachia indivisa were still 
firmly anchored in the gill filaments on northern rock sole, 
were evenly distributed per side (60), and showed a prefer- 
ence for the outer arches over the inner arches on both sides 
of the fish (Table 3). All Naobranchia occidentalis on the 
southern rock sole from the Gulf of Alaska were unattached. 
Four Nectobrachia indivisa were found still attached on the 
blind side of southern rock sole from the Gulf of Alaska, but 
this small sample was insufficient for analysis. 
Aleutian Islands samples were frozen for only a few 
weeks prior to examination in the laboratory and were in 
much better condition than samples from the Gulf of Alas- 
ka survey. The infested gill arch was determined for 83%' 
of the Naobranchia occidentalis and for 97% of the Nec- 
tobrachia indivisa (Table 3). For northern rock sole from 
the Aleutian Islands, the total number of Naobranchia oc- 
cidentalis was higher on the blind side (452) than the eyed 
side (414) and equal on both sides for Nectobrachia indi- 
visa (33); these results nearly duplicated those observed 
for northern rock sole from the Gulf of Alaska. Naobran- 
chia occidentalis showed a strong preference for the mid- 
dle and inner gill arches on both eyed and blind sides (Ta- 
ble 3). The results were mixed for Nectobrachia indivisa ; 
parasites on the blind side showed a preference for the 
outer arches, whereas parasites on the eyed side showed 
