378 
Fishery Bulletin 99(2) 
12 
10 
8 
6 
4 3 
2 
0 
I I Total fish examined 
I | Fish with Naobranchia occidentalis 
B Fish with Nectobrachia indivisa 
10 
15 
20 
25 
I I 
30 
35 
40 
Male 
“I 1 1 1 
45 
50 
Figure 5 
Length frequency of male and female northern rock sole examined for parasites from the Aleutian Islands. Numbers 
of fish infested with one parasite species added to those infested with the other parasite species may exceed the total 
because several fish were infested with both parasite species. 
from the Aleutian Island samples had white gill filaments. 
Because of this observation and the apparent preference 
of Nectobrachia indivisa for northern rock sole, we specu- 
late that the white gill filaments in the northern rock soles 
from the Gulf of Alaska survey may have been caused by 
heavy infestation with N. indivisa. 
The decreased weight at length of parasitized male 
northern rock soles from the Gulf of Alaska indicates a 
possible effect of infestation. It is interesting to note that 
northern rock soles from both areas, which were infested 
with one parasite, had an increased chance of also being 
infested by the other parasite. Either some northern rock 
soles are more susceptible to infestation by both para- 
sites, or infestation by one parasite increases the likeli- 
hood of infestation by the other species. Because infesta- 
tion of southern rock soles by Nectobrachia indivisa was 
independent of infestation by Naobranchia occidentalis, 
it is possible that southern rock sole may be resistant to 
infestation by both parasite species. 
Apparently Naobraiichia occidentalis infests smaller 
northern than southern rock sole; thus, much of the south- 
ern rock sole population may be unavailable to successful 
infestation by this parasite. This size-dependent preva- 
lence may be related to the grasping method of attachment 
of N. occidentalis ; thus, its ability to infest rock sole may 
be related to the diameter of the gill filaments. Cressey et 
al. (1983) proposed a similar mechanical limitation for fe- 
male pseudocycnids that grasp the gill filament in scom- 
brids by partially encircling it with the lateral lobes of their 
cephalon. They suggested that hosts must reach a mini- 
mum size before the gill filament is large enough for the 
parasite to achieve a firm grip. Roubal and Graham (1999) 
noted that the smallest fish in their study were not infest- 
ed with Naobranchia variabilis , which attaches to gill fila- 
ments in a similar manner as N. occidentalis, and suggested 
it was because those fish simply had not been encoun- 
tered by a parasite. They did not suggest any sort of me- 
chanical limitations to successful infestation because the 
