Zimmermann et al.: Parasitism by Naobranchia occidentalis 
377 
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3 
Figure 4 
Length frequency of male and female southern rock sole examined for parasites from the Gulf of Alaska. 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. 
Alaska did not have significantly different length-weight 
slopes (df=l, F-ratio=0.29, P> 0.05) but had significantly 
different intercepts (df=l, F-ratio=13.74, PcO.GOl). For 
example, a 33-cm-FL parasitized male northern rock sole 
from the Gulf of Alaska had an estimated weight of 407.5 
grams, whereas an unparasitized fish of the same length 
weighed an estimated 451.4 grams, a difference of more 
than 10%. The length-weight slopes and intercepts were 
not significantly different for female northern or southern 
rock soles from the Gulf of Alaska. No comparison could 
be made between parasitized and unparasitized Aleutian 
Islands fish because individual weights were not taken for 
unparasitized fish. 
Discussion 
Laboratory examination confirmed our original observa- 
tions that copepod parasites were more prevalent on the 
northern rock soles from the Gulf of Alaska than on south- 
ern rock soles. There could also be a higher mean inten- 
sity for both species of parasites on the northern rock sole, 
but our analysis was limited by a small sample size and 
high variability within the sample. Of the two gill par- 
asites examined in detail, Naobranchia occidentalis was 
more prevalent and caused more intense infestations than 
did Nectobrachia indivisa. Our sampling efforts in the 
Aleutian Islands occurred beyond the geographic range of 
southern rock sole and indicated that the northern rock 
sole had a higher prevalence and greater mean intensity 
of Naobranchia occidentalis than the northern rock sole in 
the Gulf of Alaska. Differences in parasite prevalence and 
mean intensity between the Gulf of Alaska and Aleutian 
Island samples were confounded by sampling in different 
years, limited areal coverage, limited temperature range, 
and greater depths in the Aleutian samples. Also, the sig- 
nificance of the large number of juvenile N. occidentalis in 
the Aleutian Island samples is not known. Despite these 
limitations, it is noteworthy that none of the northern 
rock soles heavily infested with Naobranchia occidentalis 
