Rocinela sp. belongs to the family Aegidae. 

 Members of this family are widely recorded 

 ectoparasites of fishes. The first three pairs of 

 walking legs are prehensile and are used effec- 

 tively for attachment. 



We originally felt that the attacks on our 

 salmon fry and fingerlings by R. belliceps puget- 

 tensis might have been the result of confinement 

 and would not be likely to occur with such fre- 

 quency in open waters. However, on at least 

 three occasions, we have observed wild chum 

 salmon (O. keta) and pink salmon fry and finger- 

 lings attacked by R. belliceps pngettensis. These 

 attacks occurred at night, under a light, that 

 probably attracted larger numbers of fish and 

 isopods than would normally occur in open 

 waters. In each instance, an isopod attached 

 itself at a point just posterior to the dorsal fin 

 and on, or slightly below, the lateral line. Af- 

 flicted fish could not maintain a normal swim- 

 ming position in a school and darted about in 

 erratic patterns. On one occasion, an afflicted 

 fish was seen to leave the school and disappear. 

 Even if a wound inflicted by an isopod were not 

 fatal, it is possible that the erratic behavior of 

 a fish trying to dislodge the parasite might at- 

 tract predators. 



On another occasion, in one of our large float- 

 ing pens, we found a juvenile coho salmon with 

 R. belliceps pugettensis attached anterior to the 

 dorsal fin and just above the lateral line (Figure 

 1). This pen was not near our night light and 

 there was a large amount of free space compared 

 with our tanks or cages. 



Also, in a cage that had a submerged light to 

 attract plankton, we found R. belliceps puget- 

 tensis on an immature coho salmon weighing 

 about 200 g. The fish appeared to be in some 

 distress, but perhaps because of its size, the 

 wound was not fatal. 



Aral (1969) reported on (i8 taxa of parasites 

 recovered fi-om 61 species of fish in British Co- 

 lumbia, but these did not include R. belliceps 

 belliceps. Aral's collections were made by seine 

 and trawl; we found that R. belliceps pugettensis 

 will release its grasp if the host is forced into 



a restrictive net. Hatch (1947) states that R. 

 belliceps belliceps is found in 9 to 1250 m of 

 water and that it is an ectoparasite of cod, 

 sculpin, halibut, skate, and other bottom fish. 

 At Manchester, \\'here our observations were 

 made, the water depth is 9 to 13 m. The abun- 

 dance of R. belliceps pugettensis varies between 

 seasons, with the greatest abundance from April 

 through August. 



Although we have no way to judge the extent 

 of natural predation of Pacific salmon by R. bel- 

 liceps belliceps or R. belliceps pugettensis in the 

 wild, we think that, especially under the con- 

 fined conditions of pen rearing, the fry and ju- 

 veniles of Pacific salmon should be included as 

 possible prey of R. belliceps pugettensis and 

 probably R. belliceps belliceps. 



The present instance clearly points up the 

 possible misapplication of the term "parasitism" 

 in certain cases of specialized predation. The 

 isopod R. belliceps pugettensis is not a permanent 

 symbiont of a fish and is thus not properly re- 

 ferred to as a parasite. 



Figure 1. — Rocinela belliceps pugettensis on a juvenile 

 coho salmon in one of the large growing pens. This 

 specimen was anterior to the dorsal fin, whereas most 

 of the others w-ere attached in a posterior position. 

 Note the blood-filled gut of the isopod. 



The authors would like to thank Dr. Paul Ilg 

 of the Department of Zoology, University of 

 Washington, for his suggestions on terminology 

 and Dr. Thomas E. Bowman of the National 

 Museum of Natural History, Washington, D.C., 

 for verifying our identification of the isopod. 



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