NOTE 



PREDATION ON JUVENILE PACIFIC 



SALMON BY A MARINE ISOPOD 



Rocinela bellkeps pugettensis 



(CRUSTACEA, ISOPODA) 



Observations were made of predation by a ma- 

 rine isopod on both captive and wild Pacific 

 salmon. Pacific salmon are known to be hosts 

 to a number of ectoparasites, esjiecially the 

 Copepoda. However, we have been unable to 

 find any reported incidents of predation or para- 

 sitism of Pacific salmon by the Isopoda. 



In July 1969, we began a series of experiments 

 in Pug-et Sound to evaluate the feasibility of 

 saltwater rearing of Pacific salmon within float- 

 ing pens. Stocks of young salmon were weighed 

 and measured several times during the year at 

 the Manchester, Wash., e.xperimental station of 

 the National Marine Fisheries Service and rou- 

 tinely examined for ectoparasites visible to the 

 naked eye. A large Branchiuran parasite, Ar- 

 gtdus sp., was found on a 600-g chinook salmon 

 {OncorhyncMis tshaivytscha) ; the Branchiuran 

 was removed from the fish and kept for study. 

 Later, another Branchiuran was found in the 

 same body position on this fish. There were no 

 other visible ectoparasites. 



In March 1970, we began collecting zooplank- 

 ton to provide live food for both salmon fry and 

 newly metamorphosed flatfish. Collections were 

 made during the night; a surface light was used 

 to attract the plankton and an airlift pump to 

 draw them into a net suspended at the sea 

 surface. 



The collected organisms were poured into a 

 tank containing over 1000 young pink salmon ( 0. 

 (jorbiischa) , 30 to 45 mm long. Among the un- 

 sorted plankton were 8 or 10 isopods of the same 

 species. The pink salmon fed vigorously on large 

 numbers of amphipods in the tank, whereas the 

 isopods alternately rested on the sides of the tank 

 and swam about in random patterns. One iso- 

 pod quickly attached itself to a young pink 



salmon. Within minutes, two more of the young 

 salmon were similarly attacked. The fish be- 

 came distressed, swam erratically about the tank, 

 and drifted listlessly to the bottom, where they 

 died within a few miiuites after settling. The 

 point of attack by the isopod on each young 

 salmon was lateral, just above or slightly poste- 

 rior to the pelvic fin. Small, but deep wounds 

 that penetrated the body wall were found on 

 each aflfected fish. The gut of each of the isopods 

 removed from the dead fish contained blood. 



One of the isopods was placed in another tank 

 containing several dozen coho salmon (0. 

 kisntch), slightly larger than the pinks (100- 

 140 mm long). An hour later, the isopod was 

 found firmly attached to the head of one fish, 

 immediately posterior to and between the eyes. 



Isopods are now removed before any plankton 

 are fed to our fish. If we should overlook even 

 the smallest of these isopods, we can expect 

 either death or injury to some fish. 



We examined several specimens of the hun- 

 dreds of isopods we have collected and have 

 identified them as Rocinela belliceps pugettensis 

 (Stimpson), a subspecies of Rocinela belliceps. 

 R. belliceps is widespread in northern coastal 

 areas of the North Pacific Ocean, from the Be- 

 ring Strait southward along the North American 

 coast to California and southward along the 

 Asian coast to Korea (Pavlovskii, 1955) . Hatch 

 ( 1947) feels that most of the specimens in Puget 

 Sound are the subspecies, pugettensis. He as- 

 signs subspecific rank on the basis of the number 

 of spines on the propodite of the prehensile legs 

 — six spines on the propodite of pugettensis, 

 whereas belliceps has three to four. A zone of 

 mixed populations of R. belliceps belliceps and 

 R. belliceps pugettensis exists near the entrance 

 to Hood Canal, just inside the Strait of Juan de 

 Fuca. All of the specimens we examined from 

 our collections at the Manchester station (which 

 is on the western side of central Puget Sound) 

 had six spines on the propodite of the prehensile 

 legs. 



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