PARASITES IN SUMMER-CAUGHT PACIFIC ROCKFISHES^ 



by 

 John Listen, John Peters, and Joseph A. Stern 



ABSTRACT 



This investigation was carried out in the summer of 1958 on 

 rockfishes caught in the area extending from Hecate Strait to Cape 

 Blanco, Oregon. The purpose of the investigation was to study (1) 

 the relationship between the area of catch and the degree to w^hich 

 these fish are infested with parasites, (2) the distribution of the para- 

 sites in the fillets of the fish, and (3) the method of detecting the 

 parasites and of removing them from the fillets. 



Parasites in the fillets of rockfishes 

 are not harmful to man but they present 

 an aesthetic problem. Little information 

 has been published concerning the inci- 

 dence and distribution of parasites in 

 these fish or on the feasibility of re - 

 nnoving the parasites from the infested 

 fish muscle. Accordingly, an investiga- 

 tion was undertaken to study the degree 

 of infestation in relationship to the area 

 in which the fish are caught, the distri- 

 bution of the parasites in the fillets, and 

 the problem of detecting and removing 

 the parasites. 



SPECIES OF ROCKFISH STUDIED 



Most of the information obtained con- 

 cerned Pacific ocea-nperch. (Se bastode s 

 alutus), since this ^vas the principal 

 species of rockfish being landed at the 

 time of the survey ( July and August 

 1958). Furthermore, a common prac- 

 tice in the industry is to distinguish 

 only Pacific ocean perch from the 

 various species of Scorpaenidae landed. 

 Other species usually are loosely 

 grouped as snapper, red rockfish, and 

 black rockfish. The results of this 

 survey therefore are reported pri- 

 marily in relation S. alutus, though 

 some information is included concern- 

 ing other individual species. 



SPECIES OF PARASITES STUDIED 



The most common muscle parasite 

 of S. alutus has been reported to be a 

 trematode, Prosorhynchus sp. The larval 

 stage of this parasite occurs as cysts 

 in the muscle tissue of the fishandmay 

 be observed as whitish to black spots, 

 Z to 5 millinneters in length. Prosorhynchus 

 gains entry to the muscle in the worm- 

 like cercarial stage by boring under 

 the scales and into the tissues, where it 

 encysts. There is no evidence that this 

 trematode can infest man. The objec- 

 tion to its presence in fish marketed for 

 human consumption is based on aes- 

 thetic grounds. 



The larva of the trematode Porracaecum 

 decipieiis, know^n as the cod-worm, is 

 reported to occur in Pacific ocean 

 perch and other rockfish. In this case 

 also the fish is an intermediate host. 

 The larvae, which occur in the muscle 

 of the fish, are wormlike, white to 

 reddish brown in color, and 5 milli- 

 meters to 9 centimeters in length. They 

 gain access to the tissues from the 

 alimentary canal. These parasites are 

 particularly objectionable aesthetic- 

 ally, since after the fish fillet has been 

 cut, the parasites tend to migrate fronn 

 the interior of the fillet and appear 

 between the plastic or cellophane wrap- 



iContribution No. 47, College of Fisheries, University of Washington, Seattle, Washington. 



