Differential parasitism by Naobranchia occidentalis and Nectobrachia indivisa on northern rock sole and southern rock sole in Alaskan waters - Copepoda: Naobranchiidae - Copepoda: Lernaeopodidae - Lepidopsetta polyxystra Orr and Matarese, 2000 - L. bilineata Ayres, 1855 - Statistical Data Included

Fishery Bulletin, April, 2001 by Mark Zimmermann, Robin C. Harrison, Anthony F. Jones

Abstract--Northern rock sole (Lepidopsetta polyxystra Orr and Matarese, 2000) and southern rock sole (L. bilineata Ayres, 1855) from the Gulf of Alaska and northern rock sole from the Aleutian Islands were examined for gill parasites. Four species of copepod parasites were identified: Naobranchia occidentalis and Nectobrachia indivisa were the most common. Both parasites were more prevalent on northern rock sole (22% and 15%, respectively) than on southern rock sole (5% and 1%, respectively) in the Gulf of Alaska samples. Northern rock sole tended to have a greater mean intensity of Naobranchia occidentalis than southern rock sole but there was not a significant difference because of the high variance about the means; too few southern rock sole were infested by Nectobrachia indivisa for comparison. Northern rock sole from the Aleutian Islands region had a significantly greater prevalence (36%) and mean intensity (10.2/infested fish) of Naobranchia occidentalis than northern rock sole from the Gulf of Alaska (22%, and 4.4, respectively) but did not differ significantly in prevalence and mean intensity of Nectobrachia indivisa. Parasitized male northern rock sole from the Gulf of Alaska had a significantly reduced weight at length, indicating a possible effect of parasitism. Naobranchia occidentalis selectively infested larger northern rock sole and only the largest southern rock sole. Nectobrachia indivisa also were found on larger northern rock sole but did not infest enough southern rock sole to describe a trend. Southern rock sole males were not infested by either parasite. Naobranchia occidentalis preferred to infest the middle gill arches of hosts and Nectobrachia indivisa preferred to infest the exterior gill arches of hosts.

The rock soles (Lepidopsetta spp.) are important commercial flatfish species that inhabit the continental shelf of the north Pacific Ocean (Hart, 1973). Stock assessment bottom trawl surveys conducted by the Alaska Fisheries Science Center (AFSC) historically recognized only one species of rock sole occurring in four survey areas: eastern Bering Sea, Aleutian Islands, Gulf of Alaska, and western U.S. coast. Recent larval morphometrics by Matarese et al. (1989) and a generic revision by Orr and Matarese (2000) have demonstrated that there are actually two species of rock sole that overlap in the AFSC survey areas. The northern rock sole (L. polyxystra Orr and Matarese, 2000) ranges throughout the Bering Sea and Aleutian Islands to Puget Sound, whereas the southern rock sole (L. bilineata Ayres, 1855) ranges from the Islands of Four Mountains in the eastern Aleutian Islands to Mexico (Orr and Matarese, 2000). With the knowledge of this recent research, field biologists began separating the two rock sole species in AFSC bottom trawl surveys starting in 1996. The northern rock sole is distinguished by higher gill-raker counts and a whiter blind side than southern rock sole occurring in the same trawl hauls (Orr and Matarese, 2000).

While examining gill rakers to identify specimens of rock soles captured near the Shumagin Islands during the 1996 Gulf of Alaska survey, we noticed that several live, apparently healthy, northern rock soles had pink or white gill filaments, a condition normally associated with dead fish. On close examination, we found small copepod parasites attached to the gill filaments of these fish. Southern rock soles from the same trawl hauls usually had red, apparently healthy gill filaments and were less frequently parasitized. Therefore we surveyed northern and southern rock soles in selected hauls from the Gulf of Alaska in 1996 for the presence of these parasites and saved infested heads for parasite identification and enumeration in the laboratory. In 1997, we conducted the same parasite investigation in several hauls in the Aleutian Islands region. Our main objective was to quantify differences in the prevalence (percent fish infested) and mean intensity (average number of parasites per infested fish) of gill parasites infesting the northern and southern rock soles (see Margolis et al., 1982; Bush et al., 1997, for complete definitions of ecological terms in parasitology). Secondary objectives were to test for regional differences in prevalence and mean intensity, to describe and compare site preferences of the parasites, and to determine if parasites influenced weight at length for parasitized fish.

Materials and methods

Between 8 and 18 June 1996 (Table 1), northern and southern rock soles captured in a series of trawl hauls from the western Gulf of Alaska were examined for parasites (Fig. 1). Between 25 and 27 June 1997 (Table 1), we examined northern rock soles from a series of trawl hauls around Seguam Island and the east end of Amlia Island in the central Aleutian Islands. The two sampling areas are approximately 850 km apart. We scanned the interior and exterior surfaces of gill filaments on all gill arches for parasites. All rock soles were examined in hauls with few fish (n [is less than or equal to] 50), and a random selection of fish in larger hauls were subsampled in a manner similar to that used for obtaining a random subsample for lengths (see Martin, 1997, for Gulf of Alaska bottom trawl survey methods). For each parasitized fish, we measured the fork length (FL) in centimeters, weight ([+ or -] 2 grams), determined the sex, and removed and froze the entire head, including gill arches, at sea. Lengths and weights were also collected from uninfested northern and southern rock soles in the Gulf of Alaska survey for comparisons with parasitized fish.