Distribution and abundance of common parasitic nematodes in marine fishes is not well documented in many geographic regions. Understanding the influence of large-scale environmental changes on infection rates of fish by nematodes requires quantitative assessments of parasite abundance for multiple host species. We collected samples of two species of cod and eight species of rockfish (total of 232 specimens) from waters near Kodiak Island, Alaska, USA during Spring and Summer of 2015, and dissected and recorded all internal nematode parasites. We quantified the prevalence and intensity of nematode parasites in the ten host species, and tested for differences in prevalence among host species. We found three species of nematode: Anisakis simplex, sensu lato (Van Thiel), Pseudoterranova decipiens, sensu lato (Krabbe), and Hysterothylacium sp. (Ward and Magath). Eighty-two percent of the examined fish were infected with at least one parasitic nematode. The overall prevalence of P. decipiens, A. simplex, and Hysterothylacium sp. was 56%, 62%, and 2%, respectively. Anisakis simplex and P. decipiens were abundant and present in all ten species of host fish examined, whereas Hysterothylacium sp. was rare and found in only five of the host fish species. Prevalence and mean intensity of P. decipiens and A. simplex varied across the ten host species, and the number of parasites varied substantially among individual hosts within host species. The mean intensity of P. terranova and A. simplex in our study was substantially higher than the mean intensity for these same species from multiple other locations in a recent meta-analysis. This study provides a baseline of nematode parasite abundance in long-lived fish in waters near Kodiak Island, AK, and fills an important gap in our quantitative understanding of patterns of occurrence and abundance of these common and widespread parasites of marine fish.

Kodiak Island is a large island off the coast of southeastern Alaska, USA. The island is home to the largest fishing port in Alaska, and is one of the largest fishery ports in the USA. We examined a total of 232 fish of ten species as follows: Pacific ocean perch, Sebastes alutus (Gilbert; n = 20; May 2015); silvergray rockfish, S. brevispinis (Bean; n = 20; June 2015); dark rockfish, S. ciliatus (Tiliesius; n = 49; May, August, September 2015); black rockfish, S. melanops (Girard; n = 23; June 2015); northern rockfish, S. polyspinis (Taranets and Mosiev; n = 20; May, June 2015); redstripe rockfish, S. proriger (Jordan and Gilbert; n = 20; June, July 2015); yelloweye rockfish, S. ruberrimus (Cramer; n = 20; June, July 2015); harlequin rockfish, S. variegatus (Quast; n = 20; June 2015); Pacific cod, Gadus macrocephalus (Tilesius; n = 20; September 2015); and Alaska pollock, Gadus chalcogrammus (Pallas; n = 20; September 2015). We purchased specimens of nine of the ten species from the local fish processing facility of Trident Seafoods Corporation on Kodiak Island, and we captured specimens of S. melanops by hook and line from a charter boat. The samples we acquired from Trident Seafoods were caught by net trawlers as part of commercial fishing operations in the area surrounding Kodiak Island (Figure 1). Gadus chalcogramus was collected in the Shelikhov Strait west of Kodiak Island in relatively shallow depths (>200 m). We purchased freshly killed specimens immediately upon their arrival at the fish processing facility in which parasites were still alive and active. We obtained all samples between May and October of 2015, and fish dissections, parasite collection, and preservation were performed at the Kodiak Seafood and Marine Science Center on Near Island, Alaska, USA.

To facilitate the detection of parasites, we carefully searched all internal organs in the body cavity according to standard dissection methods (14). Briefly, we split the digestive tract along its entire length and searched both the internal and external surfaces. We cut all solid organs, such as the liver and gonads, into small pieces and examined each piece under a Nikon MK II Microscope. We then filleted the flesh off both sides and visually searched the muscle for parasites. We received the S. ruberrimus samples after they had been filleted, so only the internal organs were available for inspection. We found the vast majority of nematodes within organ structures and only rarely in the muscle tissue, and we assume that the locations of nematodes found within S. ruberrimus are consistent with this trend. As nematodes were detected, we cleared and preserved them according to methods outlined in [14]. Briefly, we cleared specimens in a 1:1 solution of EtOH and glycerol. As the ethanol evaporated, we gradually added glycerol to prevent the specimens from collapsing. After they were cleared, we stored them in ethanol. Identification and description of nematodes was based on [15,16] and http://www.marinespecies.org/ (accessed on 16 June 2018). Specifically, we used characteristics of the caecum, general size, and morphology of the anterior region to differentiate among the three observed species of nematodes [15,16]. We did not use molecular methods to differentiate nematode species; however, differentiation of these species is generally clear based on the characters reviewed [15,16] and seen in Figure 2. We acknowledge that there may be hidden diversity that we were unable to discern given our reliance on morphology for identification, so we indicate this potential uncertainty by the use of the term sensu lato after species names. We calculated quantitative descriptors of the parasite populations according to [12,13], and for mean intensity we included confidence intervals of the mean.

We performed chi-square tests to determine differences in prevalence among host species and among parasite species. To better understand how the number of parasites varies within and among host species, and to address the possibility that parasites exhibit a clumped distribution of individuals within a few individual hosts, we characterized the frequency distribution of parasites within host species [13]. Specifically, to determine if distributions were clumped, we calculated the percentage of the total number of a given parasite found within a host species that was contained in the most highly infected individual host. (We define “clumping” as a distribution of parasites in which a few fish have most of the parasites.) Obviously, this clumping index is not very useful when the total number of parasites is low.

Dataset has seven columns:

column 1 = unique identifier for host fish

column 2 = species identifier abbreviation, first letter of genus and species, see article for full host species names

column 3 = total length of host

column 4 = standard length of host

column 5 = sex of host

column 6 = number of Psuedoterranova decipiens found in host

column 7 = number of Anisakis simplex found in host