DOI: 10.5061/dryad.rbnzs7hc0
title: Nematode parasites of rockfish (Sebastes spp.) and cod (Gadus spp.) from waters near Kodiak Island Alaska, USA

description: 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.
description: 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 (&gt;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 <i>S. ruberrimus</i> 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 <i>S. ruberrimus</i> 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 <i>sensu lato </i>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.
description: 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 <em>Psuedoterranova decipiens </em>found in host
 column 7 = number of <em>Anisakis simplex</em> found
 in host