Sarcotaces arcticus (Collett, 1874) is a relatively large, internal parasitic copepod that infects many marine fishes. Although its large size suggests it may have a negative effect on host reproduction by reducing space available in the abdominal cavity (i.e., volumetric effect), such quantitative aspects of host-parasite relationships for S. arcticus have never been documented. We compared the prevalence and the ratio of live to dead parasites among sizes and sexes of two species of rockfish hosts (Sebastes ciliatus, Tilesius, 1813, dark rockfish; and Sebastes variabilis, Pallas, 1814, dusky rockfish) and quantified the reduction of internal space available in infected hosts. Samples were collected in southeast Alaskan waters where the two host rockfish species coexist in sympatry. Both total prevalence and prevalence of live S. arcticus were significantly higher in S. variabilis compared to S. ciliatus, because of higher prevalence in female S. variabilis. The relationship between body cavity volume, volume available for reproduction, and total length was isometric for both host species combined. An average live S. arcticus with a volume of 8.1 milliliters occupied about 45% in smaller hosts and about 5% in larger hosts of the volume available for reproductive organs. The high prevalence and large size of this parasite could significantly reduce fecundity and fitness of rockfish hosts through reductions in internal volume available for reproduction.

To assess the prevalence and effects on internal volume of hosts of S. arcticus, we surveyed two species of rockfish: S. variabilis and S. ciliatus. Sebastes ciliatus and S. variabilis are sister species that are commonly caught in the same areas. Both species are pelagic, occur in large aggregations, and feed on pelagic crustaceans in the water column. They are sympatric and syntopic in the areas we surveyed, such that fishing yields a mix of both species and sexes, and the depth at which both species are caught does not differ. We caught both species via hook and line in Frederick Sound, near Admiralty Island, AK, USA (56°57′14.76″ N, 134°10′36.19″ W). We were not able to collect individuals from the smallest size class (<240 mm TL; ages 1–4 yr) of either host fish species. This study focuses on individuals larger than about 240 mm TL, which are the smallest individuals available in the large aggregations where we fished.

Study system

We caught a total of 182 S. ciliatus and 135 S. variabilis from 7–10 July 2017. In addition to capturing the two host species, we caught 31 S. ruberrimus, 29 S. maliger, and small samples of other co-occurring rockfish species (S. brevispinis, S. caurinus, S. flavidus, S. melanops, S. miniatus, and S. nigrocinctus). On the same day the fish were caught, we dissected each fish for internal observation and detection of S. arcticus parasites. We recorded total length and standard length to the nearest millimeter of the host fish, determined the sex, collected muscle tissues and otolith samples, recorded whether each fish was parasitized by S. arcticus, and counted the total number of galls of S. arcticus. Only *S. ciliatus *and S. variabilis were parasitized by S. arcticus; therefore, none of the other Sebastes species were used for further analysis. We identified S. arcticus parasites in S. ciliatus and S. variabilis by the presence of an internal gall(s) near the anal vent of the fish. Then, we carefully extracted the galls to prevent rupture of the parasites and preserved them in 90% ethyl alcohol. In the lab, we dissected the gall and characterized each parasite by life stage. We separated S. arcticus into three life stages at the time of the collection: live small, live large, and dead. We classified stage one as a gall, up to 10 mm in length, containing a female but no black fluid (hemosiderin). Stage two was classified as a gall (>10 mm in length) containing a female and hemosiderin, as well as the presence of eggs or nauplii. We classified stage three as the “scar” or dead stage, which we identified by a relatively small and hard gall, with no identifiable female, often with the presence of solidified hemosiderin (ranging from 5 to 20 mm in length). Additionally, we classified each fish into one of three size classes based on total length: small (fish between 24 to 37 cm; 5–10 years old), intermediate (fish between 37 to 44 cm;10–15 years old), and large (44+ cm; 15+ years old) [11].

In 2018, we collected 21 S. ciliatus and 10 S. variabilis to assess the volumetric effects of S. arcticus infection. To measure available space, we extracted the organs of each fish and measured their volume in milliliters via water displacement. Major organs such as the heart, kidneys, and stomach were included in the measurement of organ volume. The stomachs were always empty due to barotrauma-induced extrusion during capture. The swim bladder was excluded from our measurements, since its volume is highly variable depending on the location of the fish in the water column. If the individual was reproductively mature, we removed their enlarged gonads before calculating organ volume. We did not remove gonads of reproductively immature individuals because they were only small, thread-like structures that would not significantly affect the volume available. After all the organs were removed, we measured the total volume of each fish’s body cavity in milliliters. To adjust the volume of the nonreproductive internal organs to account for the swim bladder, we took the volume of nonreproductive organs and added an estimated 20% of the total cavity volume to represent the swim bladder. We then divided nonreproductive organ volume by total cavity volume (×100) to get the percentage of total cavity volume occupied by nonreproductive organs.

Further, to calculate volume available for reproduction, we subtracted nonreproductive organ volume from total cavity volume. From a separate set of host individuals, we collected 19 galls of S. arcticus. The volume of these galls was measured in milliliters via water displacement to establish an average volume occupied by a single S. arcticus infection. We then divided the average volume of a single S. arcticus individual by the calculated volume available for reproductive organs (×100) to get the percentage of normally available volume for reproductive organs that would be occupied by an individual S. arcticus.

Analysis

We calculated prevalence of S. arcticus for both species of host rockfish in relation to sex and size class. Prevalence was measured as the number of infected hosts divided by the total number of hosts examined for the specified group [14]. We compared prevalence of S. arcticus between species and sexes, and among size classes within species by sex combinations using contingency table analysis in Proc FREQ in SAS (SAS version 9.4, SAS Inc., Cary, NC, USA). Because some categories had an expected value < 5, we used a one-tailed version of Fisher’s exact test for comparison. We calculated prevalence of live S. arcticus in relation to sex and size class of host in the same way as total prevalence using contingency table analysis in Proc FREQ in SAS (SAS version 9.4, SAS Inc., Cary, NC, USA). To determine differences in the distribution of live and dead parasites, we mapped parasite mortality structure (live or dead) among size classes of the host fishes by comparing the number of parasites that were live to the number that were dead within each host species by sex and by size class of host fishes. We used a contingency table analysis to test for deviation from the overall expected proportion of live and dead parasites among host groups using Proc FREQ in SAS (SAS version 9.4, SAS Inc., Cary, NC, USA).

We performed an Analysis of Covariance (ANCOVA) to evaluate how total cavity volume and nonreproductive organ volume differed between host species and with total length (Table 1; R software, version 4.4.0). Because there was no significant species effect nor a significant species by total length interaction, we combined species for further analyses. We used linear regression to test against an expected slope of 3 (isometry for a length/volume comparison) between the (ln transformed) total length (mm) and (ln transformed) host cavity, and (ln transformed) nonreproductive organ volume. In addition, we used linear regression to determine if the percent of cavity volume available for reproduction occupied by an individual S. arcticus varied with host body size, as well as the percent of organ volume out of total cavity volume.

We estimated the volume of the two dead parasite galls as 4 milliliters each (half of the average live parasite volume) and subtracted them from the total measured gall volumes. For fish with multiple live parasites, we evenly distributed the volume to separate them into individual parasites. Subsequently, we determined the mean, minimum, and maximum percent of the potential reproductive volume an average S. arcticus occupied in the host species combined.

We estimated the volume of the two dead parasite galls as 4 milliliters each (half of the average live parasite volume) and subtracted them from the total measured gall volumes. For fish with multiple live parasites, we evenly distributed the volume to separate them into individual parasites. Subsequently, we determined the mean, minimum, and maximum percent of the potential reproductive volume an average S. arcticus occupied in the host species combined.