https://cpw.cvlcollections.org/files/original/2ea3fd5daebe48685d5ae068d3218e71.pdf 3164fa7d2a61d47abba2c0ecac854216 PDF Text Text The research in this publication was partially or fully funded by Colorado Parks and Wildlife. Dan Prenzlow, Director, Colorado Parks and Wildlife • Parks and Wildlife Commission: Marvin McDaniel, Chair • Carrie Besnette Hauser, Vice-Chair Marie Haskett, Secretary • Taishya Adams • Betsy Blecha • Charles Garcia • Dallas May • Duke Phillips, IV • Luke B. Schafer • James Jay Tutchton • Eden Vardy �Received: 28 June 2021 | Revised: 5 September 2021 | Accepted: 8 September 2021 DOI: 10.1111/jfd.13539 RESEARCH ARTICLE Rapid proliferation of the parasitic copepod, Salmincola californiensis (Dana), on kokanee salmon, Oncorhynchus nerka (Walbaum), in a large Colorado reservoir Jesse M. Lepak1 | Adam G. Hansen1 Estevan M. Vigil4 1 Colorado Parks and Wildlife, Aquatic Research Section, Fort Collins, Colorado, USA 2 | Mevin B. Hooten2 | Daniel Brauch3 | Abstract Ecologically and economically valuable Pacific salmon and trout (Oncorhynchus spp.) Department of Statistics and Data Sciences, The University of Texas at Austin, Austin, Texas, USA are widespread and susceptible to the ectoparasite Salmincola californiensis (Dana). 3 was observed in Blue Mesa Reservoir, Colorado, USA, an important kokanee salmon Colorado Parks and Wildlife, Gunnison, Colorado, USA 4 Colorado Parks and Wildlife, Monte Vista, Colorado, USA Correspondence Jesse M. Lepak, Colorado Parks and Wildlife, Aquatic Research Section, 317 West Prospect Road, Fort Collins, CO 80526, USA. Email: salvelinus2005@gmail.com Funding information Financial and material support was provided by Colorado Parks and Wildlife. All aspects of the project were completed under appropriate animal care and use protocols, and no human subjects or clinical trials were conducted. Materials are not reproduced from others. The range of this freshwater copepod has expanded, and in 2015, S. californiensis (O. nerka, Walbaum) egg source for sustaining fisheries. Few S. californiensis were detected on kokanee salmon in 2016 (<10% prevalence; 2 adult S. californiensis maximum). By 2020, age-3 kokanee salmon had 100% S. californiensis prevalence and mean intensity exceeding 50 adult copepods. Year and kokanee salmon age/maturity (older/mature) were consistently identified as significant predictors of S. californiensis prevalence/intensity. There was evidence that S. californiensis spread rapidly, but their population growth was maximized at the initiation (the first 2–3 years) of the invasion. Gills and heads of kokanee salmon carried the highest S. californiensis loads. S. californiensis population growth appears to be slowing, but S. californiensis expansion occurred concomitant with myriad environmental/biological factors. These factors and inherent variance in S. californiensis count data may have obscured patterns that continued monitoring of parasite–host dynamics, when S. californiensis abundance is more stable, might reveal. The rapid proliferation of S. californiensis indicates that in 5 years a system can go from a light infestation to supporting hosts carrying hundreds of parasites, and concern remains about the sustainability of this kokanee salmon population. KEYWORDS gill lice, intensity, invasion, maturity, prevalence 1 | I NTRO D U C TI O N become an important sport and food fish in many North American lakes (Biser, 1998; Nelson, 1968; Wydoski & Bennett, 1981). They Salmon and trout of the genus Oncorhynchus are widely distributed have been introduced in systems as large as the Great Lakes and throughout the Pacific Rim region, historically supporting large- many smaller waterbodies in western states such as California, scale fisheries in their native ranges. Managers have introduced Colorado, Idaho, Montana, New Mexico, Utah and Wyoming, as Oncorhynchus salmon and trout widely in freshwater systems to cre- well as in north-eastern states such as Connecticut, Maine and New ate new recreational and sport fishing opportunities. For example, York (United States Geological Survey, 2021). Kokanee salmon are kokanee salmon (lacustrine sockeye salmon, O. nerka, Walbaum) have often heavily managed in these systems as well as others in western J Fish Dis. 2021;00:1–10. wileyonlinelibrary.com/journal/jfd� © 2021 John Wiley & Sons Ltd | 1 �2 | LEPAK et al. Canada (e.g. British Columbia) where egg collection and/or stock- uncertain, deleterious impacts are evident (e.g. Herron et al., 2018) ing takes place regularly in multiple waterbodies (pers. comm. BC, and could be detrimental to some reservoir populations of eco- CO, ID, NM, WY management personnel). Although kokanee salmon logically and/or economically important Pacific salmon and trout represent valuable sport fish and management tools, they (and other throughout their native and introduced ranges. Oncorhynchus species) are susceptible to the freshwater ectopara- In 2015, S. californiensis were confirmed in Blue Mesa Reservoir sitic copepod Salmincola californiensis (Dana), also known as “gill-lice” (Gunnison County, Colorado) with an observation of an adult S. cal- or “gill-maggots” (Kabata, 1969). Kokanee salmon and anadromous iforniensis on a rainbow trout that was captured in a gill net during sockeye salmon interact with the ectoparasite S. californiensis within a survey in May (C. Gunn, Colorado Parks and Wildlife, Aquatic their native ranges on the Pacific Coast (Bailey & Margolis, 1987; Animal Health Laboratory Case # 15–102, pers. comm.). There is a Chigbu, 2001; Kabata, 1969). However, the distribution of S. californ- lack of information about early S. californiensis invasion dynamics in iensis has expanded eastward through the movement and stocking of reservoirs, with evaluations of infestations largely focused on es- infected fish (Kamerath et al., 2009; Ruiz et al., 2017; Sutherland and tablished populations (e.g. Hargis et al., 2014; Monzyk et al., 2015; Whittrock, 1985) and now represents an emerging concern for some Murphy, Gerth, Pauk, et al., 2020). We describe the rapid prolifera- populations of kokanee salmon and other Oncorhynchus salmon and tion of S. californiensis in the Blue Mesa Reservoir kokanee salmon trout. population. We hypothesized 1) the spread (increase in prevalence Despite wide distribution and potential impacts on valuable fish and intensity) of S. californiensis infestation in Blue Mesa Reservoir populations, relatively little is known about S. californiensis (Murphy would occur, and be most evident in older, mature kokanee salmon, et al., 2020). The adult stage of the ectoparasite is visible with the 2) there are differences in S. californiensis dynamics on male and fe- naked eye, appearing similar to a grain of rice. They attach to host male kokanee salmon, 3) there are seasonal components to S. cal- fish in a variety of locations including the gill filaments, branchial iforniensis infestations, 4) mature kokanee salmon would be more cavity and fin rays (Kabata and Cousens, 1977). Although low-level susceptible to S. californiensis relative to others, and 5) accumulation infections are not thought to cause mortality directly, anaemia, re- of S. californiensis will be highest on the gills relative to other tissues duced gas exchange and osmotic regulation, and poor swimming en- of kokanee salmon. We describe our findings and the potential im- durance are associated with severe infections (Herron et al., 2018; plications of this infestation and others, highlighting the need to limit Pawaputanon, 1980). These sublethal effects may contribute to further spread of S. californiensis. higher mortality under stressful environmental conditions (Hargis et al., 2014; Vaughn and Coble, 1975) or during critical life stages such as spawning or smoltification (Barnett et al., 2020; Herron et al., 2018). Further, high prevalence (proportion of individuals parasitized) and intensity (number of adult parasites per fish) of S. cal- 2 | M E TH O DS 2.1 | Study system iforniensis infestations may impede recovery efforts for threatened species like Chinook salmon (O. tshawytscha, Walbaum) and have Blue Mesa Reservoir (38°28′30″ N and 107°12′30″ W) is a meso- been implicated in the deterioration of some recreational fisher- trophic, 3,793-ha impoundment with a maximum depth of 101 m, ies concomitant with other stressors (Hargis et al., 2014; Monzyk mean depth of 28 m and an approximate hydraulic retention time of et al., 2015; Vigil et al., 2016). 0.8 years (the reservoir can fluctuate widely; see Results). The reser- Prevalence and intensity of S. californiensis can be highly vari- voir thermally stratifies, generally between May and October, with able on their hosts, and multiple mechanisms may be contributing peak surface temperatures reaching 20–22°C. Densities of Daphnia to this observed variability. For example, S. californiensis preva- spp and typically peak in June at ~10–30 individuals/L in surface wa- lence, intensity and rate of accumulation were higher in reser- ters (Hansen et al., 2019). Kokanee salmon are stocked annually in voirs versus streams, and disparate among species and life stage Blue Mesa Reservoir, and overall, feeding and growth conditions in in Pacific salmon and trout including kokanee salmon, Chinook the reservoir support high kokanee salmon growth rates (Stockwell & salmon, cutthroat trout O. clarkia (Richardson) and rainbow trout Johnson, 1997). Kokanee salmon dominate angler catch and harvest O. mykiss (Walbaum) (Monzyk et al., 2015). Additionally, water (averaged 64.3% of total catch between 1993 and 2012), and Blue temperature in the laboratory has been shown to influence S. cali- Mesa Reservoir fish often provide a large portion of the kokanee forniensis reproduction/fecundity rates, the length of time for free salmon eggs needed to meet the stocking objectives for Colorado. swimming parasites to successfully attach to a host and the inten- There is also a trophy lake trout Salvelinus namaycush (Walbaum) sity of infections on hosts (Murphy et al., 2020; Neal et al., 2021; fishery sustained by a naturalized population supported in large part Vigil et al., 2016). S. californiensis has been found to increase in by energetically dense kokanee salmon that serve as prey (Johnson prevalence and intensity on larger kokanee salmon (Barndt & et al., 2017; Pate et al., 2014). Large individual lake trout exhibit some Stone, 2003; Neal et al., 2021) and with increasing salmon age of the fastest growth rates measured in North America (Martinez (Hargis et al., 2014). Further, host maturity or sex might play a role et al., 2009). These trophy fish are an important component of the in host–parasite dynamics (Rolff, 2002; Sheldon & Verhulst, 1996). fishery overall, and the presence of S. californiensis adds even more Although the mechanisms driving S. californiensis variability are complexity to successfully balancing a hatchery-sustained kokanee �| LEPAK et al. 3 salmon population and a naturalized lake trout population in Blue locations where they were attached to kokanee salmon were docu- Mesa Reservoir (Pate et al., 2014). mented. Attachment points were categorized as adipose fin, anal fin, body, cleithra, dorsal fin, gills, gill arches, isthmus, mouth, opercles, 2.2 | Kokanee salmon collection pectoral fins, pelvic fins and vent. These categories were condensed into three: 1) gills (gills and gill arches); 2) head (cleithra, isthmus, mouth and opercles); and 3) body/fins (adipose fin, anal fin, body, Kokanee salmon were collected with three different methods dur- pectoral fins, pelvic fins and vent). All S. californiensis were counted ing this study, all in accordance with required Colorado Parks and in the laboratory setting. The ice water holding kokanee salmon Wildlife animal care protocols. Approximately 100 kokanee salmon captured in vertical gill nets and transported to the laboratory was per week (~ 1:1 male and female fish) were collected upstream from filtered occasionally after laboratory processing was complete to as- Blue Mesa Reservoir at the Roaring Judy Hatchery along the East sess whether any adult S. californiensis were shed, but none were River (Gunnison County, Colorado, USA) during the fall spawning observed. runs of salmon from 2016 through 2020 (n = 2,329). In fall 2020, a floating trap net called a Merwin trap (Hubert et al., 2012) was deployed nearshore and retrieved on 19 October and 26 October 2.5 | Statistical analyses to collect mature kokanee salmon (n = 146) in addition to those collected during the salmon run to Roaring Judy Hatchery. Finally, from All statistical analyses were conducted using R 4.0.5 (R Core Team, 2018 to 2020, vertical gill nets, a standard suite of six multi-mesh 2021). nets described in Hansen (2019), were deployed overnight in off- We applied a suite of generalized linear models (GLMs) to draw shore regions of Blue Mesa Reservoir in the spring (May/June), sum- inference about patterns in the prevalence and intensity of S. cal- mer (July/August) and fall (November) to collect both mature and iforniensis on kokanee salmon through time and to evaluate the immature kokanee salmon (n = 580). importance of key biological factors. Based on the distribution of adult S. californiensis count data, we accounted for overdispersion in 2.3 | Kokanee salmon processing each analysis of prevalence and intensity on hosts by using logistic regression for prevalence analysis, and the more appropriate negative binomial distribution (versus the Poisson) for intensity analyses. Kokanee salmon collected at the hatchery were measured for We used Akaike's information criterion (corrected for small sample total length (TL; to the nearest millimetre), and these fish were as- size; AICc) to determine the most parsimonious (GLM) characterizing sumed to be mature. Sex was determined visually and confirmed each dataset described below (Burnham & Anderson, 2002) and in- with gamete expression. Age was estimated (due to large sample terpreted results from the reduced GLMs. size) with a machine-learning technique using fish length, sex, as For prevalence analyses, we used information from kokanee well as the weights of sagittal otoliths extracted from each indi- salmon collected from 2016 through 2020 during the fall spawning vidual as predictor variables (Lepak et al., 2012). Kokanee salmon run upstream from Blue Mesa Reservoir at the Roaring Judy Hatchery. collected in the Merwin trap were processed in the same way as Year of collection, length, estimated age (Lepak et al., 2012), sex, those collected in the hatchery. Kokanee salmon collected from and the presence or absence of adult S. californiensis were noted for vertical gill net sampling were put in ice water and transported to each individual. A strong correlation between length and estimated the laboratory where they were measured for TL and wet weight age precluded the inclusion of both variables (note: the age estimate (WW; nearest gram). Reproductive condition (immature versus ma- calculation incorporates kokanee salmon length; Lepak et al., 2012) ture, or maturing) was assessed across all seasons for these fish in the same models. Thus, kokanee salmon length was removed and and confirmed based on gamete presence/production during dis- estimated age was retained. We used logistic regression to evaluate section. Ages were assessed in these individuals by extracting sag- the additive effects of year of collection, estimated age, a quadratic ittal otoliths and visually inspecting their surfaces for annuli under year term and sex on the probability of adult S. californiensis pres- a dissecting microscope. ence on these fish. Information collected from 2016 through 2020 during the fall 2.4 | Salmincola californiensis counts spawning run at Roaring Judy Hatchery (as described in the prevalence analysis) was used to quantify how the intensity of S. californiensis infection on mature kokanee salmon changed over the study In 2016 and 2017, kokanee salmon (collected at the Roaring Judy period. We fit our models to the adult S. californiensis total counts Hatchery) were examined for the total number of adult S. californien- (yi for I = 1,..., n; n = total count) on individual kokanee salmon using sis attached. Adult S. californiensis were assessed because they could several covariates, including year of collection, estimated age (Lepak be seen relatively easily with the naked eye, though we acknowledge et al., 2012), a quadratic year term and sex. A reduced negative bino- juveniles may have been present and impacting kokanee salmon. In mial GLM was fit to draw inference about covariates with significant 2018, 2019 and 2020, total counts of adult S. californiensis and the explanatory value. �4 | LEPAK et al. To evaluate whether the progression of S. californiensis infec- Prevalence reached 100% for all age classes sampled 4 years after tion intensity varied seasonally and whether the maturation process initial detection in 2015 (Figure 1). The most parsimonious logistic modified dynamics, we used data collected from kokanee salmon regression model indicated that year of collection, estimated age captured in Blue Mesa Reservoir from 2018 to 2020. These data arose from fish collected using vertical gill nets (2018–2020) and supplemented with fish captured in fall 2020 using the Merwin trap (set nearshore in the reservoir proper) since no mature kokanee salmon were captured in vertical gill nets in fall 2020. The Merwin trap was only used in 2020 and precluded a relevant comparison of fish collected in the Merwin trap to others; however, lice counts on fish collected with the Merwin trap were not significantly different than those collected at the hatchery in 2020. Information was available for each individual kokanee salmon about the year of collection, length, estimated (or interpreted in the case of individuals from the gill nets) age, sex, maturity and season of capture. Length and estimated (or interpreted) age were correlated (as expected) with maturity. Because kokanee salmon maturity was of interest, it was retained in analyses while length and estimated (or interpreted) age were excluded. We fit these data by modelling the adult S. californiensis total counts (yi for i = 1,..., n) on individual kokanee salmon using several covariates, including year of collection, maturity, a quadratic year term, sex and season of capture. A reduced negative binomial GLM was fit to draw inference about covariates with significant explanatory value. Because age and maturity could not be included in the same model, we parsed and plotted data from 2018 to 2020 by estimated (or interpreted in the case of individuals from the gill nets) age and maturity to visualize differences in the effects of these correlated covariates. From 2018 to 2020, adult S. californiensis on kokanee salmon sampled were categorized as being located on the gills, head and body/fins because there was increasing concern about sublethal effects and the importance of parasitism on the gills in particular. This was in contrast to data from 2016 and 2017 (used in the analyses described above), which were total counts and not location-specific. Kokanee salmon collected at the hatchery, in vertical gill nets, and the Merwin trap were included in analyses to evaluate S. californiensis intensity at these locations. We evaluated the influence of the year of collection, kokanee salmon maturity (versus correlated length and age covariates which were excluded from the model set), a quadratic year term and kokanee salmon sex, on the counts of adult S. californiensis on each of the three locations (gills, head and body/fins). A reduced negative binomial GLM was fit to draw inference about covariates with significant explanatory value. 3 | R E S U LT S 3.1 | Salmincola californiensis prevalence and intensity The prevalence of adult S. californiensis infections increased significantly on mature Blue Mesa Reservoir kokanee salmon collected during the egg-t ake operation at the hatchery from 2016 to 2020. F I G U R E 1 Prevalence and intensity of adult S. californiensis on kokanee salmon collected during the Blue Mesa Reservoir spawning run at the Roaring Judy Hatchery from 2016 to 2020. Essentially all kokanee salmon were estimated to be age-2 (light grey bars) or age-3 (black bars). Note the difference in scales on the x-axes for per cent frequency of adult S. californiensis counts on kokanee salmon. In the upper right of each panel, sample sizes are provided by age class, followed by the prevalence of adult S. californiensis infections on kokanee salmon, and the standard deviation (SD) of adult S. californiensis counts each year (upper left) �| LEPAK et al. and sex were significant factors characterizing prevalence of adult 5 (±0.09 S.E.), respectively, while kokanee salmon sex and season of S. californiensis (z-values = 20.3, 3.24 and 2.65, and p-values <.01, capture were not retained as significant predictors of S. californien- respectively), with estimated regression coefficients of 2.95 (±0.15, sis intensity after accounting for the other factors. Based on these S.E.), 0.43 (±0.16, S.E.) and 0.53 (±0.16, S.E.), while the quadratic results, for every year that passed from 2018 to 2020, the log mean year term was not retained as a significant predictor of S. californien- S. californiensis intensity increased 2,671, and mature fish had log sis prevalence after accounting for the other factors. Based on these mean S. californiensis intensities that were 1.55 higher than immature results, for every year that passed from 2016 to 2020, the log odds fish. The negative quadratic year term indicates again that there is a of S. californiensis being present on hosts increased by 2.95, female slowing in the increase in S. californiensis intensity over time (2018– fish had significantly higher log odds of having S. californiensis pre- 2020 in this case). This adds further support to the idea that S. cali- sent 0.43, and as fish increased a year in age, their log odds of having forniensis intensity is nearing an asymptote in Blue Mesa Reservoir. S. californiensis present increased 0.53. Kokanee salmon age and maturity were positively correlated and The intensity of adult S. californiensis infections increased signifi- could not be included in the same models. However, we were in- cantly on Blue Mesa Reservoir kokanee salmon collected during the terested in the relative importance of each factor for determining egg-take operation at the hatchery from 2016 to 2020 (Figure 1). S. californiensis intensity. Thus, Figure 3 was created showing mean The variation in S. californiensis counts also increased significantly adult S. californiensis counts by year, age and maturity to qualitatively throughout the study period (Figure 1). We evaluated the influence visualize the effects of these factors from 2018 to 2020 using fish of year of collection, kokanee salmon estimated age, a quadratic year collected from the reservoir (vertical gill nets and the Merwin trap). term and kokanee salmon sex on the intensity of adult S. californ- In some cases, it was challenging to obtain rare samples (e.g. age-1 iensis. The most parsimonious negative binomial GLM included the mature fish and age-3 immature fish), and sample sizes were limited effect of year of collection (z-value = 27.4 and p-value <.01), esti- for some age–maturity combinations (Figure 3). However, the mean mated age (z-value = 9.1 and p-value <.01) and the quadratic year infection intensity of mature fish generally exceeded those of their term (z-value = −27.4 and p-value <.01), with estimated regression similarly aged counterparts over the 2018–2020 period, suggesting coefficients of 1647 (±61.1 S.E.), 0.30 (±0.03 S.E.) and −0.41 (±0.02 that the effects of being mature (or maturing) can increase levels S.E.), respectively, while kokanee salmon sex was not retained as a of host infection beyond those expected by age alone. Mean age-0 significant predictor of S. californiensis intensity after accounting for and age-1 (immature) fish infection intensities remained relatively the other factors. Based on these results, for every year that passed low and stable from 2018 through 2020. Conversely, mean infec- from 2016 to 2020, the log mean S. californiensis intensity increased tion intensities for older, mature fish increased over the study period 1,647, and as fish increased a year in age, their log mean S. californ- and remained elevated above the immature members or their same iensis intensity increased 0.30. The negative quadratic year term cohort. One exception to this pattern was age-2 mature fish cap- indicates a slowing in the increase in S. californiensis intensity over tured in vertical gill nets in 2020, but mature fish were rare in the time. Indeed, unadjusted, mean (±S.D.), yearly intensities of S. cali- catch that year and sample size was small (n = 2). However, numer- forniensis on mature kokanee salmon collected at the hatchery were ous age-2 mature kokanee salmon were captured in the Merwin trap (0.1 ± 0.3), (1.6 ± 2.0), (9.2 ± 7.7), (36.9 ± 19.1), (48.1 ± 23.7) in 2016– and these fish had an elevated mean infection intensity (Figure 3). 2020, respectively. This corresponds to increases in over an order of Interestingly, age-4 kokanee salmon (n = 4) were captured in 2018, magnitude, nearly an order of magnitude, 4.0-fold and 1.3-fold, each and these exhibited relatively low S. californiensis counts (1, 2, 7 and year, respectively. Thus, S. californiensis intensity should be nearing 24), while a single mature age-1 fish documented in 2020 had a count an asymptote. of 16. We acknowledge the limited sample sizes reflected in some of these comparisons as well as multiple interacting factors potentially 3.2 | The influence of season and kokanee salmon maturity on S. californiensis influencing the observations (e.g. S. californiensis population dynamics and kokanee salmon ageing and maturation), but the representation provides some indication for the potential effect of maturation after accounting for age. The negative binomial GLM demonstrated that the intensity of adult S. californiensis infections increased significantly on Blue Mesa Reservoir kokanee salmon collected from the reservoir from 2018 3.3 | Salmincola californiensis attachment location to 2020 (Figure 2). We evaluated the influence of year of collection, kokanee salmon maturity, a quadratic year term, kokanee salmon sex The intensity of adult S. californiensis infections increased signifi- and season of capture (spring, summer or fall) on the intensity of cantly on kokanee salmon gills, heads and fins/body collected from adult S. californiensis. The most parsimonious negative binomial GLM the reservoir, and at the hatchery, from 2018 to 2020, and focused included the effect of year of collection (z-value = 7.6 and p-value largely around the head and gills (Figure 4). The negative binomial <.01), maturity (z-value = 20.7 and p-value <.01) and the quadratic GLMs describing S. californiensis intensity on gills, heads and fins/ year term (z-value = −7.5 and p-value <.01), with estimated regres- bodies included year of collection, kokanee salmon maturity, a quad- sion coefficients of 2,671 (±353 S.E.), 1.55 (±0.08 S.E.) and −0.66 ratic year term and kokanee salmon sex on the intensity of adult �6 | LEPAK et al. log mean intensity on host fins increased 0.007 (±0.002, S.E.) and 0.03 (±0.003, S.E.) with increases in S. californiensis on host gills (z- value = 3.6, and p-value <.01) and head (z-value = 12.0 and p-value <.01), respectively. This suggests that if individual fish are prone to high infections, those infections will likely be high across multiple attachment locations. We visualized means and standard deviations of the S. californiensis count data for immature and mature kokanee salmon separately by year and by attachment location to provide an indication of the shape of the quadratic term suggesting that S. californiensis intensity increase is slowing/declining (Figure 5). At all attachment locations evaluated for immature individuals, mean S. californiensis intensities appeared similar or lower in 2020 than in 2019. Conversely, in mature kokanee salmon, mean S. californiensis intensities at all locations were higher in 2020 than in 2019, but with a lower proportional increase from 2019 to 2020 relative to the period from 2018 to 2019 (Figure 5). 4 | D I S CU S S I O N To our knowledge, this work represents the first rigorous documentation and extensive monitoring of the early invasion dynamics of S. californiensis on kokanee salmon in a large inland reservoir. In less than 5 years, Blue Mesa Reservoir went from having kokanee salmon with very few S. californiensis to supporting heavy parasite loads similar to those found in reservoirs with established S. californiensis populations (Hargis et al., 2014). As expected, the most infected fish tended to be older, mature individuals that carried over 100 individual adult parasites, largely focused around the head and gills. Though older, mature individuals tended to carry more parasites, there was also high variation in infection intensity among individuals. Higher inF I G U R E 2 Intensity of adult S. californiensis on immature (light grey bars) and mature (black bars) kokanee salmon collected in Blue Mesa Reservoir from 2018 to 2020. Sample sizes are provided for immature and mature kokanee salmon followed by adult S. californiensis prevalence for each year (upper left), respectively tensities of S. californiensis were observed on older, mature kokanee salmon, but the increase in S. californiensis abundance can largely be described as a function of time, and it appears that the S. californiensis population is nearing an asymptote in Blue Mesa Reservoir under current conditions. These findings suggest that when S. californiensis first invade a system, there may be only a few years for managers S. californiensis and showed that the same factors found to be impor- and biologists to react before they can become well-established on tant predictors of S. californiensis intensity in the previous analysis host fish populations. (year of collection, kokanee salmon maturity and a quadratic year Based on our observations from Blue Mesa Reservoir, there was term) were also important within these data. After accounting for a rapid increase of S. californiensis on spawning kokanee salmon these effects, we found that S. californiensis attachment locations from 2016 to 2020. As hypothesized, year of collection was a sig- were not independent, and characterizing S. californiensis at one at- nificant predictor of prevalence and/or intensity of S. californiensis tachment location provides information for predicting intensities at on kokanee salmon in all cases evaluated, indicating a strong effect other locations. For example, S. californiensis log mean intensity on of S. californiensis population growth on the results presented here. host gills increased 0.05 (±0.002, S.E.) and 0.03 (±0.008, S.E.) with In 2016, relatively few S. californiensis were detected, and by 2020, increases in S. californiensis intensities on host heads (z-value = 23.6 age-2 through age-4 individuals had 100% prevalence, and in 2020, and p-value <.01) and fins (z-value = 3.6 and p-value <.01), respec- age-3 spawning kokanee salmon had a mean adult S. californiensis tively. Similarly, S. californiensis log mean intensity on host heads in- intensity exceeding 50, relative to a mean of less than one in 2016 creased 0.02 (±0.001, S.E.) and 0.05 (±0.006, S.E.) with increases in when data were first being collected. S. californiensis on host gills (z-value = 17.8 and p-value <.01) and fins The relatively large variance and broad distribution of S. cal- (z-value = 8.2 and p-value <.01), respectively. Finally, S. californiensis iforniensis count data from hosts were indicative of a parasite �| LEPAK et al. 7 F I G U R E 3 Mean intensity of adult S. californiensis on kokanee salmon collected in Blue Mesa Reservoir from 2018 to 2020. Mean S. californiensis counts from fish collected in the Merwin trap in 2020 are shown at the right and represented by solid boxes corresponding in colour and symbol to the age, maturity status and sample size listed in the legend. Standard deviations (when calculable) from left to right and top to bottom corresponding to parenthetical sample sizes listed in the legend are 0.5, 0.6, 0.7, 1.5, 2.0, 2.5, 4.1, 13.3, 10.9, 3.8, 11.7, NA, 1.5, NA, 18.6, 6.6, 19.6. 26.0, 10.7, NA, 21.6, 29.7, respectively. Mean S. californiensis counts from fish collected with vertical gill nets in 2018–2020 are represented with circles and solid lines (mature fish) and broken lines (immature fish) corresponding in colour and symbol to the age, maturity status and sample size listed in the legend. Age-4 mature fish are a single circle as they were only captured in 2018 F I G U R E 4 Attachment location of adult S. californiensis on immature (left) and mature (right) kokanee salmon by year. Adult S. californiensis attached to kokanee salmon gills (black bars) were those found on gills and gill arches of hosts. Adult S. californiensis attached to kokanee salmon heads (dark grey bars) were those found on the cleithra, isthmus, mouth and opercles of hosts. Adult S. californiensis attached to kokanee salmon fins/body (light grey bars) were those found on the adipose fin, anal fin, body, pectoral fins, pelvic fins and vent of hosts. Sample sizes are provided for each year (upper right of each panel). Note that the per cent frequencies (calculated location specifically) are presented cumulatively across locations and can therefore exceed 100% when multiple locations are considered simultaneously for a given adult S. californiensis count. Also, note the difference in scales on the x-axes for per cent frequency of adult S. californiensis counts on kokanee salmon by location with relatively high host specificity (Combes, 1997; Poulin, 1998). note that kokanee salmon maturity is strongly correlated to length Based on the findings of others (i.e. Barndt & Stone, 2003; Hargis and age, and S. californiensis prevalence and intensity have been re- et al., 2014), we expected that the oldest (and generally largest) ported to increase with length as well as age (Barndt & Stone, 2003; hosts had the potential to carry the highest numbers of S. californ- Hargis et al., 2014; Neal et al., 2021). Given the inherent variability iensis. Indeed, numbers of S. californiensis increased with kokanee in the data, it is not surprising that even larger, older fish had the salmon estimated age and when hosts were mature. However, we potential to carry light infections in Blue Mesa Reservoir. �8 | LEPAK et al. body (adipose fin, anal fin, body, pectoral fins, pelvic fins and vent) of kokanee salmon had more habitat (albeit of unknown suitability) to colonize on individual hosts. This was also the case (but to a lesser extent) for kokanee salmon gills and heads, and there may be some upper limit to the number of adult S. californiensis that can occupy a host in a single area. However, it appears that something (perhaps physiological/biological/behavioural factors at the host and/or parasite level) is limiting (or enhancing) larger build-ups of adult S. californiensis at some locations versus others on kokanee salmon. We were interested in potential differences in adult S. californiensis on male and female kokanee salmon (e.g. Rolff, 2002), and also potential seasonal patterns of adult S. californiensis intensity. F I G U R E 5 Adult S. californiensis attachment location: immature kokanee salmon sample sizes were 89, 92 and 156 in 2018 (light grey bars), 2019 (dark grey bars) and 2020 (black bars), respectively. Mature kokanee salmon sample sizes were 648, 487 and 601 in 2018, 2019 and 2020, respectively. Means and standard deviations of adult S. californiensis counts on kokanee salmon gills (those found on gills and gill arches of hosts), heads (those found on the cleithra, isthmus, mouth and opercles of hosts) and fins/body (those found on the adipose fin, anal fin, body, pectoral fins, pelvic fins and vent of hosts) are shown Kokanee salmon sex was a significant predictor of adult S. californiensis prevalence, with females having increased prevalence compared to males. However, sex was not found to be a significant predictor of S. californiensis intensity in any analysis. Thus, we are cautious about drawing inference from these results. There may be mechanism(s) influencing these data beyond the covariates evaluated, and we also acknowledge that the population of S. californiensis (as indicated by adults on kokanee salmon) was growing rapidly during this time period, and this effect may be obscuring more subtle processes related to kokanee salmon sex. Seasonal patterns were not evident Kokanee salmon maturity was a significant predictor of adult after accounting for the effects of year, kokanee salmon maturity S. californiensis intensity, with mature individuals carrying higher and the quadratic year term on adult S. californiensis intensity data. If parasite loads. Blue Mesa Reservoir salmon and trout do not transi- seasonal effects like increased water temperatures driving increased tion from fresh to saltwater and back, but they do divert energy to S. californiensis intensities were present, they were likely being ob- maturation, reproductive processes/behaviours, as well as gamete scured by S. californiensis population growth, and perhaps they will production. As they mature, kokanee salmon change morphologi- become evident as the S. californiensis population reaches a more cally and also congregate prior to spawning and running upstream. stable state. The physiological and behavioural characteristics associated with The expansion of S. californiensis to new, available habitat the process of maturation could be contributing mechanistically to patches (hosts), was expected, though the expansion was relatively this observation (Sheldon & Verhulst, 1996). We note that maturity rapid. The growth of the S. californiensis population in Blue Mesa is correlated with age (and length), and host–parasite relationships Reservoir occurred concomitant with several factors that may have influenced by these factors could be confounding. For example, influenced some of our observations. Stocking of kokanee salmon one could reasonably assume that physically larger hosts (presum- was conducted throughout this period, adding potential hosts to the ably with more parasite habitat) would have higher intensities of system. In 2018, the estimate of Blue Mesa Reservoir volume above S. californiensis (Neal et al., 2021). If there are behavioural or phys- deadpool was the lowest in the data series (reduced by over half) iological differences related to age/maturity that could influence based on data available from the US Department of Interior, Bureau S. californiensis intensity, those mechanistic effects could be ob- of Reclamation unpublished data (K. Rogers, Colorado Parks and scured by larger fish having higher surface area and parasite habitat. Wildlife, pers. comm.). This likely resulted in warmer overall water We evaluated kokanee salmon age and maturity separately, and also temperatures (Johnson & Martinez, 2012) and higher host and para- attempted to represent these effects simultaneously, and found ev- site densities within the water column. idence that both are likely influencing S. californiensis counts despite These factors likely created conditions that were more ideal for low sample size in some cases for direct comparison, and we note S. californiensis and exacerbated their spread. But the combination that correlations between host size, age and maturity may confound of a lack of some data, correlations between many of the metrics the interpretation of results. used to indicate the data available, and the rapid population growth Currently, external (fins, body and vent) attachments of S. cali- of S. californiensis, made it challenging to determine the importance forniensis on kokanee salmon in Blue Mesa Reservoir appear to reach of factors like water temperature and reservoir volume on S. californ- a maximum of about 15–20 adult individuals. By comparison, head iensis spread. Thus, continued monitoring of evolving host-predator (cleithra, isthmus, mouth and opercles) and gill (gills and gill arches) dynamics could provide more insight on factors exacerbating nega- tissues of kokanee salmon supported higher numbers of S. cali- tive impacts from S. californiensis. forniensis, in some cases exceeding 50 adult individuals. Based on In 2020, the highest intensities of adult S. californiensis were surface area alone, the adult S. californiensis attached on the fins/ observed on mature kokanee salmon during the egg-t ake operation �| LEPAK et al. 9 at Blue Mesa Reservoir. The kokanee salmon run in 2020 produced C O N FL I C T O F I N T E R E S T the lowest number of eggs since 1984 because of a lack of spawn- To our knowledge, there are no conflicts of interest with any of the ing adults (D. Brauch, Colorado Parks and Wildlife, pers. comm.). As authors. in the past in Colorado, the S. californiensis infestation in Blue Mesa Reservoir occurred concomitant with a variety of potentially influ- DATA AVA I L A B I L I T Y S TAT E M E N T ential factors. Angling pressure and entrainment are factors to con- Data were not made accessible publicly for this manuscript. sider in Blue Mesa Reservoir, as well as a dynamic predatory lake trout population that consumes kokanee salmon (Pate et al., 2014). ORCID Further, heavily infected individuals may be more vulnerable to Adam G. Hansen https://orcid.org/0000-0001-5360-6530 some of these stressors and predation, and it has been shown that once a host is infected with S. californiensis, they may be more sus- REFERENCES ceptible to repeated infection (Neal et al., 2021), though these fac- Bailey, R. E., & Margolis, L. (1987). Comparison of parasite fauna of juvenile sockeye salmon (Oncorhynchus nerka) from southern British Columbian and Washington State lakes. Canadian Journal of Zoology, 65, 420–431. Barndt, S., & Stone, J. (2003). Infestation of Salmincola californiensis (Copepoda: Lernaeopodidae) in wild coho salmon, steelhead, and coastal cutthroat trout juveniles in a small Columbia River tributary. Transactions of the American Fisheries Society, 132, 1027–1032. Barnett, H. K., Quinn, T. P., Bhuthimethee, M., & Winton, J. R. (2020). 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The Blue Mesa Reservoir kokanee salmon population is particularly important for sustaining approximately two dozen kokanee salmon fisheries throughout Colorado and demonstrates the potential for wide- reaching impacts of S. californiensis infestation. Conditions in Blue Mesa Reservoir are such that managers may have selective breeding options to promote S. californiensis resistance. Blue Mesa Reservoir is also a highly managed, non-native fishery, so some management options might be considered more feasible there relative to other systems. However, managers may face challenges at larger scales associated with changing climate conditions like warmer temperatures and elongated growing seasons (potentially favourable for S. californiensis life history; Murphy, Gerth, & Arismendi, 2020; Neal et al., 2021; Vigil et al., 2016) that may exacerbate the spread and ultimate impacts of S. californiensis. Further, Marcogliese (2001) described how warmer temperatures from climate change could intensify fish crowding, negatively influence fish immune response and increase parasite transmission. The combination of pre-existing/recurring stressors and the new threat from introduced S. californiensis could prove detrimental to the Blue Mesa kokanee salmon population (and others) in the future. Due to the continued spread of S. californiensis outside their native range (Kamerath et al., 2009; Ruiz et al., 2017; Sutherland and Whittrock, 1985), there is a need to better understand (through observation and experimentation) host–S. californiensis dynamics to avoid negative outcomes associated with infestations. AC K N OW L E D G E M E N T S We acknowledge the long- term efforts of Colorado Parks and Wildlife personnel. We thank Kevin Rogers and William Pate for consultations about S. californiensis host abundance and reservoir volume. 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Journal of Fish Biology, 7, 283–294. https://doi. org/10.1111/j.1095-8649.1975.tb04601.x Vigil, E. M., Christianson, K. R., Lepak, J. M., & Williams, P. J. (2016). Temperature effects on hatching and viability of juvenile gill lice, Salmincola californiensis. Journal of Fish Diseases, 39, 899–905. Wydoski, R. S., & Bennett, D. H. (1981). Forage species in lakes and reservoirs of the western United States. Transactions of the American Fisheries Society, 110, 764–771. How to cite this article: Lepak, J. M., Hansen, A. G., Hooten, M. B., Brauch, D., & Vigil, E. M. (2021). Rapid proliferation of the parasitic copepod, Salmincola californiensis (Dana), on kokanee salmon, Oncorhynchus nerka (Walbaum), in a large Colorado reservoir. Journal of Fish Diseases, 00, 1–10. https:// doi.org/10.1111/jfd.13539 � Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Journal Articles Description An account of the resource CPW peer-reviewed journal publications Text A resource consisting primarily of words for reading. Examples include books, letters, dissertations, poems, newspapers, articles, archives of mailing lists. Note that facsimiles or images of texts are still of the genre Text. Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Rapid proliferation of the parasitic copepod, Salmincola californiensis (Dana), on kokanee salmon, Oncorhynchus nerka (Walbaum), in a large Colorado reservoir Description An account of the resource Ecologically and economically valuable Pacific salmon and trout (Oncorhynchus spp.) are widespread and susceptible to the ectoparasite Salmincola californiensis (Dana). The range of this freshwater copepod has expanded, and in 2015, S. californiensis was observed in Blue Mesa Reservoir, Colorado, USA, an important kokanee salmon (O. nerka, Walbaum) egg source for sustaining fisheries. Few S. californiensis were detected on kokanee salmon in 2016 (<10% prevalence; 2 adult S. californiensis maximum). By 2020, age-3 kokanee salmon had 100% S. californiensis prevalence and mean intensity exceeding 50 adult copepods. Year and kokanee salmon age/maturity (older/mature) were consistently identified as significant predictors of S. californiensis prevalence/intensity. There was evidence that S. californiensis spread rapidly, but their population growth was maximized at the initiation (the first 2–3 years) of the invasion. Gills and heads of kokanee salmon carried the highest S. californiensis loads. S. californiensis population growth appears to be slowing, but S. californiensis expansion occurred concomitant with myriad environmental/biological factors. These factors and inherent variance in S. californiensis count data may have obscured patterns that continued monitoring of parasite–host dynamics, when S. californiensis abundance is more stable, might reveal. The rapid proliferation of S. californiensis indicates that in 5 years a system can go from a light infestation to supporting hosts carrying hundreds of parasites, and concern remains about the sustainability of this kokanee salmon population. Creator An entity primarily responsible for making the resource Lepak, Jesse M. Hansen, Adam G. Hooten, Mevin B. Brauch, Daniel Vigil, Estevan M. Subject The topic of the resource Gill lice Intensity Invasion Maturity Prevalence Extent The size or duration of the resource. 10 pages Date Created Date of creation of the resource. 2021-09-28 Rights Information about rights held in and over the resource <a href="http://rightsstatements.org/vocab/InC-NC/1.0/" target="_blank" rel="noreferrer noopener">In Copyright - Non-Commercial Use Permitted</a> Format The file format, physical medium, or dimensions of the resource application/pdf Language A language of the resource English Is Part Of A related resource in which the described resource is physically or logically included. Journal of Fish Diseases Type The nature or genre of the resource Article