Seasonality of salmonid parasites from flow‐through aquaculture in northern Germany: Emphasis on pathogenicity of Diplostomum spp. metacercaria

Abstract

The aim of the present study was to identify the parasite fauna of cultured rainbow trout (Oncorhynchus mykiss Walbaum, 1792) and Arctic charr (Salvelinus alpinus (L.)) from a flow‐through aquaculture system at Lake Tollense in northern Germany. The fish were sampled during different seasons in 2018 from an open freshwater raceway. For rainbow trout, 10 different parasite species were isolated, belonging to Monogenea (1), Cestoda (2), Crustacea (2) and Digenea (5), including the eye flukes Tylodelphys clavata Niewiadomska, 1984, Diplostomum pseudospathaceum and Diplostomum spathaceum (Rudolphi, 1819) Olsson, 1876, the latter representing a potential pathogenic species, which was molecularly identified and reported as a core species with prevalences of 80.0%–100.0% and high mean abundances of 42.8–55.0. Arctic charr only harboured T. clavata and D. spathaceum. Other taxa were found with low prevalence in the summer season exclusively, indicating strong seasonality of the detected parasites. Statistical correlation was positively tested for coherences of eye fluke burden and fish growth, weight and performance between seasons, recommending implementation of an enhanced aquaculture management for the surface water–dependent flow‐through aquaculture system at Lake Tollense.

INTRODUCTION

Freshwater fish represent two-thirds (50.4 million tons in 2018) of the world aquaculture production (FAO 2021). In Europe, 0.51 million tons were produced in freshwater, with Germany contributing only 18,252 tons (FAO 2021). Rainbow trout (7852 to) and Arctic charr (1700 to) contributed 9552 tons or 54% to the German aquaculture production, most commonly produced in flow-through raceway systems or in fish ponds (Rosenthal & Hilge, 2000). According to Shinn et al. (2015), parasites are responsible for annual costs in hatchery and grow out aquaculture estimated at USD $1.05–$9.58 billion. Thus, diseases and parasites are a major obstacle for the future sustainable development of the aquaculture industry.

Besides bacterial (e.g. furunculosis), virus (e.g. VHS, ISAV, IHNV) or fungal (e.g. saprolegniose) diseases, fish parasites are a major threat to fish health in European freshwater aquaculture (Paperna, 1991). Initial parasitic infections are often the entry point for secondary infections with bacteria (Bürger et al., 2006). Aquatic fish parasitology is one of the most important but often neglected research areas in aquatic ecology, because fish parasites link the different components of the aquatic food web. Parasites and pathogens can easily spread within the aquatic ecosystems. Especially open aquaculture systems, such as net cages or flow-through raceways, provide risks of parasite exchange with wild populations. These transmissions may occur as spillover from cultured to wild fish via introduced infected fish or vice versa as spillback from wild to cultured fish (Bouwmeester et al., 2021).

Relevant parasite taxa belong to the Protozoa, Trematoda, Cestoda, Nematoda, Acanthocephala and Crustacea, with most severe disease outbreaks associated with infections of ectoparasites (Protozoa and Crustacea with direct life cycles) (Bouwmeester et al., 2021). In salmonids, Buchmann and Bresciani (1997) reported a total of 12 protozoan and 10 metazoan species of parasites from inland pond-reared rainbow trout in Denmark. Among these, the trematode eye flukes belonging to the genus Diplostomum von Nordmann, 1832 caused exophthalmos and blindness in trout and are important salmonid fish parasites. Diplostomum spathaceum (Rudolphi, 1819) Olson 1876 causes pathogenic effects to their fish hosts as well as manipulates their behaviour (Palm et al., 2018). They are common endoparasites of fish-eating birds as final hosts with a cosmopolitan distribution in freshwater, especially of the Nearctic and Palearctic regions (Shigin, 1986, 1993). Diplostomum commonly uses freshwater snails as first and finfish as second intermediate hosts (Niewiadomska, 2002). Especially fishes from the littoral region, where the snail intermediate hosts are highly abundant, are getting parasitised (Pikalov, 2017).

Within a recent population data set study from 2016, D. spathaceum, Posthodiplostomum cuticula Nordmann, 1832 and Tylodelphys clavata have shown to be the most frequently recorded trematodes in European freshwater systems (Faltýnková et al., 2016). Previous records of freshwater fish parasites from 10 wild living fish species in Mecklenburg-Western Pomerania, north-eastern Germany were summarised by Pikalov (2017).

However, knowledge on the parasite community in Lake Tollense (53° 30′ N, 13° 13′ O) is relatively scarce. Wysujack et al. (2014) studied European eels for the presence of Anguillicola crassus Kuwahara, Niimi and Hagaki, 1974 from numerous water bodies, rivers and coastal waters, including Lake Tollense. A recent study of Suthar et al. (2021) analysed the parasite fauna of bream, Eurasian perch and roach in Lake Tollense in order to evaluate the parasite community and to compare them to other freshwater systems in the state. Altogether, 32 parasite species were found by Suthar et al. (2021) including seven species which were also recorded in this study (D. spathaceum, T. clavata, Tylodelphys podicipina, Ichthyocotylurus variegatus, Triaenophorus nodulosus, Argulus foliaceus and Ergasilus sieboldi). The aim of the present study was to document the parasite infections of cultured salmonids in an open flow-through system in Mecklenburg-Western Pomerania that utilises surface water from Lake Tollense and to enlighten the interaction of cultured and free-living fish. Special emphasis is given to the identification of pathogenic parasites and their seasonal variation during 2018.

MATERIAL AND METHODS

Sample collection and examination

In spring and summer 2018 (April and July), specimens of Oncorhynchus mykiss Walbaum, 1792 (n = 64) and Salvelinus alpinus (L. 1758) (n = 10) were taken from raceways of an aquaculture system in Neubrandenburg, Mecklenburg-Western Pomerania (see Figure 1). Salmonids were kept with an average stocking density of 25 kg/m³ and no anti-helminth treatments were given. The water reaches the farm through the Ölmühlenbach and originates from the Lake Tollense. Additional 42 specimens of O. mykiss were studied in November 2018. Fish were caught from two raceways during sampling and killed immediately after catch. Rainbow trout and Arctic charr have been bred from eggs at the same facility in January 2017. Smears of the gills from freshly dead fishes were taken and fish where transported to the laboratory at University of Rostock on ice and frozen at –20°C for subsequent parasitological examination. Morphometrically data including the total length (TL), standard length (SL), total weight (TW) and gutted weight (GW) were recorded to the nearest 0.1 cm and 0.1 g.

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Parasitological examination

Standardised parasitological examination followed Palm and Bray (2014). First, skin, fins, eyes, gills, nostrils and buccal cavity of each fish were examined for ectoparasites. Afterwards, the body cavity was opened and examined for parasites that moved freely in the body cavity (rinsed with water) or were attached. The liver, stomach, intestine, kidney, gall bladder, spleen, swim bladder, pyloric caeca and gonads were examined microscopically, followed by an examination of the musculature (on candling table). The contents of the stomach, intestine and pyloric caeca were further checked by using the ‘gut-wash’ method, described in Cribb and Bray (2010). Most isolated parasites were cleaned and fixed in 4% borax-buffered formalin and preserved in 70% ethanol. Digenea and Acanthocephala were permanently mounted in 98% glycerine surrounded by paraffin. Subsamples of the parasites were stained with acetic carmine, dehydrated and mounted in Canada balsam. Parasite identification literature included original descriptions.

Collection of molecular data

The Cytochrome c oxidase subunit I (Cox1) of mitochondrial genes was analysed by extracting the DNA of metacercaria of Diplostomum spp. that were fixed and stored in absolute ethanol. The DNA was extracted (Qiagen Blood and Tissue Kit) and the PCR was run using diplostomid-specific primers Plat-diploCOX1F (5′-CGT TTR AAT TAT ACG GAT CC-3′ (forward)) and reverse Plat-diploCOX1R (5′-AGC ATA GTA ATM GCA GCA GC-3′ (reverse)), designed by Moszczynska et al. (2009). Following settings were applied: denaturation at 94°C for 2 min followed by 35 cycles (94°C for 30 s, 50°C for 30 s, 72°C for 60 s) and a final extension step at 72°C for 10 min. PCR-Mastermix had a volume of 45 μl, including 3 μl of primers and 5 μl of DNA each. Successful amplification was checked by gel electrophoresis, after which the purification of samples was done (Qiagen Purification Kit). For Sanger sequencing, samples were sent to Microsynth Seqlab, Göttingen, Germany.

Molecular data analysis

As we found more than 7000 specimens of Diplostomum spp., we selected a subsample of 20 specimens per season and fish species according to Jovani and Tella (2006). We chose one to five parasite specimens per fish to check for potential parallel infection with several Diplostomum species. The resulting sequences were subjected to nucleotide BLAST searches to find the most closely matched sequences available in GenBank.

Parasitological and ecological parameters

The parasitological and ecological terms including prevalence (P), mean intensity (mI), intensity (I) and mean abundance (mA) followed Bush et al. (1997). The prevalence of the different parasites was rated after Holmes (1991) into core species (>60%), secondary species (40%–60%), satellite species (5%–40%) and rare species (<5%). Additionally, the Shannon–Wiener Index of species diversity (Shannon, 1948), Evenness Index of Pielou (Pielou 1966) and the Berger–Parker Index (Berger & Parker, 1970) were calculated.

Statistics and calculations

We made use of the DMU package for multivariate mixed models (Madsen & Jensen, 2013) using a restricted maximum likelihood method (REML) to study the influence of Diplostomum spp. on the weight gain of the fish. We applied the DMU software with a Newton–Fisher REML estimation to derive the regression of total weight (on y-axis) against increasing parasite counts (on x-axis) for a likelihood function with Season (1 = spring, 2 = summer, 3 = autumn) as a fixed class variable. The slope of regression can be used as a quantifier for fish tolerance against Diplostomum spp. (Kuukka-Anttila et al., 2020; Kause, 2011; Simms, 2000). We interpret the intercept of regression as a projection of the data to the ideal parasite-free environment. We also derived the ordinary least square (OLS) regression of weight (on y-axis) against increasing number of parasites (on x-axis) for each season separately to study the seasonality of the effect of Diplostomum spp. on the weight gain. In case of independent samples, Mann–Whitney U-test was done to test seasonal variations of parasite burdens within the examined fish species.

RESULTS

Sampled fish

The total length of the examined rainbow trout ranged between 20.0 and 29.5 cm with a mean of 24.6 ± 2.5 cm (± SD) in spring, 20.0 and 31.5 cm with a mean of 26.4 ± 2.1 cm in summer and 18.0 and 30.0 cm with a mean of 23.9 ± 3.5 cm in autumn. The total length of the examined Arctic charr ranged between 28.0 and 35.0 cm with a mean of 31.2 ± 2.1 cm in spring and 22.0 and 25.0 cm with a mean of 24.0 ± 0.94 cm during summer sampling (find detailed information in the supplementary data).

Parasite fauna of rainbow trout

The parasite fauna of rainbow trout differed throughout the year. In the spring sample, four different parasite species were detected, including molecular records of Diplostomum spathaceum (n = 19) and D. pseudospathaceum Niewiadomska, 1984 (n = 1). The detected trematode larvae Diplostomum spp. (P = 100%) and Tylodelphys clavata Niewiadomska, 1984 (P = 60.9) represented a core species according to Holmes (1991). Additionally, the Monogenea Gyrodactylus sp. was found with a prevalence of 9.4% on the gills of trout (only hooks were found; see Table 1).

TABLE 1 Overview of the detected parasites in trout and charr during the three seasons (spring summer and autumn). Larval (l) and adult (a) developmental stages of different parasites were found

		Spring sampling 2018	Summer sampling 2018	Autumn sampling 2018
		Oncorhynchus mykiss, n = 64	Salvelinus alpinus, n = 10*	Oncorhynchus mykiss, n = 64	Salvelinus alpinus, n = 10*	Oncorhynchus mykiss, n = 42
Parasite species	Stage	P (%)	mI (I)	mA	P (%)	mI (I)	mA	P (%)	mI (I)	mA	P (%)	mI (I)	mA	P (%)	mI (I)	mA
Monogenea
Gyrodactylus sp.	a	9.4	1.5 (1–3)	0.1	–	–	–	3.1	1.5 (1–2)	0.1	–	–	–	–	–	–
Digenea
Diplostomum spp.	l	100.0	47.3 (1–128)	47.3	90.0	14.6 (3–43)	11.7	96.9	50.0 (9–125)	47.3	80.0	1.8 (1–4)	1.4	92.9	35.3 (8–91)	32.7
Ichthyocotylurus variegatus	l	–	–	–	–	–	–	1.6	3.0 (3)	0.1	–	–	–	–	–	–
Tylodelphys clavata	l	60.9	3.4 (1–11)	2.1	40.0	2.0 (1–2)	0.4	28.1	4.6 (1–19)	1.3	10.0	1.0 (1)	0.1	73.8	1.8 (1–15)	1.3
Tylodelphys podicipina	l	–	–	–	–	–	–	1.6	1.0 (1)	0.1	–	–	–	1.6	1.0 (1)	0.1
Cestoda
Eubothrium crassum	a	–	–	–	–	–	–	1.6	1.0 (1)	0.1	–	–	–	–	–	–
Triaenophorus nodulosus	l	–	–	–	–	–	–	1.6	1.0 (1)	0.1	–	–	–	–	–	–
Crustacea
Argulus foliaceus	a	–	–	–	–	–	–	21.9	1.4 (1–4)	0.1	–	–	–	–	–	–
Ergasilus sieboldi	a	–	–	–	–	–	–	3.1	1.0 (1)	0.1	–	–	–	–	–	–
Shannon–Wiener Diversity Index	H	0.19			0.15			0.18			0.25			0.18
Eveness of Pielou	E	0.14			0.21			0.08			0.35			0.13
Berger–Parker Index	BP	0.96			0.97			0.97			0.93			0.96

During summer, the number of parasite taxa was significantly higher, because nine species could be detected. Beside T. clavata, only D. spathaceum (n = 20) was molecularly identified in the summer subsample. Thus, D. spathaceum. represented a core species with P = 96.9%, though additional infection with other Diplostomum species such as D. pseudospathaceum cannot be excluded. All other parasites were found with a low or moderate prevalence of infection. Two crustacean parasites, A. foliaceus (L., 1758) and Ergasilus sieboldi sieboldi (Nordmann, 1832), were found parasitising the gills of the rainbow trout. In the case of the detected cestodes (Eubothrium crassum (Bloch, 1779) Nybelin, 1922, Triaenophorus nodulosus (Pallas, 1781) Rudolphi, 1793) and the larval trematode Ichthyocotylurus variegatus (Creplin, 1825) Odening, 1969, only single specimens were found inside the studied aquaculture farm (P = 1.6%).

In the autumn sampling, parasite diversity was similar to the spring sample, consisting of the four species D. spathaceum, D. pseudospathaceum, T. clavata and T. podicipina Kozicka and Niewiadomska, 1960. The prevalence and intensity of the most abundant parasite species appeared comparable to those in both spring and summer seasons. Only the prevalence of T. clavata was higher, representing a core species with a prevalence of 73.8%.

Parasite fauna of Arctic charr

In spring and summer 2018, two species of Digenea were found in Arctic charr. The eye fluke, Diplostomum sp., was frequently recorded in the eyes of S. alpinus in both seasons with a prevalence of 80% in spring and 90% in summer. A subsample from each season was molecularly identified as D. spathaceum (spring, n = 7; summer, n = 1). The mean intensity of Diplostomum sp. was significantly higher (Mann–Whitney U-test; p = 0.01) in spring (mI = 14.6) compared with summer (mI = 1.8). According to the low sampling size, the mean Abundance and (mean) Intensity may be biased. Tylodelphys clavata was found inside the eyes of Arctic charr with a prevalence of 40% in spring and 10% in summer.

Molecular identification of diplostomids

Overall, 60 sample identifications were performed for diplostomids from O. mykiss, of which 58 were D. spathaceum and two specimens belonged to D. pseudospathaceum (see Table 2). For Arctic charr, only eight parasite sequences were obtained from both the spring and autumn samples. All obtained parasitic sequences were identified as D. spathaceum. The sequence data of both D. pseudospathaceum and of a single specimen of D. spathaceum from each season and fish species were deposited in GenBank (Accession numbers MW549874–MW549876 (D. spathaceum), MW558116 and MW558117 (D. pseudospathaceum) for rainbow trout and MW559541 and MW559542 for Arctic charr (D. spathaceum)).

TABLE 2 Results of molecular identification of Diplostomum spp

	O. mykiss Spring(n = 20)	O. mykiss Summer(n = 20)	O. mykiss Autumn(n = 20)	S. alpinus Spring(n = 7)	S. alpinus Summer(n = 1)
D. spathaceum	19 (95%)	20 (100%)	19 (95%)	7 (100%)	1 (100%)
D. pseudospathaceum	1 (5%)	0 (0%)	1 (5%)	0 (0%)	0 (0%)

Ecological parameters

Shannon–Wiener Diversity Index and Evenness Index of Pielou were very low (maxima of 0.25 and 0.35, respectively) at all sampling seasons, indicating a low diversity with Diplostomum spp. predominating the parasite community of trout and charr inside the aquaculture farm. Uniformly high values of Berger–Parker Index (<0.93) likewise indicate the strong predominance of Diplostomum spp. within the parasite communities of both fish and seasons.

Statistics and calculations

For the OLS method, the slope of regression of total weight (on y-axis) against increasing parasite count (on x-axis) in spring (0.015 ± 0.29) was near zero; in summer (–0.54 ± 0.29) and autumn (–0.73 ± 0.74), the slope decreased and turned negative. Thus, a negative correlation between Diplostomum infection and the total weight of the rainbow trout was observed in the summer and autumn samples of rainbow trout, showing an intensifying trend with fish weight. The slopes of regression, the intercepts of regression and the weight means for all seasons are summarised in Table 3 and the data points with the interpolating regression lines are displayed in Figure 2a–c. The restricted maximum likelihood method for the likelihood function with season as a fixed class variable applied on the whole data yields one common negative slope of regression (–0.30 ± 0.22) and separate intercepts of regression for each season (spring: 229.63 ± 14.23, summer: 253.83 ± 14.17, autumn: 233.48 ± 13.75) which surpass the corresponding weight means (spring: 214.97, summer: 239.29, autumn: 223.69). The values are given in Table 4 and all data points with the three resulting regression lines (spring in red, summer in black, autumn in blue) are displayed Figure 2d.

TABLE 3 Correlation of total fish weight in gram (g) and number of Diplostomum spp. of trout in three different seasons (spring, summer and autumn) with calculated standard error (±SE) for slope of regression (regr.) and intercept of regression (regr.)

	Spring	Summer	Autumn
Slope of regr. (±SE)	0.015 ± 0.29	–0.54 ± 0.29	–0.73 ± 0.74
Intercept of regr. (±SE)	214.25 ± 16.80	265.16 ± 16.29	247.40 ± 28.56
Weight mean (g)	214.97	239.29	223.69
Sample size	64	64	42

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TABLE 4 Correlation of total fish weight in gram (g) and number of Diplostomum spp. of trout in three different seasons (spring, summer and autumn) with calculated standard error (±SE) for slope of regression (regr.) and intercept of regression (regr.) using a restricted maximum likelihood method for the likelihood function with Season as a fixed class variable

	Spring	Summer	Autumn
Slope of regr.	–0.30 ± 0.22
Intercept of regr. (±SE)	229.63 ± 14.23	253.83 ± 14.17	233.48 ± 13.75

DISCUSSION

The aim of the present study was to analyse the parasite fauna of rainbow trout and Arctic charr from a flow-through aquaculture system in northern Germany during 2018, displaying seasonal patterns and the occurrence of potentially harmful and/or pathogenic species. For rainbow trout, the parasite diversity was highest in summer (nine taxa), whereas in spring and autumn only four parasite species were found. In Arctic charr, only two parasite taxa were recorded each in spring and summer seasons. The parasites originated from Lake Tollense because the aquaculture farm is using surface water through an inflowing stream (Ölmühlenbach), and most of the recorded parasites have been reported also from free-living fish from Lake Tollense by Suthar et al. (2021). Although the cestodes Triaenophorus nodulosus and Eubothrium crassum either enter the farm through the copepod first intermediate or the infected teleosts as second intermediate hosts (e.g. perch), the metacercariae of the bird infecting diplostomids and of Ichthyocotylurus variegatus can infect the salmonids through the cercariae released from the snail first intermediate hosts. Suthar et al. (2021) recorded a T. nodulosus infection in wild perch from Lake Tollense and E. crassum was recorded from a brown trout April 2019 (own observation), demonstrating a reverse-spillover infection from the wild into the farm (Daszak et al., 2000). The bottom of the inflowing stream is covered with different snail species, for example Viviparus viviparus (L., 1758) and Radix auricularia (L., 1758) were observed (present study). A water flow-through of average 650l/s. rapidly washes the parasite larvae through the system, significantly increasing the exposure of the fish to cercariae and explaining the high prevalence of infections and accumulation during the summer.

In comparison with the studied wild fish populations, especially of the cyprinids and perch from the neighbouring Lake Tollense (Suthar et al. 2021), the infection level inside the aquaculture farm was rather low. However, this does not refer to the diplostomids that were highly abundant in the sampled salmonids. Suthar et al. (2021) recorded D. spathaceum exclusively in roach and bream from Lake Tollense, whereas Diplostomum baeri Dubois, 1937 was only found in perch. Because perch as a widespread predator in natural habitats was observed to enter the flow-through aquaculture system at a small size, it was surprising not to find any D. baeri inside the farm. This diplostomid was earlier recorded to infect salmonids by Landeryou et al. (2020) and Unger & Palm (2017). Consequently, the snail intermediate hosts of D. baeri might be less abundant in the inlet and the surroundings of the farm (the shallow outflow river Ölmühlenbach from Lake Tollense that supplies the aquaculture farm), most probably preventing its transmission.

We did not find any zoonotic fish parasites that might get transferred to humans through the aquaculture fish. According to Santos et al. (2017), several fish borne zoonotic parasites have been recorded in various fish species from different habitats and countries. For example, in wild rainbow trout, cases of infections with larval Dibothriocephalus dendriticus (Nitzsch, 1824) have been observed in many freshwater habitats in northern Europe and northern America, suggesting its potential to invade aquaculture. Dibothriocephalus dendriticus causes the human disease diphyllobothriasis, which is considered to be a mild disease, infected people being symptomless or showing diarrhoea, abdominal pain and anaemia (Scholz et al., 2009). Fish parasites can be also problematic for their teleost hosts especially under culture conditions and if they cause pathogenic effects. Infestations of the gills with herewith recorded monogenean Gyrodactylus sp. and crustacean A. foliaceus ectoparasites bear high risk for salmonid fish, especially under highly dense culture conditions and high intensities of infection. However, both ectoparasitic species were only found in low numbers in summer as earlier described by Hakalahti and Valtonen (2003) and Gault et al. (2002) for Argulus coregoni Thorell, 1866 and A. foliaceus in rainbow trout from a finish inland fish farm and a water reservoir in Ireland, which is due to the life cycle of lice, as the females detach from fish host to lay eggs in summer. Consequently, both species caused no major risk for the sampled aquaculture farm at Lake Tollense during 2018.

Diplostomum spp. infections have been reported from aquaculture farms around the world, including industrial as well as developing countries, for instance in Russia (Shigin, 1980), the United States (in channel catfish) (Overstreet & Curran, 2004), England (Betterton, 1974), Turkey (Avsever et al., 2016), Nigeria (Ibrahim et al., 2016) and Argentina (Semenas, 1998). Eye flukes infect several fish species but mainly salmonids like rainbow trout are susceptible to these parasites (Speed & Pauley, 1984; Voutilainen et al., 2009). Similar to Kuukka-Anttila et al. (2020) who did not find a correlation between the number of parasites and weight gain in small trout, the younger fish showed no significant weight loss with increasing Diplostomum infection within the present study (spring sample, regr. slope 0.0123 ± 0.292 (±SE), regr. intercept 214.2 ± 16.8, weight mean 215.0; see Figure 2). However, in summer and autumn with increasing fish age we found a negative correlation between eye fluke infections and weight gain (summer sample, –0.535 ± 0.293, 265.2 ± 16.3, 239.3, autumn sample, –0.729 ± 0.744, 247.4 ± 28.6, 223.7; see Figure 2). This negative correlation was strongest in the autumn sample, demonstrating an accumulation of this effect in older fish and throughout the year. According to Buchmann and Uldal (1994) and Kuukka-Anttila et al. (2020), rainbow trout with a lower resistance to parasites (with higher metacercaria infection rates) had a lower body weight. However, the rainbow trout might also accumulate the eye flukes over time and independent of differences in resistance, getting exposed to a constant infection rate with the cercariae throughout the year. In both cases, Diplostomum spp. negatively correlates with fish weight gain and the productivity of the aquaculture farm is reduced. The number of available cercaria might even be influenced by the surface water temperature in the inlet, a possible explanation for the stable infection in summer and autumn. In both cases, diplostomid infection negatively correlates with fish weight gain.

Severe diplostomid infections cause the diplostomiasis, where the metacercariae disrupt the lens of the fish as well as its structure and therefore inducing cataract to its host (Avsever et al., 2016; Chappell, Hardie, & Secombes, 1994; Palm et al., 2018; Seppälä, 2011). This restricted vision enhances the chance of getting eaten by the fish-eating bird final hosts (Palm et al., 2018) such as gulls. This results in changed behaviour of escape reactions because the bird as predator is not fully recognised anymore (Kuhn et al., 2015; Palm et al., 2018). Also, the recognition of surrounding brightness and darkness of the infected fish is impaired and leads to a reduced camouflage ability (Palm et al., 2018). Under aquaculture conditions and without exposure to predatory birds, the number of diplostomids accumulates and reduces the visual sense of the fish. Under high stocking densities such as inside the sampled farm with 25–60 kg/m³ and with a water flow through of 0.65 m³/s, the infected fish have more difficulties to compete for pellet feed. This results in reduced weight gain and negatively affects the efficiency of the aquaculture production (Chappell, 1995).

Mortalities of fish associated with heavy infection of eye flukes have been reported from common suckers (Catostomus commersonii (Lacepède, 1803)) and hog-nosed suckers (Hypentelium nigricans (Lesueur, 1817)) (Ferguson & Hayford, 1941) and in experimentally infected fish (Brassard, Rau, & Curtis, 1982). The cercaria enter the fish and migrate through the subcutaneous connective tissue and muscles into the eye lenses (Ratanarat-Brockelman, 1974). The migration distance is limited by glycogen reserves and secretions of the penetration glands. The maximum distance is given with approximately 10 cm by Ratanarat-Brockelman (1974). As the trematode cercaria are released from snails into the water column from spring to autumn when water temperature is above 10°C (Hakalahti, Karvonen, & Valtonen, 2006; Karvonen, Seppälä, & Valtonen, 2004) and it is evident that smaller or younger fish are more susceptible for infections (Ratanarat-Brockelman, 1974), distinct aquaculture management suggestions can be made. First, indoor hatchery and juvenile RAS systems that use water from a well and not from surface waters are highly recommended for minimising diplostomid infections in small fish. Secondly, a colder temperature regime can reduce the prevalence of Diplostomum spp. because the cercaria are only released above 10°C (Kuukka-Antilla et al., 2020). However, this would limit the potential growth of the trout. Larsen et al. (2005) suggested that installing mechanical filters (32 μm mesh size) or the usage of sodium percarbonate (≥20 mg/l) may improve animal welfare and reduces parasite load. However, this is not applicable under the observed flow-through rates and in a natural environment. Finally, a better hygiene and management of the inlet environment will be essential to control the snail population and its species composition.

CONCLUSION

In summary, the studied rainbow trout and Arctic charr from the flow-through aquaculture system in northern Germany were moderately infected with a diversity of parasites, originating from the nearby Lake Tollense. Depending on their life cycles, especially diplostomids were highly abundant inside the trout with the highest infection level in summer. Zoonotic parasites were not recorded, indicating no risk for the human consumers. The eye fluke infection correlated with a weight loss of the infected trout resulting from the impaired visual sense and competition for pellet feed under high stocking densities. Possible management issues include a better parasite–host control in the inlet and water management in the studied aquaculture farm.

ACKNOWLEDGEMENTS

We are grateful to the operators of the accompanied freshwater facility in Neubrandenburg, Mecklenburg-Western Pomerania. The present study was funded by the European Maritime and Fisheries Fund (EMFF) and the Ministry of Agriculture and Environment, Mecklenburg-Western Pomerania as a part of the Project Hygiene management and health concept for surface water-dependent partial circulation systems in MV (MV-II. 12-LM-03).

Open access funding enabled and organized by Projekt DEAL.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ETHICS

The Research was conducted under collaboration between University Rostock, Aquaculture and Sea-Ranching and Forellenzucht Uhthoff Neubrandenburg.

AUTHOR CONTRIBUTIONS

P.U. contributed in research idea, sampling organisation, lab work and analysis for parasite identification and writing, correction, editing and improvement of the manuscript. J.S. helped in partly writing, correction, editing and improvement of the manuscript. F.B. was involved in statistics and calculations and partly writing and correction of the manuscript. X.N.-D. helped in lab work and analysis for parasite identification and writing, correction, editing and improvement of the manuscript. S.K. contributed in overall project and research idea and partly writing, correction, editing and improvement of manuscript. H.W.P. contributed in overall project and research idea, providing laboratory space materials and equipment and partly writing, correction, editing and improvement of the manuscript.

DATA AVAILABILITY STATEMENT

Supplementary data are available at zenodo.org (10.5281/zenodo.5220908).

References

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