Bull. Eur. Ass. Fish Pathol., 28(4) 2008, 138 The distribution of Eustrongylides sp. (Nematoda: Dioctophymatoidea) in brown trout Salmo trutta L. in the River in southern

P. Haugen1, W. Hemmingsen1* and K. MacKenzie2

1Institute of Biology, Department of Ecology and Zoology, University of Tromsø, 9037 Tromsø, Norway; 2School of Biological Sciences (Zoology), University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK.

Abstract A total of 77 trout from the River Otra in in Norway was examined for the nematode Eustrongylides sp. This nematode was found primarily in the gut of fish, but some of the trout examined had the nematode encysted in its muscles. The objective of the survey was to do a sample of two trout populations semi separated by a barrier, in this case a large waterfall, to see differences and similarities in the distribution of Eustrongylides sp. larvae in the trout populations. A sub-sample of 11 larval nematodes from trout and minnow were all identified as Eustrongylides tubifex. The present study shows that Eustrongylides sp has a high prevalence in the two basins examined. The intensity of Eustrongylides sp shows a strong correlation to fish length and it is evident that trout in Otra accumulate Eustrongylides sp over time. There is no clear pattern between the sex of the trout and the mean intensity of Eustrongylides sp. There is a good correlation between trout length and the mean intensity of the parasite, and thus the age of the trout.

Introduction Kennedy & Lie (1976) found no relationship A total of 18 species of the nematode genus between the age of infected trout and intensity Eustrongylides has been reported infecting fish of Eustrongylides infection, and found mean worldwide (Kennedy & Lie, 1976; Moravec, parasite burdens to be higher in female than 1994). Moravec (1994) described six species in male trout. They further concluded that from fish in Europe: E. africanus, E. excisus, E. trout could only be infected in lakes and not ignotus, E. mergorum, E. papillosus and E. in streams, while Elnan (1995) showed by a tubifex, with E. excisus, E. mergorum and E. swimming endurance test that heavily tubifex considered to be the most common. infected trout lost much of their ability to Eustrongylides spp. uses aquatic oligochaetes tolerate fast streaming water, which indicated as first, and fish as second, intermediate hosts. that infected trout are less likely to be found Piscivorous birds serve as final hosts, except under such conditions. Elnan (1995) found for E. excisus, which can reproduce in sturgeon that the nematode could survive in trout for Acipenser sturio L. at least five years.

* First co-author’s E-mail: [email protected] Bull. Eur. Ass. Fish Pathol., 28(4) 2008, 139

In the River Otra in , brown trout Salmo trutta L. are heavily infected with Eustrongylides sp., found mostly in the visceral cavity of the fish (Elnan, 1995). The same author found four piscivorous birds infected with Eustrongylides: one goosander Mergus merganser, one grey heron Ardea cinera and two herring gulls Larus argentatus. The heron and the gulls were found dead and their deaths were caused by the migration of the Eustrongylides larva (Elnan, 1995).

The aim of the present study was to investigate the distribution of Eustrongylides sp. in brown trout in two parts of the River Otra separated by a physical barrier in the form of a large waterfall.

Materials and methods The River Otra flows through Aust- and Vest-Agder counties in southern Norway from Hovden in Setesdal to Kristiansand. It is 243 km long and has a catchment area of Figure 1. The study area in southern Norway showing the northernmost basin Harstad and the 2 3738 km . The river flow has been regulated southern basin Flåne. and the construction of small dams divides the river into a number of basins. Some of 10.5 to 52.0 mm. The nets were placed at these basins are large and slow-flowing and random in the two basins, left overnight, and so more like small lakes than sections of a lifted the following morning. All trout caught river. The basins sampled for the present were killed by a blow to the head and kept on study, Harstad and Flåne, are separated by a ice until examination in the laboratory. large waterfall, which acts as a physical Each fish was weighed and measured. barrier between them. Harstad is upstream Otoliths and scales were removed for age of Flåne, and the latter is larger but shallower determination. The visceral organs were than Harstad (Figure 1). Both basins have exposed by making a cut from around the large populations of both brown trout and anus to the gills, thus enabling the entire minnow Phoxinus phoxinus (L). viscera to be removed for examination under A total of 77 brown trout were caught using a dissecting microscope. Cysts in the an expanded Jensen series of nets three times musculature containing Eustrongylides could during July 2005. Each series consisted of 10 be felt by hand and observed by naked eye nets with different mesh sizes ranging from examination. Nematodes found in the Bull. Eur. Ass. Fish Pathol., 28(4) 2008, 140

No. of trout No. of trout Mean Mean intensity Location Prevalence (%) examined infected abundance (range) Flåne 46 46 100 15.1 15.1 (1-65)

Harstad 31 28 90.3 6.4 7.1 (1-26)

Table 1. Eustrongylides sp. infections at the two locations sampled. abdominal cavity and muscles were counted et al. (1997). Abundance data were compared separately. A sub sample of seven nematodes using ANCOVA, the dependent variable from trout, four from the abdominal cavity being log (n+1), n being number of parasites and three from the musculature, and four per host, and the independent variables being from the abdominal cavity of minnows, were fish length and sampling site. All statistical kept and stored in tubes of 70% ethanol for analyses were carried out using Systat specific identification. The nematodes were Statistical Package (Wilkinson, 1989). first cleared in a mixture of ethanol and glycerol until the ethanol evaporated Results completely. The anterior and posterior In the basin Flåne all fish caught were infected extremities were then removed and examined with the L-3 and L-4 stage larva from under a compound microscope at X200 – Eustrongylides sp. X400. Identification was according to Measures (1988) and Moravec (1994). In the basin Harstad the prevalence was found to be at 90,3%. The mean abundance in the Measures of infection used were prevalence, two basins differed with Harstad having abundance and intensity, as described in Bush lower total number of parasites (Table 1).

Figure 2. Correlation between trout length and the intensity of Eustrongylides sp infections. Bull. Eur. Ass. Fish Pathol., 28(4) 2008, 141

When the logarithm of the intensity of cation from sewage discharge can result in infection was plotted against the length of increased densities of fish that will attract the fish a correlation was found between these larger numbers of piscivorous birds, which two parameters (Figure 2). in turn increases the chances of infected birds contaminating the site. The difference in mean abundance between the two sexes varied in the two basins. In Flåne In both basins the mean infection of male trout had a higher mean abundance than Eustrongylides sp increases with the length of the female. In Harstad however mean the fish. In general larval nematodes show a abundance was higher in female fish, and this long lifespan in their hosts. Elnan (1995) basin has a lower infection in general. showed that Eustrongylides sp. could survive in trout for at least five years and Moravec The 11 nematodes kept for specific (1994) suggested that the main factor limiting identification were all identified as their survival time in fish is the life span of Eustrongylides tubifex (Nitzsch, 1819). the host. Rohde (1993) suggested very long life spans for nematode larvae in marine fish, Discussion and Polyansky (1955), Smith (1984) and Brown trout in both basins had high Hemmingsen et al. (1993) showed that prevalences of infection with Eustrongylides ascaridoid nematode larvae are cumulative sp. and infections seem cumulative with time. with age in marine fish. The results from this The high levels of infection in the basins may survey show that trout from the river Otra be at least partly the result of migration of accumulate larval Eustrongylides sp. with time heavily infected trout from areas of fast- (Figure 2). flowing water to the calmer basins, as Elnan (1995) reported reduced swimming ability in Although all individuals in the sample of 11 trout infected with more than five nematodes specifically identified were of E. Eustrongylides sp. larvae. Infections were tubifex, it is possible that more than one heavier in the lower basin Flåne than in species is present in the River Otra. This may Harstad. This difference may be due to also have been the case in the study location different host densities, of both fish of Kennedy & Lie (1976). There are varying intermediate and avian final hosts, in the two reports of the effects of Eustrongylides spp. on basins. Conditions for the oligochaete first both fish and birds (Moravec, 1994; Elnan, intermediate hosts may be made more 1995) and the effects may vary with nematode favourable downstream in Flåne by the species. In general, however, disease in wild release of treated sewage into the river at the populations is difficult to evaluate because the lower (southern) part of Harstad. Coyner et most heavily infected hosts are likely to be al. (2003) concluded that treated sewage could selectively predated upon before clinical result in an increased number of fish infected symptoms appear. with larvae of Eustrongylides ignotus at an Contradictory results were obtained between urban watershed in Florida. The eutrophi- the two basins in Eustronglides abundances in Bull. Eur. Ass. Fish Pathol., 28(4) 2008, 142 male and female fish, which is difficult to Bush A, Lafferty K, Lotz J & Shostak A (1997). Parasitology meets ecology on its own terms: explain. Differences in levels of parasitic Margolis et al. revisited. Journal of Parasitology infection between male and female hosts have 83, 575-583. been reported many times (Poulin, 1996a,b; Zuk & McKean, 1996). These studies have Coyner DF, Spalding MG & Forrester DJ (2003). Influence of treated sewage on shown that males, particularly in mammals, infections of Eustrongylides ignotus tend to have greater prevalences and (Nematoda: Dioctophymatoidea) in eastern intensities of parasitic infection than females. mosquitofish (Gambusia holbrooki) in an urban The few reports of sexual differences in watershed. Comparative Parasitology 70, 205- 210. parasitic infections in fish, however, show almost as many examples of female fish being Donnelly RE & Reynolds JD (1994). the more heavily parasitized (Thomas, 1964; Occurrence and distribution of the parasitic isopod Leptosphilus labrei on corkwing wrasse Paling, 1965; Williams, 1965; Borgström & (Crenilabrus melops) from Mulroy Bay, Ireland. Halvorsen, 1968; Kennedy, 1968; Borgström, Journal of Parasitology 80, 331-332. 1970; Donnelly & Reynolds, 1994). Elnan SD (1995). Aspects of the ecology of Eustrongylides sp. (Nematoda: Dioctophy- Acknowledgements matoidea), parasite of brown trout (Salmo community provided the economic trutta L.) and fish-eating birds. Thesis. Department of Animal Ecology, Institute of resources for this study. Thanks to Valle Zoology, University of Bergen, Norway, 38 pp. fisheries administration (VAFA) and Bø grunneigarlag for permission to collect fish Hemmingsen W, Lysne DA, Eidnes T & Skorping A (1993). The occurrence of larval from the River Otra, and to VAFA ascaridoid nematodes in wild-caught and in representative Torjus Uppstad for the loan of caged and artificially fed Atlantic cod. Gadus equipment for the field work. The help from morhua L., in Norwegian waters. Fisheries Tom Arild Homme in collecting and Research 15, 379-386. dissecting fish and with the use of equipment Kennedy CR (1968). Population biology of the was much appreciated. cestode Caryophyllaeus laticeps (Pallas, 1781) in dace, Leuciscus leuciscus L., of the River Avon. Journal of Parasitology 54, 538-543. References Borgström R (1970). Studies of the helminth Kennedy CR & Lie SF (1976). The distribution fauna of Norway. XVI. Triaenophorus nodulosus and pathogenicity of larvae of Eustrongylides (Pallas, 1760) (cestoda) in Bogstad Lake. III. (Nematoda) in brown trout Salmo trutta L. in Occurrence in pike, Esox lucius L. Nytt magasin Fernworthy Reservoir, Devon. Journal of Fish for Zoologi 18, 209-216. Biology 8, 293-302.

Borgström R & Halvorsen O (1968). Studies Measures LN (1988). Epizootiology, of the helminth fauna of Norway. XI. pathology and description of Eustrongylides Caryophyllaeides fennica (Schneider) (Cestoda: tubifex (Nematoda: Dioctophymatoidea) in Caryphyllidea) in Lake Bogstad. Nytt magasin fish. Canadian Journal of Zoology 66, 2212-2222. for Zoologi 16, 20-23. Moravec F (1994). Parasitic nematodes of freshwater fishes of Europe. Kluwer Academic Publishers, Dordrecht, 473 pp. Bull. Eur. Ass. Fish Pathol., 28(4) 2008, 143

Paling JE (1965). The population dynamics of Smith JW (1984). Anisakis simplex (Rudolphi, the monogenean gill parasite Discocotyle 1809 det. Krabbe, 1878): length distribution sagittata Leuckart on Windermere trout, Salmo and variability of known minimum age from trutta L. Parasitology 55, 667-694. herring Clupea harengus L. Journal of Helminthology 58, 337-340. Polyansky YuL (1955). Parasites of the fish of the Barents Sea. Zoologischeskogo Instituta Thomas JD (1964). Studies on the growth of Akademiya Nauk SSSR, Moscow-Leningrad trout, Salmo trutta, rom four contrasting (English translation 1966 by Israel Program habitats. Proceedings of the Zoological Society of for Scientific Translations, 158 pp). London 142, 459-509.

Poulin R. (1996a). Sexual inequalities in Wilkinson L (1989). Systat: The system for helminth infections: a cost of being a male? statistics. SYSTAT, Evanson, Illinois, 638 pp. American Naturalist 147, 287-295. Williams HH (1965). Observations on the Poulin, R (1996b). Helminth growth in occurrence of Dictyocotyle coeliaca and vertebrate hosts: does host sex matter? Calicotyle kroyeri (Trematoda: Monogenea). International Journal for Parasitology 26, 1311- Parasitology 55, 201-207. 1315. Zuk M & McKean KA (1996). Sex differences Rohde K (1993). Ecology of marine parasites. in parasite infections: patterns and processes. An introduction to marine parasitology, 2nd International Journal for Parasitology 26, 1009- Edition. CAB International, Wallingford, UK, 1024. 298 pp.