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A moderate raise in temperature was suggested to enhance the impact of parasites on aquatic ecosystems. Under higher temperatures, poikilothermic animals (e.g. fish) increase their activity, which can result in a more frequent encounter with parasites. However, temperature increase may also trigger processes counteracting an increased risk of parasitic infections. Thus, the removal of free‐living stages of parasites by filter‐feeding organisms can increase with temperature and potentially mitigate disease risk in ecosystems under climate change. We aimed to study whether an increased infection transmission under higher temperatures can be compensated by the increased removal of parasitic larvae by aquatic predators. In addition, we planned to reveal the behavioral mechanism underlying the more successful transmission of the parasite at higher temperatures. We studied experimentally how temperature, the behavior of fish hosts (rainbow trout), and the presence of filter‐feeding mussels in the environment influence the infection success of trematode larvae Diplostomum pseudospathaceum cercariae. We found that temperature raise increased while the presence of filter‐feeding mussels in the environment decreased infection intensities in fish. However, the effect of mussel's presence was constant within the tested range of water temperatures (15–23°C), which suggests that it cannot compensate for the observed increased transmission of parasites under temperature raise. Fish activity before the exposure to parasites was a substantial factor affecting the host's vulnerability to infection. However, fish activity only weakly correlated with temperature, therefore, it is unlikely to be the only factor responsible for the increased infection success under warmer conditions. After the exposure, fish activity decreased and did not influence the infection's success. This decrease was temperature‐dependent and more pronounced in more active fish. In general, we showed that the elimination of trematode larvae by filter‐feeders is unlikely to deter the potential effects of global warming on host–parasite interactions in temperate freshwater ecosystems. peerReviewed

Influence of fish personality on infection rate is almost not studied. In the experiments on the young-of-the-year Oncorhynchus mykiss and cercariae of a trematode Diplostomum pseudospathaceum we tested the hypothesis that infection rate differs between more and less active (“bold” and “shy”) fish. Will individual differences in infection persist upon re-infection? Fish serve as a second intermediate host for this trematode. A positive correlation was found between the results of consecutive infections. Accumulation of parasites with successive infections leads to an aggregated distribution of D. pseudospathaceum among the hosts, affecting individual fitness and polymorphism in fish populations. Persistent individual differences in parasite burden among fish and, as a result, vulnerability for predators confirms the role of parasites as an important factor of natural selection. peerReviewed

Fish parasites may potentially harm fisheries and aquaculture. Infected fish are unusable, even though they would not pose a direct risk to human health. The aim of this thesis was to investigate the ecology and epidemiology of three previously unknown or poorly known spore-forming parasites infecting muscles of economically important fish species. The first novel microsporidian species, here described as Microsporidium luciopercae, was found from pike-perch (Sander lucioperca) and European perch (Perca fluviatilis) and formed opaque-looking patches in the muscles. The second novel microsporidian species, Myosporidium spraguei, was found from pike-perch and burbot (Lota lota) and occurred within xenomas in the musculature. The third muscle-dwelling parasite, Henneguya zschokkei (Cnidaria), formed white plasmodia in the muscle of whitefish (Coregonus lavaretus). Among the six study lakes, the prevalence of M. luciopercae in wild pike-perch and perch varied from 0 to 1 %, while M. spraguei was more common (prevalence of 5–26 % in pike-perch and 65 % in burbot). M. spraguei was strongly aggregated and most abundant in the middle-sized (37–45 cm) pike-perch and in larger burbot (> 35 cm), but the infection was not related to host sex, sampling season or host condition. In the case of H. zschokkei, in two inland fish farms, one year-old whitefish were virtually uninfected, but the prevalence of infection was higher in 2 and 3 year old fish (up to 36 % of 3 year old fish were infected). New infections appeared seasonally in July-August. Neither the size of plasmodia nor the proportion of different plasmodium types changed from 2 to 3 year old fish, suggesting slow development of H. zschokkei in whitefish. However, the proportion of infected fish with melanised plasmodia increased with fish age, suggesting an age-dependent host response against this strongly aggregated parasite.

Projektissa tehtiin kartoitus Suomessa tehdystä Gyrodactylus salaris – lohiloisen vastustyöstä. 80-luvun puolivälistä asti on pohjoisia Atlantin lohen jokiamme, Tenoa ja Näätämöä, suojeltu elävän kalan siirtokielloin ja suosituksin tai säädöksin kalastusvälineistön kuivaamisesta tai desinfioinnista. Suomen integroituminen Euroopan unioniin on tuonut lainsäädäntötyöhön kansainvälisen ulottuvuuden: on pitänyt vaikuttaa unionin kalatautilainsäädäntöön niin, että tehokas lohiloisen vastustus on edelleen ollut mahdollista. Ehkäisytyötä on ollut tekemässä suuri joukko paikallisista kalastusluvanmyyjistä keskushallintoviranomaisiin ja tieteelliseen tutkimukseen. Kansainvälinen yhteistyö on luonnollisesti ollut vilkkainta Teno- ja Näätämöjokien alueiden naapurimme, Norjan, kanssa. Norjalaisiin arviointeihin ja Tenojoen vesistön sekä Tenon lohen biologian ominaisuuksiin pohjautuen esiselvityksessä todettiin, että valmiussuunnittelussa ei voida tähdätä Teno- tai Näätämöjoen vapauttamiseen lohiloistartunnasta, jos tartunta sinne pääsisi. Sen sijaan täytyisi ilmeisesti pyrkiä pelastamaan lohen geneettinen materiaali eläviin geenipankkeihin. Valmiussuunnittelulla tulisi selvittää, voitaisiinko istutuksilla ja mahdollisesti joitain loisesta puhdistettuja vesistön osia vaelluspoikasten kasvualueina säilyttämällä antaa lohikannalle tekohengitystä. Lohikannan ja lohenkalastuksen mahdollinen palauttaminen edellyttänee nykyistä paremmin lohiloista kestävän lohikannan aikaansaamista. Lohiloistartunnan aiheuttamien näkymien lohduttomuus alleviivaa ehkäisytoimien suurta merkitystä Teno- ja Näätämöjokien lohelle, lohenkalastukselle ja jokialueiden ihmisille. Valmiussuunnitelman teon aloittamista norjalaisten kanssa kuitenkin ehdotetaan, koska katastrofiskenaarion varalta pitäisi myös varautua. Valmiussuunnittelu lohiloisen varalta on Teno- ja Näätämöjokien alueella poikkeuksellisen monimutkainen ja laaja eläintautiin varautumistehtävä. Dán prošeavttas kártejuvvojedje Suomas dahkkojuvvon Gyrodactylus salaris –luossaparasihta dustenbarggut. 80-logu gaskkamuttu rájes Suoma davviosiid atlánttaluossajogaid Deanu ja Njávdánjoga leat geahččalan suodjalit ealli guliid sirdingieldduiguin sihke ávžžuhusaiguin dahje njuolggadusaiguin bivdoneavvuid goikadeamis dahje desinfiseremis. Go Suopma searvvai Eurohpá Uniovdnii, de dát buvttii láhkanásahanbargui riikkaidgaskasaš dimenšuvnna: galggai váikkuhit Uniovnna guolledávdaláhkamearrádusaide nu, ahte luossaparasihta lei vejolaš ainge beaktilit dustet. Dán dustenbarggu leat leamaš ollašuhttimin ee. báikkálaš bivdolohpevuovdit, guovddášhálddahuseiseválddit ja guolledutkit. Deanu ja Njávdánjoga hárrái Suopma lea bargan eanemus ovttas riikkaidgaskasaš dásis dieđusge Norggain. Norgalaččaid árvvoštallamiid mielde ja Deanu čázádaga ja deanuluosa biologalaš iešvuođaid vuođul gávnnahuvvui ovdačielggadusas ahte gearggusvuođaplánemis ii sáhte vuolgit das, ahte luossaparasihta livččii vejolaš ollásit jávkadit Deanus ja Njávdánjogas, jos dat njomošii daidda. Galggašii várra baicce viggat gádjut luosa genetihkalaš materiála ealli genabáŋkkuide. Gearggusvuođaplánema vuođul galggašii čielggadit, sáhtášiigo luossanáli ealáskahttit gilvimiiguin ja vejolaččat nu, ahte vádjolanveajehiid šaddanguovlun seailluhuvvojit parasihtas buhtistuvvon čázádatoasit. Luossanáli ja luossabivddu vejolaš máhcaheami várás galggašii bargat dan ovdii ahte ožžojuvvo áigái luossanálli, mii gierdá buorebut luossaparasihta. Olbmot ballet luossaparasihta njoammumis Detnui ja Njávdánjohkii, ja danin parasihta dustendoaimmat mearkkašit ollu dáid jogaid lussii ja luossabivdui sihke johkaguovlluid olbmuide. Lea goittotge evttohuvvon ahte galggašii dahkkojuvvot gearggusvuođaplána norgalaččaiguin, dasgo vejolaš luossaparasihttarossui galggašii maiddái ráhkkanit. Gearggusvuođaplána dahkan luossaparasihta dusten dihte Deanus ja Njávdánjogas lea hui mohkkás ja viiddis bargu. I projektet gjorde man en översikt av bekämpningsarbetet mot Gyrodactylus salaris i Finland. Från mitten av 80-talet har våra nordliga laxälvar, Tana och Nejden, skyddats med hjälp av transportförbud av levande fisk till dessa. Samtidigt har det funnits rekommendationer och senare bestämmelser att fiskeredskap, före införes till älvarna, ska torkas eller desinficeras. Finlands integration till den Europeiska Unionen medförde en internationell utveckling av lagstiftningen: det var viktigt att få unionens fisksjukdomslagstiftning att ta i beaktande en fortsatt effektiv prevention av laxparasiten. Bekämpningsarbetet har utförts av ett stort antal grupper från lokala fiskelovsförsäljare till centrala myndigheter och vetenskaplig forskning. Med hjälp av norska evalueringar samt de biologiska egenskaperna av Tanaälvens vattendrag och Tanaälvs laxestam, drogs det slutsatsen, att man i beredskapsplanen inte kan ha som mål att bli av med G. salaris om den ges tillfälle att överföras till Tana- eller Neidenälven. I stället borde man troligen etablera en levande genbank av laxens arvsmaterial för att rädda laxstammen. Med beredskapsplanering kunde man klarlägga, om det vore möjligt att förbättra laxstammens överlevnadsmöjligheter genom att förinta parasiten från delar av vattendraget samt utplantera migrerande laxyngel till dessa delar. Återställandet av laxbeståndet och laxfisket skulle förmodligen förutsätta ett motståndskraftigtigare laxbestånd än nuförtiden. Tröstlösheten bakom följderna av en möjlig G.salaris infektion betonar den stora betydelsen av preventiva åtgärder för Tana- och Neidenälvens lax, laxfiske och människor. I arbetet föreslås det emellertid, att man skulle påbörja en beredskapsplanering med Norge, eftersom man även bör förbereda sig på ett skräckscenarie. Beredskapsplaneringen mot G. salaris vid Tana- och Neidenälvarnas områden är en ovanligt komplicerad och vid uppgift. An overview of the prevention of Gyrodactylus salaris from the mid 80’s in Finland was performed in the project. There has been a ban of transportation of live fish to the two Atlantic salmon rivers of Finland, Rivers Teno and Näätämö (Tana and Neiden in Norwegian). Also recommendations and later statutes of the drying or disinfection of the fishing equipment has been in force at the same time. The joining of Finland to the European Union (EU) possessed a task of influencing the legislation of the EU in order to be able to continue effective prevention. A large number of players from the local fishing license sellers to central government authorities and scientific research has participated in this prevention work. The international cooperation has been closest with the neighboring country of the river areas, Norway. In the event of a Gyrodactylus salaris infection in the river Teno, there will not be possibilities to the total eradication of the disease. This conclusion was based on the evaluation reports of the treatments of the Norwegian Atlantic salmon rivers and the conditions of the river and biology of River Teno salmon. An attempt to conserve the genetic material to live gene banks would probably be the option of choice in such case. In a contingency plan the possibility of keeping certain parts of the water system free of the infection and compensatory restocking programs should be analyzed. This kind of work could perhaps serve as artificial respiration for the salmon in the river system. In the long run, a more resistant stock of the River Teno salmon would presumably be needed for the restoration of the salmon population and fishing. The desolate sight of the Gyrodactylus salaris infection in River Teno or Näätämö underlines the importance of the prevention work. Commencement of contingency planning with Norway is, however, proposed. There is a need to prepare also against the catastrophe scenario. The contingency planning addressing the infection of Gyrodactylus salaris is an extraordinary and extensive task of preparation against an animal disease.

FM Matthias Bandilla osoittaa väitöstutkimuksessaan, että kirjolohen kalatäitartunnalla on selkeä yhteys Flavobacterium columnare -infektioon, joka on yksi ongelmallisimmista bakteeritaudeista suomalaisessa kalanviljelyssä tällä hetkellä. Vaikka heikko kalatäitartunta ei aiheuta kalan kuolemaa, yhdessä bakteerialtistuksen kanssa se voi olla tappava. Vastaavien vuorovaikutusten ymmärtäminen olisi tärkeää vesiviljelyn kannalta, koska erilaisten mikrobien aiheuttamien tautien ja loisten yhteisesiintyminen kalanviljelylaitoksella on ennemminkin sääntö kuin poikkeus.Kalatäi (Argulus coregoni) on pääasiassa lohikaloilla esiintyvä äyriäispintaloinen, jonka levinneisyys ulottuu koko pohjoiselle vyöhykkeelle. Täin toukat kiinnittyvät kalaisännän pintaan ja syövät isännästään verta ja kudoksia. Kalatäikoiraat etsivät yleensä kalan pinnalta naaraan pariutuakseen, jonka jälkeen naaraat irrottautuvat kalasta muniakseen keskimäärin 300 munaa. – Kalatäit kuolevat avovesikauden loppuun mennessä ja talvehtivat ainoastaan munina. Suomessa lajilla on yksi sukupolvi vuoden aikana, mutta ilmaston lämpeneminen voisi mahdollistaa myös toisen sukupolven, joka toisi väistämättä lisää ongelmia kalankasvatukseen, Bandilla toteaa.Tutkimuksen kohteena myös kalatäin ekologiaVäitöskirjatutkimuksessaan Bandilla tutki paitsi kalatäiloisinnan yhteyttä bakteeritautiin, myös kalatäin siirtymistä kalaisäntään ja pariutumista. Tutkimuksessa selvisi, että pieni osa kalayksilöistä kantoi suurimman osan loistaakasta. Näköaisti osoittautui hajuaistia tärkeämmäksi, kun täit etsivät isäntää. Vain koiraat reagoivat vastakkaisen sukupuolen tuottamaan hajuun. Kalatäikoirailla ja -naarailla oli erilaiset lisääntymisstrategiat: koiraat toimivat aktiivisina parittelukumppanin etsijänä ja naaraat passiivisempina pysytellen kalan pinnalla. – Torjuntastrategiat, joissa yhdistetään tehokkaiden kemiallisten torjunta-aineiden käyttö ja loiseliöiden biologian tunteminen, voivat vähentää tappioita ja ympäristövaikutuksia. Argulus coregoni -kalatäin biologian tutkimus on johtanut lois-isäntäpopulaatiomallin kehittämiseen, joka on ensimmäinen askel kestävän kalatäitorjunnan kehittämiseksi kalanviljelylaitoksilla, Bandilla kertoo. Argulus coregoni is an ectoparasite preferentially infecting salmonid fish. In this thesis I focussed on aspects of transmission, host and mate location of A. coregoni and its link with secondary diseases in fish. Under fish farming conditions, metanauplii and juvenile stages of the parasite were aggregated among the host population; i.e. a relatively small proportion of the host population carried the majority of the parasite population. In laboratory experiments I showed that such aggregated distribution was likely to be driven by differences in exposure to the parasite and not by differences in susceptibility or by acquired immunity. The transmission of A. coregoni metanauplii in the laboratory was highly successful, with 80% attaching to the host within 1-2 h. Extended transmission periods due to host absence did not decrease the infectivity of the parasite, but some consequences of a prolonged transmission process became apparent during the early development; parasites that experienced long transmission periods were smaller compared to argulids with short transmission time. However, in late phases of development the smaller parasites compensated their decreased size by an increased growth rate. Since A. coregoni is a sexually reproducing species, such “catch-up” growth might be beneficial in terms of reaching the optimal mating size, higher fecundity and sexual selection. In Y-maze experiments it was shown that visual cues are most important for host finding, however the parasite could also detect fish odour which might act as an auxiliary cue to find a host. Males responded to female odour but not vice versa, indicating that two different reproductive strategies exist, where males are the active mate searchers while females are rather stationary on the host and wait for partners. Host switching experiments support this view since adult males were more active in switching between hosts than were adult females. Knowledge gained in this study elucidates the sophisticated behavioural patterns of A. coregoni and can be used for developing sustainable control strategies at fish farms, which is important since this thesis showed that argulids can enhance the susceptibility of fish to Flavobacterium columnare, the causative agent of columnaris disease

Kalatäit ovat pintaloisia, jotka imevät kaloista verta. Ne liikkuvat kalan pinnalla ja vaurioittavat ihon pintaa. Ihovauriot ja niistä johtuvat bakteeri- ja sienitaudit heikentävät kalojen arvoa ja voivat johtaa kasvun hidastumiseen ja kalan kuolemaan. Kalatäiden torjunta kalanviljelylaitoksissa on ollut vaikeaa ja tehotonta, koska niiden ekologiaa ja elinvaiheita ei ole tunnettu, kertoo Teija Hakalahti. Hakalahti tutki Argulus coregoni -kalatäin ekologiaa ja käyttäytymistä loisen elinkierron eri vaiheissa, joita ovat munat, munista kuoriutuvat vapaana elävät nuoruusvaiheet ja kalassa aikuistuvat loiset. In this thesis, the life history traits of the ectoparasite Argulus coregoni were examined during its life cycle. Water temperature was a key determinant of temporal patterns of A. coregoni population abundance and dynamics; neither parasites attached to the host nor egg-hatching were recorded at temperatures below ~10ºC. Parasite transmission to their hosts was characterized by peak recruitment in spring followed by extended, but slower recruitment. Egg-hatching was triggered by cooling treatments, the magnitude of which was dependent on the period of cooling. Each female parasite, however, produced eggs that hatched over many hatching opportunities. This delayed emergence pattern can be seen as an adaptation to unpredictable, risky environments, where such bet-hedging may allow some infection to occur even after periodic absences of hosts. Female A. coregoni preferred to lay their eggs on dark substrates in the deepest water in a 2 m-deep canal, with the egg population showing an aggregated distribution pattern. Non-random distribution of eggs coupled with variable hatching may result in differences in host exposure to parasites, which was shown to be a dominant causative factor behind the observed aggregated distribution of A. coregoni on their rainbow trout (Oncorhynchus mykiss) hosts. Rainbow trout did not seem to develop any resistance mechanisms to prevent parasite settlement after first encounters with A. coregoni nor any general trend in repeated susceptibility of fish to parasite attachment observed. Both these findings suggest that the fish were of equal genetic susceptibility. Nevertheless, the attachment of parasite infective stages, that carried a finite energy resource for host seeking, seemed to be opportunistic and non-selective. Although the infectivity of A. coregoni remained high up to death, delayed starts with respect to host searching time were reflected on growth in the altered growth rate of settled parasites. Over time, parasites that had spent more time searching for a host reached the length of parasites that attached to a host faster. A. coregoni thus appeared to compensate for delayed growth resulting from an extended period off-host by accelerated growth, although, this was shown to incur a cost through decreased life-expectancy. This study has shown that individual parasites are more flexible in their life history strategies than has hitherto been understood. The results from this thesis provide parameter estimations for a detailed host-macroparasite model that will capture both host and parasite population dynamics, and can be used for planning cost-effective management against ectoparasite infestations at fish farms

Kilpailuyhteiskunnan säännöt eivät päde kalojen loisyhteisöissä. Sen sijaan loiset pyrkivät kaikin tavoin kerääntymään yhteen lisääntymisen varmistamiseksi, toisin sanoen ne aggregoituvat tiettyyn isäntäyksilöön ja jopa samalle paikalle isäntäkalassa, totesi Anna Bagge väitöstutkimuksessaan. Bagge tutki pääasiassa särjen ja ruutanan kiduksissa esiintyviä pieniä kidusmatoja, joita yhdessä kalassa saattoi olla jopa tuhansia. Ne eivät ole isännälle haitallisia eivätkä pysty tarttumaan ihmiseen, vaan kidusmatoja esiintyy pääasiassa särkikalojen kiduksissa ja ihon pinnalla. Kidusmadot ovat hermafrodiitteja eli kaksineuvoisia, niillä on suora elinkierto, eli ne eivät tarvitse väli-isäntiä, ja ne tarttuvat kalasta toiseen ripsitoukkavaiheessa. The central aim of community ecology is to understand how the numbers and relative abundances of species in an assemblage are determined. Parasite communities in freshwater fish are often described as unpredictable, depauperate communities. However, monogenean species on the gills of cyprinid fish often form multispecies communities with even thousands of specimens on one gill-arch. The factors affecting parasite communities were studied at infra- and component community level in two cyprinid fish species; roach (Rutilus rutilus) and crucian carp (Carassius carassius). First, I discuss the environmental factors determining the numbers and species of parasites in the community. I focus first on host population structure, but environmental diversity, host immunity and physiology are also discussed. Recruitment of the parasite species onto the gills is another important theme not previously studied. Second, I concentrate on the factors affecting the site-preference of the parasite specimen on the gills. Competition between species is one possible factor determining site preference. However, no competition between species was detected in any of the communities studied, in spite of unusually high abundances. Aggregation of parasites on certain host specimens is one of the most peculiar phenomena of parasite communities. Here I show that it is one of the most prominent features also at the infracommunity level. Individuals are more aggregated at lower abundances, in order better to find a mate. Other possible factors determining the occurrence on the gill-arch are the size of the gill-arch and the direction of the water current through the gills. Finally, the level of the study and its usefulness in other freshwater fish parasite communities are discussed

Otto Seppälä tutki väitöskirjassaan loisten kykyä manipuloida isäntiään mahdollisena strategiana tehostaa loisten siirtymistä uusiin isäntäyksilöihin. Monien loisten on havaittu vaikuttavan mm. isäntiensä käyttäytymiseen ja ulkonäköön. Erityisesti ravintoketjuissa saaliseläimistä saalistajiin kulkeutuvilla loisilla isännän manipulointia on yleisesti pidetty loisten evolutiivisena sopeumana altistaa isännät saalistajille ja täten tehostaa loisten siirtymistä elinkierrossa eteenpäin. Manipulaatioteoriaa on kuitenkin vain harvoin testattu kriittisesti selvittämällä saalistuskokein loisten aiheuttamien käyttäytymismuutosten vaikutusta isäntien saalistusalttiuteen.Seppälä selvitti useiden eri kalalajien silmissä yleisen Diplostomum spathaceum -loisen vaikutusta kalojen käyttäytymiseen, värinsäätelyyn ja saalistusalttiuteen. Trophically-transmitted parasites may predispose infected hosts to predation by altering their phenotype. This can be either an adaptation of the parasites to enhance their transmission to the next hosts in the life cycle or a non-adaptive side-effect of infection. In this thesis, I examined whether the Diplostomum spathaceum (Trematoda) eye fluke can manipulate the phenotype of its fish intermediate hosts to increase their susceptibility to predation, and if this could be an adaptation of the parasite to enhance its onward transmission to the bird definitive hosts. In laboratory experiments, I found that anti-predator behaviour of infected fish was reduced compared to uninfected fish. Parasitized fish did not prefer the surface layers of the water column more than control fish, but did show a weaker reaction to an approaching simulated avian predator. Furthermore, their ability to adjust to the environment using cryptic coloration and cryptic behaviour was reduced. These changes led to an increase in the susceptibility of fish to simulated avian predation (capture by dip-net). This result was not reproduced when fish were exposed to predation by wild birds in a field experiment, possibly because the experimental set-up allowed birds to feed on fish in an easy, unnatural manner. Catchability of fish in the laboratory increased with the coverage of parasite-induced cataracts, which suggests that impaired vision may be the definitive mechanism leading to manipulation. Moreover, cataract formation was most intensive after parasites had completed their development, resulting in host manipulation only after parasites had reached infectivity to bird hosts. Furthermore, manipulation was not observed to be costly for the parasite, because it did not predispose fish to predation by non-host piscivorous fish. These findings suggest that manipulation of the fish host may increase the probability of parasite transmission to bird hosts, and thus be a parasite strategy evolved to enhance transmission.

FM Anssi Karvonen tutki väitöskirjassaan kalojen silmän linssissä elävän Diplostomum spathaceum -imumatoloisen ekologiaa. Tutkimus tarjoaa uutta tietoa lois-isäntä -suhteiden evoluutiosta sekä perustietoa, jonka avulla voidaan suunnitella loisen torjuntamenetelmiä kalanviljelyssä. Tutkimukset ovat lisäksi osa laajempaa projektia, jossa tutkitaan D. spathaceum -loisen elinkierron ekologiaa ja evoluutiota kaikissa elinkierron vaiheissa. Sen yhtenä päämääränä on kehittää elinkierron matemaattinen malli, jonka avulla on mahdollista kehittää ekologisesti kestäviä menetelmiä D. spathaceum -loisen torjuntaan.D. spathaceum on yksi luonnonkalojen yleisimmistä loisista Suomessa ja sitä esiintyy myös paikallisesti kalanviljelylaitoksilla, jossa olosuhteista johtuen on usein paljon loisen elinkierrossaan tarvitsemia isäntälajeja: kalaa syöviä lintuja, makean veden kotiloita ja kaloja. Loinen aiheuttaa haittaa yksittäisillä laitoksilla sokeuttamalla kaloja ja heikentämällä näin niiden kasvua ja yleiskuntoa. Tutkimuksissaan Karvonen keskittyi tekijöihin, jotka vaikuttavat loisen siirtymiseen kotiloisännästä kalaisäntään (kirjolohi).Kalan silmässä loisen toukat aiheuttavat kaihia, joka näkyy silmän samentumisena. Tämä voi johtaa silmän näkökyvyn heikkenemiseen ja vaikuttaa mm. kalan kykyyn löytää ravintoa. Silmätutkimukseen tarkoitetun mikroskoopin avulla Karvonen havaitsi laboratoriossa ja luonnonoloissa suoritetuissa kokeissa, että kaihin voimakkuus kalan silmässä lisääntyi merkittävästi loismäärän kasvaessa. Lisäksi hän havaitsi, että jo pienetkin loismäärät aiheuttivat osittaisia samentumia silmän linssiin.Kalat eivät kuitenkaan ole täysin loisen armoilla, vaan ne puolustautuvat haitallista loista vastaan. Karvonen havaitsi, että ensimmäisten loisten tunkeutuminen kalaan saa aikaan vastustuskyvyn kehittymisen, mikä vähensi seuraavissa altistuksissa kalaan pääsevien loisten määrää. Vastustuskyky on kuitenkin osittainen eikä se tarjonnut riittävää suojaa kaihin muodostumista vastaan, kun kaloja pidettiin sumpuissa luonnonoloissa. Karvosen tutkimuksessa osoitetaan ensimmäistä kertaa, että kalat pystyvät myös reagoimaan loisen toukkien läsnäoloon vedessä ja pyrkivät välttämään infektiota. Transmission rate between hosts is a major determinant of fitness in parasites with complex life cycles and this has favoured the development of sophisticated adaptations in parasites to increase the probability of transmission. On the other hand, increasing pressure by parasites has forced the target hosts to develop means to control or prevent the infections. This study examined the transmission of the trematode parasite Diplostomum spathaceum between the two intermediate hosts, the freshwater snail (Lymnaea stagnalis) and rainbow trout (Oncorhynchus mykiss), focusing on the parasite strategies in cercarial production from the snail host in laboratory conditions and responses of the fish host to cercarial infection. Infected snails released cercariae for an extended period of time in accordance with the bet hedging hypothesis, which suggests spreading the risk of transmission failure in an environment with an unpredictable host contact probability. Bet hedging was not found in association with cercarial quality, but cercariae were released in higher numbers during day compared to night. High prevalence and intensity of infection in wild fish populations suggests that the parasite strategies are likely to maintain effective transmission to fish. As a response against the cataract-causing parasite, fish hosts acquired physiological resistance, which decreased the number of establishing parasites although the resistance proved to be insufficient to prevent cataract formation. However, this study demonstrated for the first time that the fish also decreased exposure to the parasite by avoiding the cercariae. Thus, a combination of resistance and avoidance behaviour may provide more efficient defence against the parasite. Individual differences between fish in exposure to the parasite and ability to avoid cercariae probably cause the parasites to aggregate in a few host individuals, which has important implications for the parasite population dynamics and transmission. As a practical application, the results of this study allow the estimation of key parameters in the parasite transmission dynamics, which can be used in designing preventative methods against D. spathaceum outbreaks in fish farming industry.

Raine Kortetin väitöstutkimus lisää tietämystä kalan kutukyhmyistä, lisääntymisbiologiasta, seksuaalivalinnasta ja immuunipuolustuksesta. Tutkimuksessa on ensimmäistä kertaa osoitettu, että sekä loisvälitteinen että ns. immunokompetenssihaittateoria voivat olla toimivia saman systeemin seksuaalivalinnassa. Tutkimustyön tuloksia voidaan mahdollisesti soveltaa esimerkiksi kalanviljelyssä ja -jalostuksessa sekä uhanalaisten kalalajien suojelussa. One of the main issues in immunological ecology is the role of immune defence in the evolution of life-history traits, i.e. the investment in reproduction at the cost of immune function. Moreover, theories of parasite-mediated sexual selection suggest that secondary sexual characters are connected to parasitism. For this thesis, I studied aspects of fish reproduction and sexual selection with respect to defence against parasites and pathogens. As a model species I used roach, Rutilus rutilus. In the seasonal cycle of fish immune defence, the relative size of the spleen and the chemotactic migration activity of head kidney granulocytes decreased immediately before and after spawning, respectively. However, breeding-related changes in other measurements (phagocytosis activity of head kidney granulocytes, blood IgM concentration, white cell count and red cell count) were absent or inconsistent. This suggests that the different parts of the immune system may have different temporal patterns. Moreover, epidermal papilloma disease outbreak peaked around the spawning period in both sexes. Males and large fish had a higher prevalence of papilloma tumours. The gender dependence in the disease prevalence found in this study may indicate differences in stress and immunocompetence between male and female roach. Sexual ornamentation of male roach, the breeding tubercles, signalled a long-term resistance of males against the locally most abundant parasite in the study, Rhipidocotyle campanula. Moreover, breeding tubercle ornamentation was associated with dominance status in intrasexual prespawning interactions. In addition, raised circulating androgen levels were related to high breeding tubercle ornamentation, but interestingly, also to increased prevalence of papilloma disease. Thus, I propose that, in the present system, the character that a male roach signals by his sexual ornamentation may be resistance against parasites (R. campanula), and that papillomatosis occurring during spawning is an element which contributes to the honesty of the signal. The results in the thesis support the hypothesis of a trade-off between reproduction and immune defence and the hypothesis of parasite-mediated sexual selection. Thus, parasites and pathogens may have an appreciable impact on the evolution of life histories and sexual selection of fish.