Downloaded from orbit.dtu.dk on: Oct 04, 2021 An Ichthyophonus hoferi epizootic in herring in the North Sea, the Skagerrak, the Kattegat and the Baltic Sea Mellergaard, Stig; Spanggaard, Bettina Published in: Diseases of Aquatic Organisms Link to article, DOI: 10.3354/dao028191 Publication date: 1997 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Mellergaard, S., & Spanggaard, B. (1997). An Ichthyophonus hoferi epizootic in herring in the North Sea, the Skagerrak, the Kattegat and the Baltic Sea. Diseases of Aquatic Organisms, 28(3), 191-199. https://doi.org/10.3354/dao028191 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. 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DK-1870 Frederiksberg C, Denmark 'Danish Institute of Fisheries Research, Department of Seafood Research, Technical University, Bldg. 221, DK-2800 Lyngby, Denmark ABSTRACT: An epizootic caused by the internal parasite Ichthyophonus hoferi in herring Clupea harengus was recorded from 1991 to 1993 in the waters around Denmark. A surveillance programme from research vessels and commercial fishing boats was conducted in the North Sea, Skagerrak, Kat- tegat and Baltic Sea. A total of 15769 hearts of adult herring were examined macroscopically for evi- dence of infection. The prevalence of the infection in this period decreased from 10.6 to 2.0% in the North Sea, from 8.0 to 5.6O!, in the Skagerrak, from 12.0 to 1.1% in the Kattegat and from 4.5 to 0.4 O/o in the Baltic Sea. Prevalence rates in research vessel catches were significantly higher (by 1.8 times) than in commercial fishing boat catches. The results show a high degree of temporal and spatial varia- tion of the prevalence of I. hoferj. Infected fish were found to be significantly longer than unaffected fish in the North Sea and the Skagerrak. The majority of the infected fish belonged to the age groups 2+, 3+ and 4+ except in the commercial catches from the North Sea where most of the affected flsh belonged to the age groups 5+, 6+ and 7+. Annual mortality in the different areas, based on the observed prevalences was roughly estimated at 12.8 to 36% in 1991, decreasing to a few percent in 1993. The spawning stock biomass of North Sea herring was reduced by 50% during the period 1990 to 1995 This reduction may have been due to a combination of increased fishing intensity and the gen- eral effect of the I. hoferi epizootic. KEY WORDS: Ichthyophonus hoferi. Epizootic . Herring. North Sea . Skagerrak . Kattegat . Baltic Sea INTRODUCTION These resting spores produce aseptate hyphae imme- diately after death of the fish host (Dorier & Degrange Ichthyophonus hoferi Plehn & Mulsow (1911) is an 1960, McVicar 1982). internal parasite of many fish species. This micro- Ichthyophonus hoferi displays a remarkable physio- organism was, until recently, believed to be a fungus logical adaptability to a wide range of environmental but new investigations have shown that I. hoferi conditions and has been reported in a broad range of belongs to the protist kingdom (Spanggaard et al. both fresh water and marine fish. The parasite is a 1996). I. hofen causes systemic infections in fish in potent pathogen in several important commercial fish which the principal infection sites are the organs richly species, e.g. herring Clupea harengus, plaice Pleuro- supplied with blood. The fish are believed to be nectes platessa, haddock Melanogrammus aeglefinus infected by ingestion of spores and affected organs and mackerel Scomber scombrus (Neish & Hughes reveal mainly thick-walled multinucleate cells of 'rest- 1980),and, recently, an epizootic among herring in the ing spores' that vary in size up to 200 pm in diameter. waters around Denmark and Norway was reported (Lang 1992a, b, Rahimian & Thulin 1996). Epizootics have previously been reported in herring in North 'Addressee for correspondence. E-mail: [email protected] American waters with consequential severe reductions O Inter-Research 1997 Resale of full article not permitted 192 Dis Aquat Org 28: 191-199, 1997 in stocks (up to 50'h) (Sindermann 1970). In the North Sea, infections by I. hoferi have been described in had- dock and plaice (McVicar 1979). Outbreaks have prob- ably previously occurred in clupeids in Danish coastal waters as evidenced by outbreaks of ichthyophoniasis in farmed rainbow trout Oncorhynchus mykiss fed on trash fish of marine origin (Rasmussen 1967). In addi- tion, Baltic cod developed ichthyophoriasis in an ex- periment in which they were held in net cages and fed on herring (Kock 1975). The infection has also been demonstrated in wild cod Gadus morhua in the Baltic Sea (Moller 1974, Reirner & Walter 1993). Until 1991, epizootics similar to those observed in North American waters had not been recorded in the North Sea area (apart from increased mortality in local plaice stocks observed along the Scottish coast; McVicar 1981). However, in the late summer of 1991, mass mortalities from ichthyophoniasis in Norwegian spring spawning herring were reported (Hjeltness & Skagen pers. comm.). A few months later, the disease was recorded among herring stocks in the Kattegat and the Sound between Denmark and Sweden. The purpose of the present study was to investigate the spatial distribution and the prevalence of the dis- ease in European herring. MATERIALS AND METHODS The surveillance programme was carried out in the North Sea, the Skagerrak, the Kattegat and the Baltic Sea. Samples were collected from trawl catches of both commercial fishing boat and research vessel catches. Total numbers of 3061, 3600 and 9474 herring were examined in 1991, 1992 and 1993, respectively. The fish were kept frozen until the time of processing. Of the examined fish, 9854 were length-determined and 10739 were age-determined. About one third of the herring were sex-determined. The hearts from sub- samples of approximately 50 herring were inspected macroscopically. Infection with Ichthyophonus hofen was diagnosed based on the presence of visible white nodules on the surface the heart as recommended by Prevalence Anonyluous (i993). -, > 10.1% H~.I-Io./,0.1-5% o X A rough estimate of the annual mortality rate of the herring stock was calculated as: Flg 1 Spatial distribution of sampling and the prevalence of Prevalence X 365 (days) Ichthyophon~~shofen ~nfectedherring in the North Sea, the Annual mortality rate = Skagerrak, the Kattegat and the Baltic from 1991 to 1993 105 (life expectancy in days)-' This formula is based on the assumptions that all infected fish which are detected in a sample die of the Data analysis were carried out using StatistixB, ver- infection within 105 d (Anonymous 1993).The estimate sion 4.1 (Analytical Software). The statistical tests of disease rate and mortal~tyat 100% are both based applled for the data treatment were chi-square test, 2- on aquarium experiments on 2000 hernng (Sinder- sample I-test and Mann-Whitney rank sum test, mann 1958). depending on the distribution of the variables. Mellergaard & Spanggaard: Icl~thyophonushoferi epizoot~c 193 RESULTS A total of 1363 herring were examined from research vessel catches from the North Sea in the Disease symptoms observed in the Ichthyophonus period 1991 to 1993 (Table 1).A significant decrease in hoferi infected herring were white granulon~aforma- the prevalence of Ichthyophonus hoferj infection from tions in the heart muscles. Granulomas were also 10.6% in 1991 to 3.2% in 1992 and 0.0% in 1993 (p < observed in the liver, on the surface of the viscera and 0.001) was observed in these catches. From commer- in the lateral red muscle. cial fishing boat catches, 2201 fish were examined and Determination of the spatial distribution of Ichthyo- the prevalence decreased from 3.7% in 1991 to 1.8% phonus hoferj infected herring was based on commer- in 1992 and 2.0% in 1993. The observed decrease was cial and research vessel catches for the last 2 quarters not statistically significant. From the Skagerrak, a total of 1991, and the years 1992 and 1993 in the North Sea, of 4805 herring originating from research vessel the Skagerrak, the Kattegat and the Baltic Sea (Fig. 1). catches were examined and, here, the infection rate The prevalence of I. hoferi was at a higher level in 1991 decreased significantly from 8.0 % in 1991 to 1.6% in compared to 1992 and 1993 and showed a decreasing 1992 (p < 0.001), whereafter the infection rate in- trend from 1991 to 1993. The I. hoferi infection was creased significantly to 5.6% in 1993 (p < 0.001).This observed in herring from the northern North Sea, the increase was due to a series of samples of highly Skagerrak, the Kattegat and eastern Baltic Sea, while infected l+ herring caught during a research vessel the southern North Sea (south of latitude 56") seemed cruise in the beginning of May.