| CHAPTER VI - Baltic Fish Stocks and Diseases |
6.3 Diseases and Parasites of Baltic Fish
The information in this sub-chapter is based on the 1995 report of the ICES Working Group on Pathology and Diseases of Marine Organisms (WGPDMO), the results of the BMB/ICES Symposium "Diseases and parasites of flounder (Platichthys flesus) in the Baltic Sea", unpublished results from the BMB/ICES sea-going workshop "Fish diseases and parasites in the Baltic Sea" and information from the literature. This information was compiled by T. Lang1, Germany, and the text has been reviewed by the ACFM and the Advisory Committee on the Marine Environment (ACME).
This chapter will provide a brief description of the prevalence and geographical distribution of selected fish diseases and parasites in the Baltic Sea, based mainly on data from the past twenty years. Information is provided for the major Baltic Sea fish species on which most research and monitoring activities have been conducted, namely, flounder (Platichthys flesus), cod (Gadus morhua), herring (Clupea harengus) and, recently, Atlantic salmon (Salmo salar). However, other species, affected by significant diseases and/or parasites, are also included in this chapter.
In addition to the grossly visible fish diseases/parasites that have been recommended by ICES for fish diseases surveys (see REF : 268) and widely used for biological effects-monitoring purposes, other diseases/parasites that have previously received considerable attention due to their possible link to marine contamination and/or their suspected impact on mortality and fish-stock size are considered here. A summary of the diseases and parasites is given in Table 6.3.
The most current data on the prevalence and spatial distribution of diseases/parasites of flounder, cod and herring are those obtained during the BMB/ICES Workshop "Fish diseases and parasites in the Baltic Sea" which was held from 25 November to 8 December 1994 on board the rv "Walther Herwig III". Practical work was conducted on a transect from the Mecklenburg Bight to the western Gulf of Finland, representing the largest area in the Baltic Sea ever studied for this purpose in a narrow time-window and using identical methods. Although the results of the workshop have not yet been published, some of them have been included here (see REF : 34).
Additional information on diseases/parasites of the Baltic flounder was presented at the BMB/ICES symposium "Diseases and parasites of the flounder (Platichthys flesus) in the Baltic Sea", held on 27-29 October 1994 at åbo Academy University, Turku, Finland (see REF : 35). In the following sections, information is given for individual Baltic fish species on their major diseases and parasites, including a compilation of selected key publications.
Information on larger-scale geographical distributions of externally visible diseases of Baltic flounder used for monitoring purposes, including data on temporal trends, has been provided by different authors (see REF : 141,142, 384,680). Other published data cover more restricted areas, including the Kattegat and the Skagerrak, and certain time periods (see REF : 147,304, 432,469,470,568,647,701,703).
A current review has been provided on the unique parasitic fauna of the Baltic flounder, which, due to the strong salinity gradient from the western to the eastern/northern parts of the Baltic Sea, consists of both marine and fresh-water species (see REF : 162). Other publications were based on more restricted areas (see REF : 431,467,568, 667,679). Of these publications, only a few (see REF : 467,667,568) contain information on temporal trends.
The viral lymphocystis disease is by far the most prevalent externally visible disease of Baltic flounder. There is general consensus about the aetiology of lymphocystis, the infectious agent of which is a virus classified as belonging to the family of Iridoviridae (see REF : 381).
The prevalences recorded in November/ December 1994 during the BMB/ICES Work-shop, according to single sampling sites, were in the range of 5-38 % in flounder ¨20 cm, with a decreasing trend from the western to the eastern stations (see REF : 34). The high prevalences, detected at the westernmost stations, were well in accordance with data from 1993 recorded in the same area (see REF : 384). A clear increase in the prevalence of lymphocystis has occurred in the southwestern parts of the Baltic Sea, i.e., southwest and southeast of Bornholm, during 1986-93, the reasons for which have not yet been elucidated (see REF : 384).
There is evidence that the prevalence of lymphocystis in Baltic flounder is influenced by both sex and size/age of the fish (males and intermediate size groups are more frequently affected than females and small/large size groups, respectively) (see REF : 384), and by seasonal effects (see REF : 304,680).
The prevalence of the bacterial skin-ulcer disease of flounder ranged from 0-12 % at the different workshop stations, with some indication of increasing prevalences from the western to the eastern sampling sites (see REF : 34). Various bacteria considered to be involved in the aetiology of the ulcer disease of Baltic flounder have been isolated (see REF : 132,354,700). However, particularly ‘atypical’ Aeromonas salmonicida was consistently isolated from diseased Baltic Sea flounder (see REF : 700), and it is therefore likely, that this bacterium is the main cause of the skin-ulcer disease. Acute as well as healed skin ulcers are more prevalent in male than in female flounder (see REF : 34,701). Seasonal changes in the prevalence were reported (see REF : 304,701). No temporal trends in the prevalence of acute/healing skin ulcers in flounder from the southwestern parts of the Baltic Sea, i.e., southwest and southeast of Bornholm, were found in the period 1986-93 (see REF : 384).
Studies on the occurrence of neoplastic liver lesions in Baltic flounder have only recently started (see REF : 94,114,384). During the BMB/ICES Workshop in November/December 1994, the prevalences of liver nodules >2 mm in diameter (which, according to ICES standard methodologies for fish disease surveys (see REF : 268), is recorded as a macroscopic indicator of the occurrence of neoplastic liver lesions) were found to be generally low (0-3 %), with a tendency for slightly increased prevalences at the easternmost stations (see REF : 34). These data are in accordance with a mean prevalence of 3 % in flounder of the same size group (>=20 cm) from Finnish coastal waters (see REF : 703). However, for flounder >=30 cm, a prevalence of 10.1 % was calculated, which is very high compared to other Baltic Sea areas (see REF : 384).
Histological confirmation of the liver nodules collected during the BMB/ICES Workshop showed, that tumours were rare and that the other nodules consisted either of unspecific storage cell changes or putative pre-neoplastic lesions, mainly clear cell foci or eosinophilic foci (see REF : 34). Other externally visible flounder diseases, such as acute/healed fin rot/erosion and skeletal deformities, were only recorded at low prevalences (<1 %), without revealing any spatial trends (see REF : 34).
Information published during the past two decades on prevalences and spatial/temporal trends of externally visible diseases and parasites of Baltic cod, including in the Kattegat and Skagerrak, are available from many sources (see REF : 119,139,141-143,147,288,289, 378,383,386,422,688). Parasitological data, mainly on parasitic helminths, were also presented (see REF : 110,160,161,182,377,468,474,655, 673,688).
The most prevalent externally visible disease of Baltic cod is the bacterial skin-ulcer disease (ulcus-syndrome) (see REF : 288). Although there is evidence that the conspicuous disease signs (open, red ulcers affecting the skin and, occasionally, even the underlying musculature) are caused by bacterial infections, the primary cause may be different. For example, Danish studies on ulcerated cod occurring at high prevalences (>40 %) around Bornholm in 1982 revealed, that these lesions were most likely primarily due to mechanical damage of the skin of specimens which escaped from fishing nets. Thereafter, the open lesions were probably invaded by pathogenic bacteria, thus leading to the occurrence of the typical skin ulcers. However, there is also evidence from the literature that pollution might have been involved in the aetiology of the disease in cod from the Danish Belt Sea in the late 1970s (see REF : 119,288). Prevalences in cod >=20 cm total length, recorded in November/December 1994 during the BMB/ICES Workshop, were in the range of 0.0-4.2 % in relation to single sampling sites, with the highest prevalences in Polish waters (see REF : 34). These results are well in accordance with data from the southwestern part of the Baltic Sea for the period 1980-88 (see REF : 139) and for December 1993 (see REF : 141), regarding both the range of prevalences recorded and the identification of regions with the highest prevalences. However, considerably higher prevalences have been found at certain sites, such as 22 % reported for 1977 from polluted sites in the Danish Belt Sea (see REF : 289), 14.1 % for the Gdansk Bight in 1983 (see REF : 688) and 15.8 % for the Polish fishery zone in 1986 (see REF : 147). From the literature, there is no clear indication of consistent temporal trends in the disease. However, it appears that the prevalences generally were higher in the 1980s compared to the 1990s.
The second most common externally visible diseases of Baltic cod are skeletal deformities, such as vertebral compression, lordosis/scoliosis and deformation of the head skeleton, which have been known for a long time to occur occasionally in certain areas in the Baltic Sea at very high prevalences.
Factors involved in the aetiology of these lesions are manifold. Unfavourable hydrographic conditions during early ontogenesis, malnutrition, parasitic infestation, effects of heavy metals and differences in migratory behaviour between healthy and deformed fish are discussed as possible causes of elevated prevalences in wild fish (see REF : 62,63,140,471,472, 383). Significantly higher cadmium contents were detected in organs of cod with skeletal deformities compared to healthy cod (see REF : 383). However, these data did not permit any conclusions about the cause-effect relationship between higher cadmium residues and the occurrence of skeletal deformities.
During the BMB/ICES Workshop, the prevalence of externally visible skeletal deformities recorded in cod ¨20 cm was 0.0-4.0 %, with the highest prevalence in the Gdansk Bight and the lowest prevalence at the western- and easternmost stations (see REF : 34). These prevalences are in accordance with other published data for cod in the southwestern parts of the Baltic Sea (see REF : 139,141,142,383). Prevalences of <1 % were reported in cod from the Polish fishery zone (see REF : 422). However, from historic data, there is information, that skeletal deformities in Baltic cod may occur at much higher prevalences, i.e., >50 % in the Sound (see REF : 429) and ¨75 % in the Gulf of Riga (see REF : 72). From the literature, there is no information on consistent trends as regards spatial and temporal distribution patterns.
The third grossly visible disease, recommended for monitoring purposes in Baltic cod, is the X-cell disease (pseudobranchial pseudotumours) causing proliferations in the pseudobranches and adjacent tissue. The epidemiology and histopathology of this disease have been described (see REF : 143,689) and additional data were presented (see REF : 139,141). Although the disease has been recorded in a large variety of fish species during the past ten years, and its histopathology has been described thoroughly, its aetiology has so far not been resolved conclusively. However, there are indications that the disease is caused by proliferative parasitic protozoans, possibly Amoebae (see REF : 690).
During the BMB/ICES Workshop in November/December 1994, the X-cell disease was only recorded at the two westernmost stations, with a prevalence of <1 % in cod >=20 cm (see REF : 34). These data again correspond to previously published information, indicating that the disease is very rare and restricted to the southwestern parts of the Baltic Sea.
The infestation of Baltic cod with conspicuous externally visible parasites, the copepod Lernaeocera branchialis in the gill chamber and encysted metacercariae of the digenean Cryptocotyle lingua in the skin, is restricted to the southwestern parts of the Baltic Sea (see REF : 34,378). During the BMB/ICES Workshop, maximum prevalences in relation to sampling sites were 20.0 % and 2.9 % for Cryptocotyle lingua and Lernaeocera branchialis, respectively. Regarding the spatial distribution of cod infested by Lernaeocera branchialis, the workshop results confirm findings based on data from 1983-88 (see REF : 378). For cod from the southwestern Baltic Sea (ICES Sub-division 22), a prevalence of 4.9 % was calculated, with some annual fluctuations in the prevalence, without, however, revealing a temporal trend (see REF : 378).
New diseases of Baltic cod causing some concern are the occurrence of a yet unknown protistan endoparasite in the yolk of cod embryos and larvae (see REF : 531), and certain indications of a possible occurrence of a M-74-like syndrome, similar to the one known to have a deleterious impact on reproduction of the Baltic stock of Atlantic salmon (Salmo salar) (see REF : 270).
The diseases and parasites of Baltic herring, that have been the subject of most scientific activities during the past twenty years, are the infestation by larvae of the parasitic nematode Anisakis sp. and, since 1991, the epizootic caused by the parasitic fungus Ichthyophonus sp. Data on the prevalence and spatial distribution of Anisakis sp. in Baltic Sea herring have been provided, for example, by several authors (see REF : 181,359,360,385,426,567,664,691). From these data, there is evidence that the infestation mainly affects the western spring-spawning herring stock which enters the Baltic Sea for spawning after leaving its feeding grounds in the Kattegat, Skagerrak and the North Sea. This migration to the Baltic Sea usually starts in late autumn and, having spawned, the herring leave the Baltic Sea in spring. Since the major spawning areas are located in the southwestern parts of the Baltic Sea, infested herring can be mainly found west of Bornholm. However, at least some of the western spring-spawners migrate further east and have been recorded off the Polish coast (see REF : 181) and the Latvian coast (see REF : 664).
The infestation with larval Anisakis sp. is restricted to this particular herring stock, and specimens, belonging to the ‘real’ Baltic herring stocks, which stay in the Baltic Sea throughout their whole life, are infested only very sporadically. Therefore, it has been concluded that the infestation of herring takes place on feeding grounds outside the Baltic Sea by feeding on infested intermediate hosts of the parasite, probably euphausids.
Mean prevalences in herring in the size range 20-27 cm total length, sampled west of Bornholm (ICES Sub-divisions 22 and 24) in December 1987 and 1988, were in the range of 30-45 %, whereas prevalences in areas east of Bornholm (ICES Sub-divisions 25 and 26) were between 0.3 and 0.7 % (see REF : 385). The same authors demonstrated a clear positive relationship between the length of the herring and both the prevalence and intensity (number of nematode larvae per infested herring) of the infestation in samples from the Kiel fishery market covering the period 1973-88. The smallest infested herring were 19 cm, a steep increase occurred in the size group 22-29 cm, and herring larger than 30 cm were infested to 100 %. Based on the same data set, an analysis of temporal trends in the prevalence was conducted. However, the results did not reveal a clear upward or downward trend. Parasitic nematode species in Baltic herring other than Anisakis sp. have been also reported (see REF : 161,169,622,668,673).
The Ichthyophonus sp. epizootic in Baltic Sea herring was noted for the first time in summer 1991, when mass mortalities occurred at the Swedish west coast, causing great concern by the public, the fishery industry, and the scientific community as well. This concern was mainly due to the well-known effects of epizootics in the 1950s that affected herring stocks in North American Atlantic coastal waters. These epizootics caused a drastic stock reduction, followed by considerable economic losses for the fishing industry (see REF : 619). From experimental studies, carried out following the first epizootics, summarized in (see REF : 619), there was evidence that the infestation was lethal for herring, in contrast to many other fish species, within a relatively short period after infestation, thus explaining the massive stock reduction recorded.
Due to the fear in the Scandinavian countries, that the European herring stocks may experience a similar decline, studies were started nearly immediately after the observation of mortalities in 1991 in order to obtain baseline data on the spatial distribution of the epizootic and the prevalences in herring stocks in the Baltic Sea as well as in the North Sea. For the southwestern Baltic Sea, it was demonstrated very quickly that herring infested by Ichthyophonus sp. belonged to the western spring-spawning stock, the same stock which is characterised by high prevalences of Anisakis sp. larvae (see REF : 380). In areas, dominated by herring of the ‘real’ Baltic stocks, the infestation was at a low level, for example, since 1991 in herring of the Gulf of Finland and the Gulf of Riga (see REF : 668).
Mean prevalences of grossly visible disease signs in the heart of herring >=20 cm total length, recorded in December 1991 west of Bornholm, were 2.3 % (ICES Sub-division 22) and 5.3 % (ICES Sub-division 24). However, the prevalences were considerably higher in single catches, revealing a patchy distribution of infested herring. In contrast, no infested herring were recorded east of Bornholm (ICES Sub-divisions 25 and 26) (see REF : 380). In the following year, the prevalences had dropped to 0 % (ICES Sub-division 22) and 0.5 % (ICES Sub-division 24). At present, there is a general consensus that the Ichthyophonus epizootic in Baltic Sea herring is over, and that the prevalences have approached natural background levels (see REF : 270). There is evidence (see REF : 468,668), that other Baltic fish species (sprat, cod) are infested at low prevalences. However, there is no indication of increased mortalities in these species.
Other significant diseases of Baltic herring include lymphocystis (see REF : 28,34,147), and a bacterial infection by Pseudomonas anguilliseptica, associated with haemorrhagic eye lesions, possibly involved in the transmission of the infection, which has been known for some time to occur in cultured Baltic salmonids between trout mariculture facilities in Finnish waters (see REF : 423). Additionally, the increased prevalence of the infestation of herring and sprat from the Gulf of Finland and the Gulf of Riga with the sporozoan parasite Eimeria sardinae (see REF : 669) has been observed.
Apart from the Ichthyophonus epizootic in herring, the most spectacular disease occurring in Baltic Sea fish species is the M-74 syndrome of Atlantic salmon, which continues to cause high mortalities (80-90 %, (see REF : 270)) in yolk-sac fry obtained from wild salmon and artificially reared salmon for re-stocking purposes within Swedish and Finnish compensatory stock-enhancement programmes. The syndrome has been known since 1974, but massive mortalities (>50 %) have only occurred since 1992 (see REF : 269).
Whilst in previous years direct effects of environmental contaminants on the reproductive success of adult wild salmon were discussed as possible causes of the M-74 syndrome, current information indicates that a vitamin deficiency (vitamin B1, thiamine), due to the feeding of the adults on clupeids with high thiaminase contents, might be the main causal factor (see REF : 270). For example, experimental studies have shown, that the addition of thiamine to the water reduces both the behavioural M-74 characteristics and the mortality of salmon fry significantly. However, although this therapy seems to be promising for the enhancement programme, the wild Baltic salmon stock will probably still continue to suffer from M-74 and natural reproduction will be seriously endangered.
Some other significant diseases or parasites studied in different Baltic Sea fish species during previous years are
From a number of studies, carried out in the Baltic Sea, there is strong indication of a link between anthropogenic environmental changes, e.g., industrial and urban pollution, eutrophication and thermal discharges, and the occurrence of elevated prevalences of certain fish diseases and parasites in certain areas. Some of these studies dealt with
The results of these studies support the assumption, that changes in the prevalence or intensity of diseases and parasites in wild fish can be used as an indicator of biological effects of contaminants and other anthropogenic activities, and that studies of fish diseases/parasites in wild fish should, therefore, be included in Baltic Sea (and other) monitoring and assessment programmes on changes of the quality of the marine environment.
There is little conclusive information regarding the impact of fish diseases or parasites on mortalities in Baltic Sea fish stocks. In only a few cases have mortalities, induced by specific diseases or environmental factors, been so obvious that the assumption seems justified that these conditions might have had a significant effect on stock size in the affected fish species. Examples are mortalities in garfish (Belone belone) at the south coast of Sweden due to gas super-saturation associated with thermal discharge effluents from a nuclear power plant, mortalities in perch (Perca fluviatilis) attributed to effects of pulp-mill effluents, and mortalities in western spring-spawning herring due to the Ichthyophonus sp. epidemic. The most recent case is the heavy mortalities in yolk-sac fry of the Baltic stock of Atlantic salmon (Salmo salar) due to the M-74 syndrome.
The Ichthyophonus sp. epidemic in herring has so far been the only disease, affecting parts of the Baltic Sea, in which attempts have been made to estimate mortality due to the infestation and the resultant impact on the stock. However, attempts to quantify disease-induced mortalities in general suffer from a number of confounding factors, such as
Because of these variables, it has so far not been possible to provide conclusive evidence for an Ichthyophonus-induced herring-stock decline nor to develop realistic models for mortality rates due to the infestation. From this experience with a disease condition, which has been very prevalent in the affected herring stocks, and which is considered to be highly lethal for infected individuals, it seems questionable whether disease-induced mortalities and associated effects on the population can be estimated by applying current methods of epidemiology and stock assessment.