Regional patterns in prevalence of principal external diseases of dab Limanda limanda in the North Sea and adjacent areas 1992-1997

The prevalence and spatial distribution of major diseases of dab Limanda limanda in the North Sea and adjacent areas were studied in the summers 1992 to 1997. Areas covered were the North Sea, Irish Sea, northern and northeastern British Waters and the English Channel. The diseases studied were lymphocystis, epidermal hyperplasia/papilloma and skin ulceration. To standardise data, results were analysed for females >15 cm (>3 yr old). Data were subjected to median polish, and additive, extended and additive plus multiplicative models were applied to best account for effects of region and year. Annual differences in disease prevalence were low whilst differences between areas were pronounced. For lymphocystis higher prevalence was observed in the northwestern sector of the North Sea, at the northern tip of Scotland and in an area south of Iceland. Prevalence was low in the Irish Sea, the English Channel and the southern North Sea, and intermediate in the German Bight. For epidermal hyperplasia/papilloma, levels were low at Icelandic stations, in the northern Irish Sea, in the southern North Sea and the English Channel, whilst levels were high in the northwestern part of the North Sea and the German Bight. Elevated levels of skin ulceration were found on the Dogger, at 1 station in the Irish Sea (off Sellafield) and at 1 station to the south of Iceland. Lower levels were detected west of Iceland. Prevalence in all other areas was intermediate. It is concluded that a detailed analysis of available data on disease prevalence and putative causative factors is desirable and, given the good availability of data, would be a promising step forward toward elucidating possible cause and effect relationships between diseases and anthropogenic factors.     

Epidemiology of pseudobranchial tumours in Atlantic cod(Gadus morhua) from the North Sea and the Baltic Sea

Investigations during several cruises (1979–1981) showed that Atlantic cod with pseudobranchial tumours occurred in all areas investigated. Statistical analysis of data revealed that there was a marked prevalence of the condition in the centre of the German Bight, whereas a lower incidence of the condition was observed on the Dogger Bank, off the Dutch coast and in the western Baltic Sea. The higher prevalence of pseudobranchial tumours in the German Bight coincides with a dense population of cod and impacts of pollution.     

Seasonal changes in frequency of diseases in dab,Limanda limanda,from the southern North Sea

Infestation rates of flatfishLimanda limanda from the southern North Sea (German Bight) were investigated in 1981 and 1982. At three stations, including a dumping area of acid-iron wastes northwest of Helgoland, dabs were examined for the occurrence of lymphocystis and epidermal papillomas. Marked seasonal changes of infestation rates were observed with maxima of diseased fish in March and May (epidermal papillomas 2 % in 1981, 4.1 % in 1982; lymphocystis 7.5 % in 1981, 17.2 % in 1982). Infestation minima were found from June to October. Differences in incidence rates were observed between stations in winter and spring, but not in summer. Both diseases turned out to be size-dependent. Seasonality of the two diseases is discussed in relation to spawning cycle and water quality, including pollution.     

Review on the effects of pollution on marine fish life and fisheries in the North Sea

Information on possible effects of pollution on fish life is reviewed with special emphasis to the North Sea. Concentrations of heavy metals are high in onshore waters, sediments and organisms. Offshore regions are considered to be not endangered. In estuaries concentrations of certain heavy metals are often surpassing thresholds, which have been experimentally demonstrated to produce sublethal effects on marine organisms. Also mussel culture areas had to be closed due to excess of cadmium contamination. Mercury in fillets of cod in the southern North Sea sometimes reaches 0.3 ppm representing 1/3 of the legal limit. Reductions of inputs are necessary due to the bioaccumulative and toxic potential of heavy metals. Organochlorine contamination of water, sediments and organisms is higher in the southern than in the northern North Sea. In offshore areas clearly elevated levels of PCB's, which are the dominating organochlorine compound in marine organisms, have been analyzed. At present only a minor fraction of total organochlorine residues accumulated in marine biota is analyzed. Margins between experimentally derived lethal levels and those occurring in marine biota are narrow suggesting that sublethal effects might occur. One of the special concerns in the context of organochlorine contamination has to be seen in the potential of these substances affecting the reproductive capacity of marine organisms. Reduction measures are urgently necessary and should include substances with bioaccumulative and toxicological potential, which are not presently included in Marine Monitoring Programmes. Impact of oil pollution is considered to be a regional problem, although especially in estuaries concentrations in the water column might be high. Input through oil platforms has led to local reductions in benthic communities. Measures have to be taken, to further reduce these inputs also in view to avoid contamination of fish in the vicinity of oil fields. Oxygen depletion in German and Danish coastal waters has been shown to be a severe degradation in coastal water quality resulting in significant impact on fish populations. Indications for large-scale avoidance reactions of low DO-areas and mortalities of fish and benthos organisms have been demonstrated. It is suspected that the low DO-situations are related to increased nutrients input and high phytoplankton production. Improvement of the situation can only be expected, if input of nutrients is reduced. Reasons for the occurrence of toxic plankton blooms are not understood. An intensified research is necessary, to avoid damage to fisheries and mariculture. Although activity of radionucleids in North Sea waters is increased due to input of radioactive material from La Hague and Sellafield reprocessing units, the present levels of activity are not considered to be a threat for marine life. The same holds for thermal pollution, which is, depending on the recipient area, expected to have only local effects. Sewage sludge dumping and discharge of untreated sewage introduce significant amounts of accompanying heavy metals, which can be expected to have a negative influence on the total metal budget of recipient waters. Nutrient input through sewage will exhibit only local effects. Dumping of wastes from titanium dioxide industry are suspected to be causally linked to the occurrence of increased prevalences of certain diseases of dab in the German Bight. Present management strategies should take account of the fact that through dumping of waste from titanium dioxide production significant quantities of heavy metal are introduced into the southern North Sea. Also the incineration of chemical wastes at sea should be reduced due to uncertainties concerning quantities and toxicity of incineration products and their potential impact on marine life. It is concluded that clear evidence exists for pollution impact in estuarine regions. Most of the major estuaries at the North Sea coast show benthos reductions, which can be related to water quality and it has to be assumed that areas of a sublethal effect are located in the vicinity of those areas of well demonstrated effects. They are of unknown size and attempts have to be undertaken to quantify areas of sublethal impact. The occurrence of fish diseases is interpreted as an expression of degraded water quality. Some of the diseases detected can produce mortality, thereby having an as yet unquantified impact on fish populations. The presence of fish diseases also reduces the marketability of catches. It cannot be excluded that changes in species composition of fish populations in German Wadden Sea areas are related to pollution. Concentrations of organochlorine substances in fishes of the southern North Sea give rise to the concern that their reproductive capability might be impaired. Future work has to be directed to this problem. Impact of pollution on populations of marine organisms has so far not been demonstrated.     

Assessment of some innate immune responses in dab (Limanda limanda L.) from the North Sea as part of an integrated biological effects monitoring

The marine flatfish dab (Limanda limanda), which lives in direct contact with contaminated sediments, is frequently used as a sentinel species in international monitoring programmes on the biological effects of contaminants. In this study, immune responses were recorded as indicators of sublethal chronic effects of contaminants, in addition to measurement of the induction of mono-oxygenase ethoxyresorufin O-deethylase (EROD) in liver cells, the inhibition of acetylcholin esterase (AChE) in muscle and a quantification of grossly visible diseases and parasites. In total, 336 dab were analysed from five sampling areas in the North Sea, including the German Bight, the Dogger Bank, the Firth of Forth, and two locations close to oil and gas platforms (Ekofisk and Danfield). When considering plasma lysozyme levels, pinocytosis and respiratory burst activity of head kidney leucocytes, a clear gradient could be observed with decreased levels in individuals collected from the Firth of Forth and locations near the oil or gas platforms compared with dab from the Dogger Bank or the German Bight. Individuals with induced EROD activity displayed reduced lysozyme and respiratory burst activities. Lysozyme levels were also reduced in dab with lymphocystis or with nematodes. The data obtained indicate that the assessment of innate immune parameters in a monitoring programme provides supplementary information about immunomodulatory effects associated with the exposure of fish to contaminants. In particular, concentrations of plasma lysozyme, which can be analysed in an easy and inexpensive assay, are considered to be an appropriate parameter for use in a battery of other bioindicators. Communicated by H. von Westernhagen and A. Diamant     

Histology of pseudobranchial tumours in Atlantic cod(Gadus morhua) from the North Sea and the Baltic Sea

Atlantic cod bearing pseudobranchial tumours were investigated from both North Sea and Baltic Sea areas. Light and electron microscopic studies revealed a tumorous structure previously described in cod from Atlantic and Pacific waters. Additionally, cysts, multinucleated cells and early stages of mitosis are described. These findings support the hypothesis that pseudobranchial tumours are instigated by a protozoan infection causing xenotumours associated with the pseudobranch organ. The cells contained in these lesions showed similarities to amoebae.     

The frequency, distribution, and pathology of three diseases of demersal fishes in the Bering Sea

During September and October, 1975, bottom-dwelling fish from the Bering Sea were examined for diseases. The three most commonly observed diseases and their frequency of occurrence were epidermal papillomas of rock sole ( Lepidopsetta bilineata Ayres), 1-0%; lymphocystis of yellowfln sole ( Limanda aspera Pallas), 2-1 %; and epidermal tumours of the pseudobranch of adult Pacific cod ( Gadus macrocephalus Tilesius), 7-4%. Both the appearance and the histological characteristics of epidermal papillomas on rock sole resembled those found on several species of flatfish in Puget Sound. The frequency of tumour-bearing rock sole ranged from 0-23%, with the highest occurrence at sampling stations around the periphery of the Bering Sea in water depths of 20-30 fathoms (36-54 m). The virus-caused lymphocystis lesions were usually on the 'blind' side fins and skin surfaces of yellowfin sole, the largest lesions being found on the operculum, and the most common site of infection was the pectoral fin. The frequency of lymphocystis was 0-15 %, with a gradual increase of prevalence from the northern to southern sampling stations. The Pacific cod tumours were always bilateral, and contained remnants of normal-appearing pseudobranchial tissue on the surface or deep inside the tumours. Portions of some tumours were necrotic and liquefied. Tumour-bearing cod were most commonly found in the south and south-eastern Bering Sea and the frequency of affected cod ranged from 0-100%.     

Dynamics of fish diseases in the lower Elbe River

As part of a survey on population dynamics and ecology of fishesin the Elbe River, seasonal and regional fluctuations of external fish diseases were studied from the open North Sea to Hamburg in 1981–1982. Clinical signs of 11 different diseases, several of them not being recorded before, were noted in 22 fish species. Averaged over all samples, the total disease prevalence was below 1 % in 16 species. Highest prevalences were found in smelt (12.7 %), eel (9.2 %), and flounder (5.5 %). The frequency of most diseases increased in larger (older) fish. High prevalences of skeletal abnormalities in cod could be related to abnormal migration habits of diseased fish. Spawning papillomatosis, skin ulceration, and fin rot in smelt occurred predominantly during the spawning season. Most diseases observed occurred at relatively high prevalences in the central Elbe estuary between Cuxhaven and Brunsbüttel. This is most obvious for lymphocystis, fin rot, skin ulceration, and bleaching syndrome in flounder, but such a tendency also seems to occur in cauliflower disease of eel, as well as spawning papillomatosis and pharyngeal granuloma in smelt. This area is less heavily polluted and less frequently affected by oxygen deficiency than the area upstream of Glückstadt, where diseases in general occurred at lower frequencies. Therefore, it is concluded that neither pollution nor lack of oxygen are the main triggers for the outbreak of diseases in Elbe fish. It is supposed that large tidal fluctuations of salinity are a major stress factor for fish in the estuary between Cuxhaven and Brunsbüttel. Flounder from this area usually are in a relatively bad nutritional state. Their condition factor increases significantly towards Hamburg, while their disease prevalence decreases in the same direction.     

Some differences in the parasitic fauna of three samples of plaice (Pleuronectesplatessa L.) from the southern North Sea

Plaice ( Pleuronectesplatessa L.) from three spawning areas in the southern North Sea were examined for six species of metazoan parasite. Differences of both incidence and level of infestation with particular parasite species were found in fish from the three areas. Neither the incidence nor the level of infestation was associated with the length of fish in the samples except in the case of the larger male fish from Flamborough, which tended to contain more larval nematodes (Anisakidae). All fish contained the intestinal nematode Cucullanus heterochrous , although fewer were found in fish from Flamborough. Southern Bight fish were more likely to contain Capillaria wickinsi than were fish from Flamborough or the German Bight. Flamborough fish were more likely to contain Anisakidae larvae than were fish from the other two areas. Fish from the three areas were similarly infested with Zoogonoides viviparus. Male fish from Flamborough were less likely to be infested with Lepeophtheirus pectoralis than male fish from the German Bight.     

Mapping the spawning grounds of North Sea cod (Gadus morhua) by direct and indirect means

Despite recent evidence for sub-stock structuring, North Sea cod are assessed as a single unit. As a consequence, knowledge of sub-stock trends is poor. In particular, there are no recent evaluations of which spawning grounds are active. Here we report results from the first ichthyoplankton survey to cover the whole North Sea. Also, this survey, conducted in 2004, was the first to make extensive use of DNA-based molecular methods to unambiguously identify early developmental stage cod eggs. We compare the findings from the plankton survey with estimated egg production inferred from the distribution of mature cod in contemporaneous trawl surveys. Results from both approaches were in general agreement and showed hot spots of egg production around the southern and eastern edges of the Dogger Bank, in the German Bight, the Moray Firth and to the east of the Shetlands. These areas broadly coincide with known spawning locations from the period 1940 to 1970. We were, however, unable to directly detect significant numbers of cod eggs at the historic spawning ground off Flamborough (northeast coast of England). The results demonstrate that most of the major spawning grounds of cod in the North Sea are still active but that some localized populations may have been reduced to the point where it is now difficult to detect the presence of eggs in the plankton.     

Prevalence of viral haemorrhagic septicaemia virus in Danish marine fishes and its occurrence in new host species

In order to analyse the occurrence of viral haemorrhagic septicaemia virus (VHSV) in the marine waters around Denmark, staff from the Danish Institute for Food and Veterinary Research participated in 5 research cruises during 1998 to 2002 as a follow-up to 4 research cruises performed in 1996 to 1997. In total, 16,655 fish were examined virologically as 3569 samples. Forty fish species and 3 invertebrate species were represented. VHSV was isolated from 133 samples representing 8 species: herring Clupea harengus, sprat Sprattus sprattus, dab Limanda limanda, flounder Platichthys flesus, plaice Pleuronectes platessa, cod Gadus morhua, sand eel Ammodytes sp. and sand goby Pomatochistus minutus. Calculations showed that VHSV was more prevalent in the Baltic Sea in an area between Zealand and the island of Bornholm and the waters surrounding Bornholm than in the Kattegat, Skagerrak and along the North Sea coast of Denmark. This is the first report on the isolation of VHSV from dab, flounder and plaice and the first publication on VHSV from sand eel from Europe and sand goby.     

Effects of survey scale and water depth on the assessment of spatial distribution patterns of selected fish in the northern North Sea showing different levels of aggregation

Geostatistics was employed to investigate spatial structuring of herring, cod, dab, haddock and whiting at different spatial scales in the northern North Sea. Additionally, a structural analysis of the maximum water depth was carried out to assess habitat associations of fish. Linear, spherical, exponential and Gaussian models were fitted to the semivariograms, showing clear spatial autocorrelations. At the smaller scale, spatial structuring was weak for haddock, herring and dab, increasing at the greater spatial scale, with the exception of whiting. Mean catch rates, estimated classically and geostatistically, were in good agreement. Corresponding variances were clearly reduced at both spatial scales, when accounting for spatial distribution of the fish. At the greater survey scale a high level of habitat association was detected for haddock and whiting, while a poor habitat association was found for cod, dab and herring. The smaller scale seems to be the threshold at which spatial structuring of “cpue” could have marked influence on estimation error. Thus, survey scale is important when analysing spatial patterns and estimating mean biomass indices, and a sound analysis of relations in spatial structuring of fish and habitat conditions is essential to derive more precise estimates.     

Salinity gradient shapes distance decay of similarity among parasite communities in three marine fishes

Published data were used to compare the distance decay of similarity in parasite communities of three marine fish hosts: Atlantic cod Gadus morhua, the dab Limanda limanda and the flounder Platichthys flesus in two adjacent areas that differ with respect to the strength of a salinity gradient. In the Baltic Sea, which exhibits a strong salinity gradient from its connection with the North Sea in the west to its head in the north‐east, parasite communities in all three fish hosts showed a significant decline of similarity with increasing distance. In contrast, among host populations in the North Sea, which is a fully marine environment, there was no such decline or only a weak relationship. The results suggest that environmental gradients like salinity can be strong driving forces behind patterns of distance decay in parasite communities of fishes.     

Species assemblages of pelagic fish embryos in the southern North Sea between 1984 and 2000

The occurrence and abundance of the pelagic eggs of southern North Sea spring-spawning fish were analysed between 1984 and 2000. Species number varied between six (1986) and 14 (1999) and was positively correlated with sea surface temperature. With one exception, dab eggs were always the most abundant and usually highly dominant. Ranking of species depended on temperature, but no significant differences in ranking between years was discernible. Although with the increase in temperature in the 1990s a change in species assemblage was evident [species belonging to the boreal-Mediterranean (Lusitanian) group became more apparent in the species assemblage] this did not lead to an increased species diversity (Shannon Index) or a change in other community parameters. It appears that the recent developments regarding spawning stock biomass of commercial North Sea fish is reflected in the declining egg abundance of the respective commercial and larger species (i.e. cod, flounder, plaice) and an increase in abundance of the eggs of small species (i.e. long rough dab, rockling) over the years. Received in revised form: 5 November 2001 Electronic Publication     

Annual cycles of apoptosis, DNA strand breaks, heat shock proteins, and metallothionen isoforms in dab (Limanda limanda): influences of natural factors and consequences for biological effect monitoring

The present study was undertaken to investigate the influence of natural and anthropogenic stressors on the induction of apoptosis, metallothionein (MT) isoforms, heat shock proteins and DNA strand breaks in the marine flatfish dab (Limbanda limanda) Seasonal changes and possible physiological influences were evaluated over a 1-year period at a fixed location northwest of Helgoland in the German Bight. These results were compared with data from sampling sites in the North Sea and the Baltic Sea. Annual cycles could be observed for all parameters except for Cd. The data revealed that changes in biomarker are not only linked to physiological processes related to reproduction but also to factors like water temperature changes, lipid content and zinc content. Cd and organochlorines had no influence on biomarkers whereas an influence of Cd on MT levels revealed in the regional observations was possibly masked by the major changes in Zn content during the annual cycle. Due to different abiotic factors we supposed that the annual cycles at each sampling site in the North and the Baltic Sea might be shifted temporally and therefore measurements at different locations during a small time window of a few weeks may lead to misinterpretation in biomarker research.     

Annual cycles of apoptosis,DNA strand breaks,heat shock proteins,and metallothionen isoforms in dab (Limanda limanda): influences of natural factors and consequences for biological effect monitoring

The present study was undertaken to investigate the influence of natural and anthropogenic stressors on the induction of apoptosis, metallothionein (MT) isoforms, heat shock proteins and DNA strand breaks in the marine flatfish dab (Limbanda limanda) Seasonal changes and possible physiological influences were evaluated over a 1-year period at a fixed location northwest of Helgoland in the German Bight. These results were compared with data from sampling sites in the North Sea and the Baltic Sea. Annual cycles could be observed for all parameters except for Cd. The data revealed that changes in biomarker are not only linked to physiological processes related to reproduction but also to factors like water temperature changes, lipid content and zinc content. Cd and organochlorines had no influence on biomarkers whereas an influence of Cd on MT levels revealed in the regional observations was possibly masked by the major changes in Zn content during the annual cycle. Due to different abiotic factors we supposed that the annual cycles at each sampling site in the North and the Baltic Sea might be shifted temporally and therefore measurements at different locations during a small time window of a few weeks may lead to misinterpretation in biomarker research.     

The forth estuary: a nursery and overwintering area for North Sea fishes

The fish community of the Forth estuary, Scotland, has several components — estuarine resident species, diadromus migratory species, marine and freshwater adventitious species, marine juvenile migrants using the area as a nursery, and adults of marine species with seasonal migrations. The population changes during the period 1981–1988 in six species representative of the last two categories are described here. The species are the juvenile gadoids, whiting, Merlangius merlangus (L.) and cod, Gadus morhua L., juvenile flatfish, plaice, Pleuronectes platessa L. and common dab, Limanda limanda (L.), and the adult clupeids, sprat, Sprattus sprattus (L.) and herring, Clupea harengus L.The species' population changes are described in relation to aspects of wider significance such as commercial fishing for these species in North Sea areas. In particular, the underlying decline of the juvenile cod populations since 1981 in the Forth Estuary is discussed in relation to the recent reduction in commercial fishing quotas.     

Humber estuary — a case study

This paper presents the results of an extensive and detailed survey conducted in 1985 on the biology, bacteriology and levels of heavy metals in subtidal sediments of the Humber estuary. The findings of the investigation are assessed with respect to environmental quality. Abundance and distribution patterns of the indicator species Capitella capitata, the numbers of faecal bacteria, and levels of heavy metals and organic matter are examined in relation to areas of sewage disposal. The possible effects of inputs from the Humber to the North Sea are discussed.     

Francisella noatunensis subsp. noatunensis is the aetiological agent of visceral granulomatosis in wild Atlantic cod Gadus morhua

During the 1980s and 1990s wild-caught cod displaying visceral granulomatosis were sporadically identified from the southern North Sea. Presumptive diagnoses at the time included mycobacterial infection, although mycobacteria were never cultivated or observed histologically from these fish. Farmed cod in Norway displaying gross pathology similar to that identified previously in cod from the southern North Sea were recently discovered to be infected with the bacterium Francisella noatunensis subsp, noatunensis. Archived formalin-fixed paraffin-embedded tissues from the original North Sea cases were investigated for the presence of Mycobacterium spp. and Francisella spp. using real-time polymerase chain reaction, DNA sequencing and immunohistochemistry. Whilst no evidence of mycobacterial infection was found, F. noatunensis subsp. noatunensis was identified in association with pathological changes consistent with Francisella infections described from farmed cod in recent years. This study shows that francisellosis occurred in wild-caught cod in the southern North Sea in the 1980s and 1990s and demonstrates that this disease predates intensive aquaculture of cod.     

Ethoxyresorufin O-deethylase (EROD) activity in the liver of dab (Limanda limanda L.) and flounder (Platichthys flesus L.) from the German Bight. EROD expression and tissue contamination

Ethoxyresorufin O-deethylase (EROD) activity was measured in the liver of dab (Limanda limanda) and flounder (Platichthys flesus) from the German Bight (southern North Sea) and compared with muscle and liver polychlorinated biphenyl (PCB) concentrations in an attempt to relate EROD activity to PCB body burden. In none of the different datasets (species-, tissue- or matrix-dependent) was a significant (P<0.05) correlation between PCB tissue contamination and EROD activity found. Yet EROD activity was significantly correlated with polycyclic aromatic hydrocarbons (PAH) levels (phenanthrene, fluoranthene, pyrene) in muscle tissue, indicating a possible dependence of EROD expression on other ubiquitous organic contaminants, thus making it a suitable biomarker for general pollution. Received: 14 April 1999 / Received in revised form: 10 July 1999 / Accepted: 15 July 1999