MARINE ECOLOGY PROGRESSSERIES Published December 31 Mar. Ecol. Prog. Ser.

Gross and in fish: summary

A. D. Vethaak

Ministry of Transport and Public Works, PublicWorks Department,Tidal Waters Division, Ecotoxicology Section, PO Box 20906,2500EX The Hague,The Netherlands

INTRODUCTION use of research vessels, often in poor working condi- tions, and does not allow the use of complex laboratory The principal aim of the group of papers on gross techniques. Furthermore, if epidemiological data are pathology and histopathology was to assess the value to be accurately interpreted, they must be based on ex- of epidemiological studies of in dab Lin~anda tensive and continued sampling lasting several years, limandaand of quantitative histological indices in the and on knowledge about the biology of the populations same species, for monitoring environmental quality in studied, particularly their movement patterns over offshore waters. long periods of time. To a certain extent, therefore, the The Bremerhaven Workshop provided a unique op- interpretation of the data collected during this study portunity for a comparative study, with a comprehen- has to depend largely on relevant published findings. sive chemical programme as a background, and with a variety of other techniques being used to study bioche- mical and molecular/cellular effects in the same dab EPIDEMIOLOGY OF EXTERNALLY VISIBLE population. AND GROSS LIVER NODULES The occurrence of the following parameters was quantified at 6 stations representing a known pollution Two replicate surveys were aimed at defining spatial gradient in the German Bight: (1) four externally visi- variation in disease prevalence along the pollution ble diseases and gross liver nodules (with associated gradient, and at establishing relationships with conta- confirmatory histological diagnosis); (2) histopatho- minant concentrations and selected other potential logically identified lesions in livers without gross nod- causal factors. Of the 4 external diseases (lyrnpho- ules; (3) melanomacrophage centres (MMCs) in the cystis, epidermal /papilloma, skin ulcera- spleen. tions (acute and healing stages),and X-cell gill lesion), The first two of these categories are dealt with in a the findings of the surveys, together with published single paper, which in view of the extensive amount of evidence, suggest that epidermal hyperplasia/papil- fieldwork involved was of necessity a team effort in- loma seems to be the most promising for biological volving all participants. The third category is closely effects monitoring. The prevalence of this disease related to the first two, but has a paper to itself. shows general agreement with the chemical data, The studies of the group were focused on a single showing decreasing levels in an offshore direction species (dab) in a specific area (the German Bight), from Stn 3 to 9. The other 3 diseases showed no clear with sampling during one particular month of the year trend, or in the case of lymphocystis, the opposite trend. (March). The results cannot necessarily be applied Gross liver nodules 2 2 mm in diameter were found more generally to other species, areas or seasons in the at most of the stations, and histological investigation North Sea. revealed that 57.5 % of them represented neoplastic It may be useful here to outline some of the limita- and putative pre-neoplastic lesions. Smaller liver tions of the epidemiological approach to fish disease nodules and other lesions detected during gross research. It is normally dependent on labour-intensive examination did not represent neoplastic lesions, but

O Inter-Research 1992 BREMERHAVEN WORKSHOP: GROSS AND HISTO-PATHOLOGY

about half the cases corresponded to putative pre- porting tests will be necessary to investigate the immu- neoplastic lesions. No spatial trends could be estab- nological significance of MMCs in spleen. The slow ac- lished for these histologically identified lesions due to cumulation of haemosiderin within MMCs, coupled their low prevalences. This was probably related to a with effects of fish migration, may make this technique scarcity of large fish, which have the highest prevalen- inappropriate for measuring short-term stress at speci- ces of liver nodules. The data do confirm, however, the fic sampling sites. importance of histological confirmation of lesions iden- tified during gross examination. The epidemiological study also provided a test of the CONCLUDING REMARKS published ICES guidelines for the implementation of fish disease surveys. It appears from the present data Despite the apparent promise of epidermal hyper- that some of the recommended minimum sample sizes plasia/papilloma for biological effects monitoring, its are too small. On the other hand, the observed scarcity aetiology, though probably infectious, remains uncer- of large fish at certain stations makes it impractical to tain and further studies and experiments are needed to attain the minimal sample sizes, thus confounding any clarify its significance as a pollution indicator. attempts to establish spatial trends for liver neoplasia. The inconclusive findings regarding liver neoplasia and associated lesions do not justify their exclusion from future monitoring studies. There is substantial HISTOPATHOLOGICAL INVESTIGATION OF evidence in the literature that similar lesions in other APPARENTLY HEALTHY LIVERS flatfish species, especially those inhabiting North American waters, have a chemical-related cause. Some of these livers were found to bear putative pre- However, further evidence is needed to establish a neoplastic lesions, but again low prevalences together causal relationship in dab. It could be argued that the with small sample sizes prevented the establishment of occurrence of liver neoplasia in North Sea dab indi- any spatial trends in relation to pollution. cates exposure to carcinogenic compounds, thereby representing one of the few critical toxicological end points proposed to date for the marine environment. MELANOMACROPHAGE CENTRES (MMCs) IN Results obtained by other techniques deployed at the THE SPLEEN workshop show clear biochemical effects and early in- dications of changes in liver function which correspond The approach consisted of investigating spatial var- well with the known pollution gradient; these changes iability in the occurrence of haemosiderin, a cellular could ultimately lead to liver diseases such as neo- breakdown product found in MMCs, in sections of dab plasia. It remains possible, however, that the chronic spleens. Attempts were then made to relate this varia- nature of liver neoplasia, coupled with the migratory bility to the presence of external diseases and to conta- beh.aviour of the target species and the possible elimi- minant concentrations in the liver. nation of diseased individuals from the population, Haemosiderin occurrence was lowest at Stns 3 & 9 at makes the disease inappropriate for measuring toxic either end of the transect. Because MMCs may be a effects. Nevertheless, liver neoplasia would still have possible indicator of the function of the non-specific toxicological and ecological significance. immune system, these findings could be interpreted as Information in several areas, essential to establishing suggesting a contamination-associated immunosup- the validity of an epidemiological approach, is needed pressive effect at these 2 stations. On the other hand, before any of the above conditions can be adopted as the spatial pattern does not correspond to the observed reliable indicators. These areas include the migratory pollution gradient and at the same time, the index patterns of North Sea dab populations, the natural based on MMCs did not appear to be linked to the background levels of disease in the absence of pollu- general health of the fish, as measured by disease tion, and th.e existence of cause-and-effect relation- prevalences. ships which can only be elucidated by experimental Due to the Limited scope of the present study, sup- work.