The fish species occurring in the area show both local and long 

 distance migrations. Pelagic species (herring, sprat, cod, etc.) show 

 the longest migrations. Littoral species and demersal fish from deep 

 soft bottoms show local migrations (Westin, unpubl.). These are sea- 

 sonal and largely a change of biotope. Thus the season governs the 

 composition of the fish fauna in any given biotope. 



The largest fish concentrations during the season of the Tsesis 

 spill occurred in the pelagic system, where large concentrations of 

 herring are normally found in the general area of the spill (Aneer et 

 al., 1978). However, echosounding surveys in the area failed to demon- 

 strate avoidance behavior by the herring and analyses showed no oil 

 contamination of locally caught herring. 



Possible effects of oil on the local fish fauna are thus likely to 

 be mainly indirect, long-term, and difficult to detect. The impoverished 

 littoral and benthic fauna in the impacted area obviously results in 

 decreased food availability. The oil contamination of bivalves such as 

 Mytilus edulis and Macoma balthica is likely to cause oil contamination 

 of fish utilizing these food items (e.g. the flounder, Platichtys 

 flesus, which preys on Macoma in this area). The most important effect 

 of the spill on fish is likely to be found in species or life stages 

 that are subject to long-term exposure. 



The herring in the Asko area spawn on exposed, fairly shallow 

 bottoms, and an investigation of the main June spawning, half a year 

 after the spill, showed both lower frequency of spawning and lower hatching 

 rate in the spill area, but not a significant increase in the number of 

 malformed larvae. The oil spill is not, however, the only possible 

 explanation for these results. 



3.3 Recommendations for spill research contingency plans 



The people involved in the Tsesis oil spill investigation have made 

 a number of observations which may help in working out contingency 

 research plans. These are: 



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