All the reference and affected stations were compared statistically 

 using the rank-sum test (Dixon and Massey, 1969:344). It was found that 

 hatching success was significantly higher in the reference area (theore- 

 tical hatching, z = -2.74, 0.007 > P > P 0.006, observed hatching, z = 

 -2.96, 0.0032 > P > 0.0026). It is worth noting, however, that one of 

 the polluted stations, Tistelholmen, gave highly variable results, with 

 significant differences between the three sampling occasions (rank-sum 

 test for several samples (Dixon and Massey 1969:345), H observed = 97.9, 

 d.f. = 2, H theoretical 94.6, d.f. = 2, p << 0.01 in both cases). 



9.4 Discussion 



The analysis of herring did not indicate any contamination by oil 

 (Boehm, pers. comro.). However it would be of great interest to trace 

 the path of the oil through the food chain by carrying out oil analyses 

 on flounder ( Pleuronectus flesus) . These fish feed mainly upon Macoma 

 balthica and the blue mussel ( Mytilus edulis ) both of which have been 

 shown to contain considerable amounts of oil. 



The low frequency of spawning by herring in the affected area may 

 indicate effects of oil pollution. It may, however, also be due to the 

 differences that undoubtedly exist between the polluted and reference 

 areas as regards exposure and sediment type distribution on the fairly 

 shallow bottoms, where herring spawn was found. 



On comparison, it is clear that the hatching of herring eggs was 

 less successful in samples from the oil affected area. Significant 

 differences also between samples from within the polluted area may, 

 however, indicate that factors other than oil pollution may have in- 

 fluenced hatching rate. One such factor, repeatedly observed in the 

 hatching experiments, was fungal infection of the roe. From previous 

 studies it is known that the presence of benthic crustaceans decreases 

 the amount of fungal growth on fish roe (Oseid, 1977). In the spring 

 after the oil spill, hardly any adult Gammarids were present in the 

 polluted area. It is therefore possible that their absence led to 

 increased fungal attack on fish roe. 



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