510 BIOLOGY Of THE SEAS OF THE U.S.S.R. 



had caused a rise of salinity. As marine biocoenoses advance, the brackish 

 water and relict biocoenoses recede eastwards. This is particularly noticeable 

 in the case of the Monodacna biocoenosis in the east and those of Cardium 

 and Syndesmya in the west. 



The benthos biomass of the Sea of Azov undergoes considerable changes 

 from spring to autumn. Generally it is doubled, but not in all sectors ; at times 

 it remains unchanged, at times it is reduced. The absence of changes in the 

 biomass may be the result either of poor productivity, or high mortality, or a 

 considerable consumption by fish. Throughout all the central part of the 

 Sea the biomass remains almost unchanged ; the cause of this must be sought 

 in the low productivity of the Hydrobia grouping as a result of unfavour- 

 able living conditions and suffocation. The absence of increase in biomass in 

 the coastal sectors of the eastern and northern part of the Sea, in the 5 to 6 m 

 zone, is attributable to considerable consumption by fish, since in the summer 

 bream, roach, starred sturgeon and bullheads are concentrated here, especially 

 in the eastern part of the Sea. In these areas benthos consumption by fish may 

 be so intensive that the biomass decreases. It is particularly intensive off the 

 Achuev and Akhtarsk inlets, on the Zhelezinskaya and Eleninskaya Banks, 

 at the entrance into the Gulf of Taganrog and in some other areas. Shoals of 

 commercial fish are most frequently found in these places. Vorobieff based the 

 organization of a commercial survey on these data which he had obtained, 

 and his expectations were to a great extent justified. In the Sea of Azov the 

 consumption of benthos by fish rarely takes on a catastrophic character. 

 Benthos left over in the autumn is represented, apart from the older age 

 groups, by the numerous young, and the biomass may not only be restored 

 later on account of its growth, but may even be increased. It may be assumed 

 that greater consumption corresponds to a greater concentration of fish. In 

 his calculations of the amount of benthos consumed by fish Vorobieff takes 

 50 g/m as unity. Vorobieff fixed the grounds where fish would probably shoal 

 for feeding in a similiar manner by examining the dynamics of benthos and 

 the transition of one community into another, as a result of fish eating the 

 benthos in spring time. 



The distribution of benthos biomass in the Sea of Azov (see Fig. 238b) is 

 very irregular and is characterized by considerable patchiness. Areas of high 

 biomass alternate with sectors of very low biomass. In the open part of the 

 Sea of Azov, in spite of its considerable variegation, it is possible to trace a 

 concentric distribution of zones of increasing biomass from the centre of the 

 Sea to its periphery, followed by a fall in biomass as the coast approaches. 

 The outline of the biomass in a latitudinal cross section passing through the 

 central impoverished zone is shown in Fig. 240 ; the ring of high biomass en- 

 circling the central deeper part is very evident here. Given the phenomenon 

 occurring in the Sea of Azov of the suffocation of bottom fauna, and the 

 equally massive phenomenon of the consumption of the fauna by fish, the 

 huge spat-fall of larvae and the subsequent development of mollusc popu- 

 lations of uniform age can proceed over the areas — and in some years they 

 are very wide areas — which have been freed from living organisms. The dis- 

 tribution of the large number of larvae is controlled by the direction of the 



