Parameter indices of Rapana population
(Rapana thomassiana Crosse) in the
Kerch Strait during different seasons of 1999/b>
O.V. Yevchenko
Southern Scientific Research Institute of Marine Fisheries
and Oceanography (YugNIRO) Kerch, Ukraine
Rapana thomassiana, being introduced accidentally into the Black Sea in the early forties, was widely distributed all over the Black Sea shelf (Drapkin 1953, Ivanov 1961 a, Gommoin 1972, Goncharov 1977, etc.). By the fifties growth, reproduction, diet of this gastropod had been studied. It proved to be the bitterest enemy of oyster and mussel banks (Chuhchin 1961 a, b,v, Ivanov 1968). Nevertheless it is known, that Rapana has valuable food and pharmacological features. In many countries it is on object of export. In connection with the appearing of commercial interest to Rapana we are continuing the works at further study of main features of Rapana biology.
In August and October 1999 six areas of the Kerch Strait with settlements of Rapana were investigated.
I. -Tuzla spit (northern and eastern parts);
II. -Kamysh Burun;
III. -Geroevskoe I (littoral part);
IV. -Geroevskoe II (offshore part);
V. -Maly cape
VI. -Takil cape
We picked out the areas by the results of inventory survey of 1998. Material collection was made from board of the cutter with cutless Khizhnjak dragnet 1.6 m width and 35 mm between rods. Such drag net allows dragging not burying into soil and to catch mollusk from the surface of bottom (Rubinstean, Zolotarjev, Litvinenko, 1984).
The survey was carried out by sections at the depth from 2 till 6 m on sandy, sandy - silt and mollusk soils. The area of one dragging was 246 m2. During each season 9 stations were carried out.
In dragging catches the quality of mollusk, their size, sex and age were inventoried. A total of 817 mollusks were analyzed.
In summer season of 1999 population was represented by individuals from 53 mm till 125 mm (Fig.1). The mollusk with 60-95 mm of shell length, making up 84.6% of total abundance, with the average length 77.5 mm presented the bulk of the population. The average weight of the mollusk soft tissues was 30.05 g.
Shell height, mm
Fig. 1. Size Structure of the Rapana settlements of the Kerch Straits in summer and autumn 1999:
—————– -summer
____________ -autumn
Abundance of Rapana in the investigated area changed from 0.010specimen/m2 till 0.340 sp/m2, average abundance index was 0.075 sp/m2, biomass varied from 1.25 g/m2 till 28.07 g/m2, average biomass was - 6.77g/m2 .In catch there were mollusks from 2 till 6 years old, the ratio of males and females in this period was 1:1.
In autumn size of mollusk varied from 45 to 110 mm. In the population the specimens of the shell length 55-95 mm prevailed. They made up 87.8% of total abundance.
The average length of shell was 70.8 mm, the average weight of 1specimen - 74.5 g, the average weight of soft tissues - 39.2 g.
In autumn the Rapana density varied from 0.004 to 0.410 sp/m2 (average - 0.064 sp/m2), biomass - from 0.49 till 41.32 g/m2 (average 5.86 g/m2). The age composition of the Gastropod in the catches was represented by specimens from 2 to 4 years old, the ratio of males and females in the period of investigations was 2:1 respectively.
While comparing the results of 2 season surveys the variations of patterns of Rapana population were observed. An average density of mollusk decreased from 0.075 to 0.064 sp/m2, biomass - from 6.77 to 5.86 g/m2. Share of mollusks with the size up to 65 mm increased from 16.8% to 29.3%. An average shell length decreased from 77.5 till 70.8 mm. An average weight of 1 specimen decreased from 79.3 g to 74.5 g and average weight of soft tissues, on the contrary, increased from 30.05 to 39.2 g. Males in the catches prevailed over females and the ratio was 2:1 respectively.
The variations of Rapana population in autumn, were prabably connected with coming of Rapana juveniles mainly of 2 years old up to the ground surface. The specimens of older size groups (shell length of 110-120 mm, 4 – 6 years old), probably, buried oneself in soil.
Comparing average indices of abundance and biomass of mollusks in different areas we conclude that while general tendency to their decreasing from summer to autumn exists, at the local parts these indices were different. To autumn in the area of the Maly cape the Rapana abundance increased from 0.34 to 0.41 sp/m2, biomass – from 28.07 to 41.32 g/m2. In August in the areas of the Tuzla spit (the western part) and the Geroevskoe I the Rapana was not dragged. In October its density was 0.06 sp./ m2 and 0.02 m2, biomass was 2.97 sp./ m2 and 0.93 sp./ m2 respectively. In Kamysh-Burun area Rapana did not occur in drag collections during all the period. In the areas of the Tuzla Split (the northern part) and Geroyevskoye II Rapana density decreased 2-50 times, biomass – 2-30 times from summer to autumn. Thus, in summer 1999 2 settlements of Rapans with high indices of the population parameters were observed: the Maly Cape – abundance 0.34 sp./ m2, biomass – 28.07 sp./ m2 and the northern part of the Tuzla split – abundance – 0.20 sp./ m2, biomass – 20.15 sp./ m2. In autumn seasom in the area of the Maly Cape the abundance 0.41 sp./ m2, biomass – 41.32 g/ m2 was recorded.
Probably Rapana settlements of the Kerch Strait differed from each other by the terms of their burying into the ground and coming out to the surface.
Perhaps the changes observed are natural seasonal fluctuations of Rapana populations connected with its biology peculiarities.
References
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