Morphometric, quantitative and population characteristics oF Rapana venosa (Valenciennes, 1846) in front of Pasha Dere

Abstract

A total of 468 individuals of the rapa whelk (Rapana venosa) were studied in terms of size, weight and sex in the sea area in front of Pasha dere (south of Varna Bay, Black Sea) in the summer of 2016. Sampling was performed by bottom trawling on three transects at different depths (from 9.0 m to 24.5 m) and different type of substrate (sand, sandy silt and silt). In the present study the average length was 6.36 cm and the absolute shell length ranged from a minimum of 3.24 cm to a maximum of 9.68 cm. The established average total weight was 47.31 g, its absolute value ranging from 9.98 g to 135.02 g. The average values of the shell length and total weight of the rapa whelk were the lowest on shallow sandy bottom (6.01 cm and 41.94 g, respectively) and the highest at 21 m depth on silty sediments (6.84 cm and 55.13 g, respectively). The size-weight relationship between all individuals was described by the formula W = 0.248L 2.797 (R 2 = 0.937), with the allometric coefficient (b) ranging from 2.817 to 3.066. On the sandy sediment, the male: female ratio was 2:1; in sandy silt, the percentages of male and female specimens were 55% and 45% respectively, and in silty sediment the sex ratio was 1:1. The male specimens were characterized by higher values of all morphometric parameters compared to the females. In the size distribution of sexes, the highest was the frequency of the males in size class of 6-7 cm, and the females with 5-6 cm shell length. In the weight distribution of the males, the individuals with total weight in the range 30-40 g dominated, while the prevailing female individuals weighed between 20 g and 30 g.