Population dynamics and ecology of the invasive veined rapa whelk, Rapana venosa in the southern Black Sea

Abstract

Spatial and temporal changes in some ecological characteristics (i.e. biomass, abundance, morphometrics, sex-composition, growth parameters and population dynamics) of the invasive veined rapa whelk (Rapana venosa) were studied off Sinop Bay, in the southern Black Sea. The whelk specimens were sampled from three depths (15 m, 25 m and 35 m) at monthly intervals between November 2005 and October 2007. Custom-made pots were deployed for the capture of R. venosa individuals. Monthly distributions in whelk biomass and abundance peaked in summer (June/July) and mid-autumn (October) and displayed no differences between 2006 and 2007. With the exception of gut weight, all morphometric variables produced three peak periods (January, June and September, respectively) over one year. Densities, morphometrics and sex composition were dictated significantly by bottom depth and monthly variation. Two stages were defined in the ovary and testis index maxima denoting longer spawning and recruitments periods starting as early as in March which is a different finding compared to previous studies from the Black Sea. For the estimation of population growth parameters, of the five morphometric variables analysed, siphon width was the best variable to produce clear size cohorts. Whelk growth in terms of shell length and width oscillated seasonally and ceased during February–March. Maximum age of the veined rapa whelk in the southern Black Sea was determined as 3.5 years corresponding to 7 cohorts. Among the environmental parameters, dissolved oxygen, temperature and salinity appeared to affect whelk densities. Results obtained here are important for better management of the whelk fishery in the Black Sea.