Rujinard Sriwoon and Naruporn Poonsud

DOI: 10.14456/sjst-psu.2019.178

pp. 1404 - 1412

Abstract

The ivory shell, Babylonia areolata, was sampled from commercially harvested resource areas in the lower part of the
Gulf of Thailand: Rusamilae (RSM) and Songkhla (SKL). This study aimed to survey the presence of imposex and compare the
size, frequency distribution, sex ratio, and gonadal stages of ivory shell. Shell length frequency distributions of B. areolata
caught in RSM from October to December 2016 (n=135) ranged from 32.04 to 58.32 mm, and in SKL from October 2016 to
January 2017 (n=149) ranged from 37.8 to 61.95 mm. The largest ivory shell that measured 61.95 mm in shell length (SL) was
recorded at SKL. The mean SL was larger at the RSM site (50.31 mm) than at the SKL site (44.67 mm). Both of the SKL and
RSM populations showed a bimodal length-frequency distribution with the smaller peak at a shell length less than the minimal
landing size (MLS) at the RSM site and a larger peak at a shell length greater than the MLS. The males and females from RSM
reached maturity at smaller sizes (44 mm and 36.49 mm, respectively), whereas the females at SKL reached maturity at 51.00
mm. In order to preserve the local fishery and to characterize the Babylon stock in the area, the reproductive biology of the
species was studied in relation to sex and commercial size (45 to 61 mm). Five reproductive stages were defined for females on
the basis of histological analysis. The shell lengths with mature gonads were 44 mm and 36.49 mm for males and females at
RSM and 51 mm for females at SKL which exceeded the minimal landing size (45 mm). The sex ratio was significantly in favor
of females; therefore, female mortality may not be directly attributable to imposex. Imposex accounted for 2.78% at RSM and
5.94% at SKL. This is the first report of imposex in B. areolata at SKL. Females from SKL showed mature gonads at a size of
about 1.5 times larger (51.00 mm) than females from RSM (36.49 mm). This finding is especially important for a sustainable
management of this fishery, taking into account that there are differences in size at maturity between populations.