The distribution and reproductive strategies of four shrimp species in the East Sea of Korea

Abstract

In this study the center of gravity (CoG) method indicated the water depths predominantly inhabited by the following species: Neocrangon communis, Argis toyamaensis, Pandalus eous, and Eualus biunguis as 494, 446, 1,002, and 836 m, respectively. N. communis and P. eous were distributed relatively in shallow water depth range while A. toyamaensis and E. biunguis occurred relatively deep water depth range. A. toyamaensis showed a positive correlation with density and depth. P. eous had the narrowest ranges while A. toyamaensis had widest in depth distribution. The sex ratio by depth was significantly difference in four shrimp species (p < 0.05). Relatively deep dwelling species, A. toyamaensis and E. biunguis, females were dominated at each water depth examined. On the other hand, relatively shallow dwelling species, N. communis and P. eous showed seasonal variations in sex ratios. The occurrence in ovigerous females of the four shrimp species varied with depth and season. Ovigerous females of N. communis appeared predominantly at depths of 700 m in summer and 300 m in winter, females with spent ovarian stage appeared predominantly at depths of 500 m in summer and 300 m in winter. Ovigerous females of P. eous also appeared at depths of 700 m in summer and 300 m in winter and at depths of 500 m in summer and 300 m in winter during the spent ovarian stage. Seasonal variations were not as obvious in A. toyamaensis and E.biunguis, although their ovigerous females appeared at a wider range of shallower depths in winter than in summer. Ovigerous females of A. toyamaensis appeared predominantly at 1,100 m. Although the majority of shrimp still appeared at 1,100 m in summer, even in their spent stage, and most of them remained at that depth during the winter, shrimp began to appear at depths as shallow as 700 m during winter. There was similar seasonality in E. biunguis, with its ovigerous females appearing predominantly at depths of 1,100 m in summer and 700 – 900 m in winter. Such seasonality is the result of migration to adapt to changes in the timing of the seasons, in order to increase the survival rate of offspring. There was no significant depth – dependent variations in mean size observed between males of four shrimp species (p > 0.05). The females mean size of N. communis, including ovigerous females, was inversely correlated with depth, i.e., their mean size decreased as depth increased (rs = -0.690, p < 0.05). A similar negative correlation was observed in females of A. toyamaensis (rs = -0.256, p < 0.05) and P. eous (rs = -0.719, p < 0.05). Large females of E. biunguis, including ovigerous females, dwelled predominantly at depths of 700 – 900 m, mean size of females and males did not have any correlation with depth (p < 0.05). The gonadosomatic indices (GSI) of N. communis was subject to seasonal variation (p < 0.05), with the highest GSI values recorded at 300 m in summer and at 700 m in winter. GSI seasonality was also observed in P. eous (p < 0.05). A. toyamaensis was also subject to GSI seasonality (p < 0.05), with maximum values recorded at 1,300 m during both seasons. For E. biunguis, a consecutive breeder species, females and ovigerous females displayed GSI differences only in winter (p < 0.05), and ovarian maturity of ovigerous females was concentrated at 700 m. In all four shrimp species, a positive relationship was observed between size and fecundity (p < 0.05). The index of reproductive potential (IRP) and productivity index (PI) were the highest when the sizes of N. communis were between 14.1 and 15.0 mm (IRP = 147, PI = 1.73) and those of A. toyamaensis were between 24.1 and 25.0 mm (IRP = 185, PI = 4.54). The IRP and PI of P. eous were the highest at 27.1 – 28.0 mm (IRP = 127) and 29.1 – 30.0 mm (PI = 1.92), and at 18.1 – 19.0 mm (IRP = 120) and 19.1 – 20.0 mm (PI = 3.39) for E. biunguis, respectively. In addition, relatively deep dwelling species, A. toyamaensis and E. biunguis showed higher PI values than those of relatively shallow dwelling species, N. communis and P. eous. Firstly, this study suggested not only specific distribution patterns for the four shrimp species within the East Sea of Korea but also a strategy to mitigate the areas competition for these species. Secondly, the study showed the significant differences of the size compositions by depth habitation and how this affects the increase in individual’s body size. Thirdly, this study suggested that species distributed in deep water are affected by seasonal variation in their reproductive characteristics. Furthermore, the study proved that the depth of habitat distribution is a reproduction strategy by choice of these species in order to improve their survival rate.