Distribution patterns and population dynamics of the pandalid shrimp Plesionika izumiae (Decapoda, Caridea) in a unique semi-enclosed deep-water bay environment

Abstract

Abstract The pandalid shrimp Plesionika izumiae Omori, 1971 has a wide bathymetric distribution in the coastal waters and the upper continental slope of west Pacific regions. The distribution patterns and population dynamics of P. izumiae were studied in a unique semi-enclosed deep-water bay, Kagoshima Bay, southern Japan, with very steep slopes on both sides. Plesionika izumiae shows a wide spatial distribution in the bay with the majority of the individuals inhabiting the channel area with sediments of almost equal portions of silt and sand, as well as in the bay head with a silty sediment. There were no significant relationships between bottom water temperature or salinity and the distribution patterns of this shrimp, but the sediment characteristics and water depth might appear to influence the distribution patterns of P. izumiae. The distribution patterns with progression of age reveal the dynamics of the P. izumiae population, which was studied by selecting one cohort from each sex and tracing them throughout their life cycle, resulting in similar patterns being shown for both sexes. Juveniles (at the age of 5.7 months) occurred throughout the distribution areas; however, there was a low catch per unit effort (CPUE) even in the main spawning grounds, probably from due to the ongoing recruitment process. The substantial increment in CPUE at the age of 8.4 months throughout the distribution areas likely indicate the stock to be fully recruited by then. A clear shift in CPUE of these cohorts was observed at the ages of 11.6 months and 14.3 months, from the deeper areas towards the shallower areas of the bay, possibly suggesting spawning migration. Individuals approaching their maximum size and longevity were found mostly in the deeper waters. A marked seasonal pattern in CPUE was noted with higher values recorded in summer and lower in winter, which could possibly be related to the life history of the shrimp.