Features of group growth for gray sea urchin (Strongylocentrotus interme¬dius) at the northwestern coast of the Japan Sea

Abstract

Comparative study of growth rate and size-age structure of gray sea urchin Strongylocentrotus intermedius is conducted for six its aggregations along Primorye coast on the data collected in diving surveys in 2015–2017. Some aspects of the method of the sea urchin individual age determination by concentric growth bands on interambulacral plates of its carapace are discussed (the plates are processed by M. Jensen method). The group growth is described with Bertalanffy equation. Considerable spatial variability of the growth rate is found between the aggregations that provides some distinctions in size structure of the sea urchins. The highest growth rate is observed for three habitats: Kreyser Rock Island — Cape Lisuchenko, Oprichnik Bay — Cape Grozny, and Cape Sosunov — Plitnyak Bay, where the sea urchins reach the commercial size mostly at the age from 3.5+ to 4.5+. On the contrary, 50 % of the sea urchins become large enough for commercial fishery at the age of 6+ in the Ezhovaya Bay, and the lowest growth rates are noted at Cape Yuzhny and Cape Nadezhda where the majority of sea urchins never reach the commercial size. The maximum age of S. intermedius (19 years) is registered for a specimen with size 73 mm caught at Cape Sosunov. Mean and maximum sizes of individuals in the aggregations of slow-growing S. intermedius are lower than in the aggregations with comparatively high growth rates; such aggregations are also distinguished by lack of commercially-sized individuals. Age structure of all aggregations shows strong irregularity of annual recruitment, particularly low portion of the sea urchins at the age of 1+ is noted. Anyway, the frequency goes down from the age of 7+...9+ that indicates mass elimination of elder animals.