Spatio-temporal distribution and seasonal population dynamics of the jellyfish Aurelia aurita s.l. studied with Dual-frequency IDentification SONar (DIDSON)

Abstract

Deployment of a Dual-frequency IDentification SONar (DIDSON) revealed the seasonal variations in spatial distribution and abundance of medusae of the common jellyfish, Aurelia aurita s.l. (detectable bell diameters: ca. ≤5 cm) in a shallow (max. depth: ∼6 m) Japanese brackish-water lake between October 2008 and October 2009. The medusae were usually in patchy aggregations, more prominently so during warm seasons. Integration of the DIDSON-detected medusae with smaller ones estimated from the bell diameter composition in net hauls enabled us to study their seasonal population dynamics. Abundance was highest in June, due to recruitment, and declined to a relatively stable level in August and thereafter. The population carbon biomass was highest in October due to somatic growth of individual medusae. The annual mean biomass (63.8 mg C m -3) was much higher than that of mesozooplankton (26.0 mg C m -3), suggesting a substantial top-down impact. The DIDSON is an effective tool for studies of the spatial distribution and population dynamics of large jellyfish species like A. aurita. Concomitant net sampling to determine the bell diameter composition is required to offset the geometric limitations of this instrument. © 2012 The Author.