The population dynamics of Balanus rostratus (Cirripedia: Thoracica) in Shizugawa Bay, northern Japan

Benthic adult individuals and planktonic larvae of the barnacle Balanus rostratus were collected monthly from April, 2003, to March, 2005, in Shizugawa Bay, Miyagi Prefecture, Japan. The adult barnacles, with a maximum base diameter of 20-60mm, exhibited an increase in gonad somatic index from October to December, and breeding individuals appeared in December and January. The occurrence of nauplius larvae of B. rostratus was restricted to winter months, from January to March, and this was followed by the recruitment of small juveniles of 1-4mm in maximum base diameter to the rocks. Under natural conditions, slow growth was observed in newly settled individuals in spring, with the mean base diameter only increasing to 7mm by December-January; however, relatively high growth rates were observed in young barnacles transplanted to plastic plates and further cultured in Shizugawa Bay. The mean maximum base diameter of these cultured barnacles increased from 6.7±1.0mm in June, 2004, to 17.7±2.6mm in December, 2004, and to 26.2±2.7mm in April, 2005. The variation in growth rates between wild and cultured specimens of B. rostratus may be attributable to differences in environmental conditions or size-dependent mortality. Under natural conditions, the number of newly settled barnacles decreased drastically after May and they almost disappeared until January during the experimental period. The cause of this decrease may be due to post-settlement mortality biased towards larger individuals (e.g., size-selective predation), which may result in underestimation of the growth rate of young barnacles in the natural population. During the two-year investigation, few individuals recruited into size classes over of 10mm in maximum base diameter because of the low survival rate. Although B. rostratus is regarded as a long-lived species with a high survival rate among its large individuals (>20mm in maximum base diameter), the high mortality of young barnacles has the potential to lead the population to extinction. Thus, the intermittent occurrence of a dominant year class may be a strategy for sustaining the population.