Hatch Dates and Growth Rates of the Cockle Fulvia mutica Estimated from Daily Growth Lines in Chondrophore in Tokyo Bay

Abstract

Based on daily growth lines and their growth increments in the chondrophore, hatch dates and growth rates of the cockle Fulvia mutica in Tokyo Bay were estimated, and factors determining the seasonality in growth were investigated. The estimated hatch dates ranged throughout the year. However, the frequency distribution of hatch dates of samples indicated that most of the cockles hatched from March to July with a peak during April and May. In particular, hatch dates of a young group with daily growth lines less than 365 (less than 1 year in age) ranged from March to August with a peak during May and June. Accordingly the main spawning season of the cockle was determined as spring in Tokyo Bay. This result is consistent with the estimation done by an alternative method. Daily growth rate varied considerably by individual samples and the mean growth rate was high in the late spring, autumn and early winter, and low in the midsummer and winter. The growth rate remained at a low and stable level after maturation. Among factors determining the seasonality in growth, for the cockles less than 1 year in age, water temperature was of major importance, while for adult cockles maturation and spawning may also have some influence.