Growth and temporal variation of two Japanese anchovy cohorts during their recruitment to the East China Sea

Abstract

This study examines some basic characteristics of the Japanese anchovy ( Engraulis japonicus S.) population arriving at the southernmost area of its distribution range in the I-lan Bay, NE Taiwan. These characteristics include recruitment dynamics, body length–sagittal radius relationship, weight–length relationship, von Bertalanffy growth curve, hatching date, and ontogenetic and time series variation of growth rate. Monotonic relationships between various parameters were first established, and then the difference between two seasonal cohorts was compared using back-calculated length and weight from sagittal increment analysis. During our study period from March to November 1998, we found two cohorts of Japanese anchovy recruited to the bay area: the spring cohort (March–May) with a hatching peak interval of 4 days, and the late-summer cohort (August–September) with that interval of 5.5 days. Most suitable models have been selected to characterize body measurement and to compare the difference between the two cohorts. Significant differences were found between the cohorts for standard length–sagittal radius and weight–length relationships. On the ontogenetic basis, the growth difference between the two cohorts occurred at the age of 4–5 weeks, where spring cohort (0.48–0.58 mm SL per day) grew faster in length than that of the late summer (0.40–0.45 mm per day). However, the late-summer cohort always increased their body weight per unit length greater than spring; the difference ranged from 0.08 to 0.59 mg per day. On weekly time series basis, when all larval size had been standardized to their age at 10 days, the overall mean daily growth in body length did not differ between the cohorts (ca. 0.67 mm per day). However, significant difference was found when the growth was measured by body weight; the growth rates were 1.6 and 2.3 mg per day for spring and late-summer cohort, respectively. We also found that the growth rates were significantly correlated with ambient water temperature, expressed by a quadratic function with an optimal temperature at about 25°C. They were also negatively correlated with catch per unit effort (CPUE), in which possible density-dependent growth was postulated. In summary, the spring cohort generally had a larger recruitment size, higher daily growth rate and bigger maximal body length, while the late-summer cohort always gained a bigger body weight per day at each unit length over the spring cohort. The differences on the growth of the two cohorts resulted from the ontogenetic differences and the temporal variations due to both the environmental temperature and its recruitment size.