Variation in the demographic characteristics of yellow-phase Japanese eels in different habitats of the Hamana Lake system, Japan

Yellow-phase Japanese eels (Anguilla japonica) were investigated in the Hamana Lake system, Japan, from 2003 to 2004 to understand how their demographic attributes vary within the lake system. A total of 779 yellow eels were collected during sampling in two inlet rivers and two brackish/saltwater lakes within the lake system. Female eels predominated, constituting 84% of the 75 sex-determined eels in the river, and 50% of the 151 sex-determined eels in the lakes. Total lengths (TL) of all eels examined ranged from 54.2 to 715.0 mm (mean = 320.4 +/- 145.4 SD). In the inlet river, the TL of eels showed a significant positive relation with the distance from the river mouth. The estimated relative abundances of eels ranged from 0 to 1.8 eels center dot m(-2) effort (mean: 0.3 +/- 0.41) in the river and was negatively correlated with the distance from the river mouth. This suggested that larger eels might tend to be distributed at lower abundances in upstream reaches of the river. Mean age of yellow eels determined by their otolith annuli was younger in the lake (N = 117, 3.3 +/- 1.4 years) than in the river (N = 214, 4.3 +/- 1.7 years). Growth rate was higher in the lake than in the river at age 1-2 years (131.9 and 104.4 mm center dot year(-1), respectively). The results of this study suggest that, although Japanese eels can adapt to various types of environments, significant differences can occur in population structures and growth patterns among habitats.