Title Study on the inshore migration mechanism of Conger myriaster leptocephali
Authers Hiroaki KUROGI
Keywords Conger myriaster , leptocephali, mtDNA, Kuroshio, inshore migration
Citation Bull. Fish. Res. Agen. No.24, 105-152, 2008
Conger myriaster is an important commercial fish species in Japan, Korea and China. The larvae have only been caught in shallow coastal areas, and most of them are large individuals (80-130mm TL)at the late leptocephalus or metamorphic stage. Because of the lack of information on distributional ecology of small leptocephali, recruitment mechanism of C. myriaster has been a mystery in spite of many catches of large leptocephali in coastal areas. Because mature individuals have not been collected from inshore waters to continental slope where commercial fishing takes place, the spawning ground has been presumed to be in deeper offshore waters. The early life history of C. myriaster from spawning in offshore waters to their inshore migration remains unknown.
The distributional pattern of leptocephali in the genus Conger was investigated around the Kuroshio, the Kuroshio Extension in the East China Sea and the western North Pacific in order to determine the inshore migration pattern of C. myriaster leptocephali. Conger leptocephali were collected along the Kuroshio, extended southerly to latitude 26° N near the Ryukyu Islands and northerly to 38° N of the Kuroshio-Oyashio transitional zone. The leptocephali were mainly caught in the mainstream and inshore side of the Kuroshio in areas south and north of latitude 29° N, respectively. Two types of C. myriaster leptocephali were recognized in a total of 211 leptocephali. External pigmentary features indicated 79 individuals were C. myriaster (Type A), most of which were larger than 80mm in total length (TL)and caught at low water temperature (<16℃). Most of non-type A leptocephali (N=132)were smaller than 80mm TL and caught at higher water temperature (16-26℃), and 89.3% of randomly subsampled individuals (N=25)were identified as C. myriaster based on mitochondrial DNA analyses (Type B). Thus, C. myriaster was estimated to occupy approximately 90% of the Conger leptocephali collected in this study. Negative relationships between TL and water temperature indicated that the inshore migration of C. myriaster leptocephali occurred along the thermal gradient from warm offshore Kuroshio areas (20- 26℃)to colder coastal areas (<16℃)with their growth; however, the mechanism of the inshore migration is still unclear. Generally, there is a negative relationship between temperature and density of sea water; therefore, the leptocephali should experience increasing environmental density during their inshore migration. It is not known whether the leptocephali, which are devoid of a swim bladder, regulate buoyancy. To examine the presence of buoyancy regulation of the leptocephali, the body density of live leptocephali caught from cold coastal areas in which sigma-t ranged from 25 to 26(13-15℃)were measured. Repetitive measurements of the same specimens (N=10)revealed that the mean body relative density was higher in the day (1.029)than in the night (1.026), indicating that the leptocephali actively regulate the body density diurnally. The body density of leptocephali at mid-night ranged from 1.025 to 1.028, and larger individuals had a higher density, indicating the increase in the body density with the body growth. The body density of the leptochphali is ascertained to be lower in the offshore areas before migration than in the coastal areas because sea water density in upper layer of the offshore Kuroshio in which the leptocephali maintain neutral buoyancy ranging from 1.023 to 1.024. Therefore, the leptocephali are considered to increase the body density through the inshore migration process. The ability of the larvae to regulate body density is important in controlling their migration from warm to cold water areas because of a negative relationship between seawater density and temperature. The findings of the present study suggest a hypothetical pathway of the migration in which the larvae vertically migrate into deeper depths to approach coastal areas. Simulations of larval dispersal or retention by hydrodynamic models should incorporate the information on larval body density fluctuation.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull24/kuroki.pdf