Research Institute of Environment, Agriculture and Fisheries, Osaka Prefectural Government, Misaki, Osaka 599-0311, Japan
Age and growth of robust tonguefish Cynoglossus robustus in Osaka Bay were studied. The technique of dyeing sectioned sagittal otoliths with methyl violet B showed distinct violet rings. These rings were confirmed to be annual and formed mainly from June to July, corresponding with the spawning season. Reading 784 specimens from 2000 to 2002, growth of the fish was expressed by von Bertalanffy's equation as TLt=352.9(1−e−0.703(t+0.528)) for males and TLt=406.6(1−e−0.638(t+0.422)) for females where TL is total length in mm and t is age in years. The females were larger than the males at each age. The total length of 1-year-old males and females reached 66% and 60% of their extremity length, suggesting the relatively fast growth of this species. The seasonal changes in the size composition of the fish caught by trawl net showed relatively large catches of low-value small fish from April to September. It was suggested that postponing the start of fishing of this fish by 4-6 months could lead to a higher fishing income.