Abstract
This is the first description of the biology of Euthynnus affinis in the Hong Kong area and is based on examination of samples of fish and on other data collected during 1967-69. A population of E. affinis inhabits the south coast of Kwangtung, China where it is the commonest tuna species. Adult fish concentrated inshore to spawn each year during June-August in water of surface temperature 25-29°C and salinity 26-29‰. Two distinct size-groups of fish made up the spawning schools; many fish of mean size 62cm long (weight 4.5kg) and lesser numbers of fish of mean size 44cm long (weight 1.9kg). These size-groups were probably 2 and 1 years-old respectively. The tuna lived dispersed over the continental shelf during other months of the year. Food eaten consisted chiefly of pelagic fish and cephalopods. The behaviour, length-weight relation and growth rate of the fish are described. Commercially useful catches could probably be taken from the summer spawning concentrations if purse seining methods were used.
