Validation of Ageing Techniques on Otoliths of Horse Mackerel ('Trachurus trachurus L.')

摘要

Except in young fish, age determination in the horse mackerel (Trachurus trachurus L.) presented considerable difficulties. At the Horse Mackerel Age Determination Workshop in Lowestoft (UK) in 1987 ten otolith readers participated. Nine used the same and one used another aging technique. The aging of these different methods differed about a factor two. The aging technique of the minority is likely to be the most accurate one based on the collected evidence. The occurrence of annual year-marks has been tested by following indentifiable year classes through successive years. Only by one of the two existing age interpretation methods this could be done. The aging technique was finally validated over an age range of up to 20 years, because the age range, over which the strong/weak year classes could be followed, overlapped the age range of the validation by the length-frequency method for the youngest age groups. It was concluded that for adult horse mackerel the age interpretation method of only one hyaline ring per annual growth zone should be adopted. If the horse mackerel is a long-lived species (up to approximately 35 years old) as suggested by present age determination, natural mortality will be much lower than assumed before.