Jack mackerel (Trachurus declivis) is a pelagic species that is found in waters off southern Australia and New Zealand. It is the subject of a major fishery, predominantly off Tasmania, with annual landings in the range of 9,000 - 42,000 tonnes. The fishery commenced in the mid 1980s and uses purse seines to target dense surface and sub-surf ace schools that are present over the shelf between September and May. The fishery is managed by a limit on the total tonnage that can be caught. This is currently set at 42,000 tonnes, which represents the largest quantity of fish that has been caught in any one year (in 1986/87). A long-term research and management objective is to develop a more scientific basis for estimating the size of the jack mackerel resource and setting the catch limit.

 

A 1994 review of jack mackerel research identified the need, in the short term, for a range of methods to indicate the condition the jack mackerel population. These included an understanding of the growth of jack mackerel, the age at which fish enter the fishery, the range of ages of fish caught in the fishery, and how these characteristics may have changed over time.

 

The age and growth of jack mackerel has been studied previously using growth increments observed in the otoliths. However, there were inconsistencies in this information among researchers within Australia and between Australia and New Zealand workers. There was a clear need to find out whether these were real differences between jack mackerel populations or were due to errors in the methods used to age the fish.

 

This study used a new technique to determine the accuracy (validate) methods used to estimate the age of jack mackerel. The technique uses the increase in levels of radioactive carbon in the atmosphere and oceans that occurred after the atmospheric nuclear tests in the 1960s. The year in which a fish is born can be estimated by matching the level of radiocarbon in their otoliths to the levels recorded in the environment.