Effect of tagging on the condition of southern bluefin tuna, Thunnus maccoyii (Castlenau)

Abstract

The results of an experiment designed to assess the effects of tagging on the condition of the southern bluefin tuna, T. maccoyii, are presented. From January to March 1983, 4590 fish were tagged off Esperance, Western Australia. Of the tagged tuna recaptured before the tagging team departed, 316 were measured for fork length and weighed to determine their relative condition factors. From a number of commercial catches that included tagged tuna, random samples of untagged tuna were taken and their relative condition factors similarly determined. The tagged tuna had significantly lower relative condition factors than the untagged tuna. This effect was greatest in those at liberty 5-20 days. It was also more pronounced in tagged tuna less than 60 cm fork length. A size-related difference in activity level during capture and tagging was suggested as a possible explanation. A change in the behaviour of the tuna immediately after tagging causing them to be less susceptible to the commercial fishery was evidenced by a relatively low number of recaptures during the first 5 days after tagging.