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Providing evidence to improve practice

Individual study: Returns of captive-reared juvenile green Chelonia mydas and hawksbill Eretmochelys imbricata turtles tagged and released in Torres Strait, Queensland, Australia

Published source details

Kowarsky J. & Capelle M. (1979) Returns of pond-reared juvenile green turtles tagged and released in Torres strait, Northern Australia. Biological Conservation, 15, 207-214

Summary

To give an insight into the potential of 'head-starting' as a marine turtle conservation measure, an analysis of the return information associated with the release in the Torres Strait (northern Queensland) in 1974 of 1,082 green Chelonia mydas and 53 hawksbill Eretmochelys imbricata turtles captive-reared from egg or hatchling for 1-2 years, was undertaken.

Turtles were captive-reared from eggs or newly emerged hatchling derived from local wild sources. The age of juvenile turtles at release was around 1-2 years old. A 'Hasco' cattle ear tag (with individual identification code and return address for tags) was attached to the trailing edge of a foreflipper of each turtle. Turtles were released in batches from beaches on Yorke Island (9°45' S, 143°24'E), Murray Island and Darnley Island (9°35'S, 143°46'E) on several dates in 1974.

Twelve green and two hawksbill turtles were subsequently ‘recaptured’ (e.g. by fishermen who returned tags) from 12 to 400 days after release; these individuals had swum minimum distances ranging from 70 to 570 km from their release point. Most turtles (11 of the 12 recaptured green turtles) were recaptured within one year of release. As well as mortality over time, other factors such as emigration from the area and tag shedding may have influenced tag returns. Information of tag shedding rates in green turtles over long periods is very limited but an estimated tag loss rate of 15-20% from another study, suggests that shedding rates of the tag-type applied may be high.
 
‘Batch 5’ green turtles had a higher rate of recapture (10 of the 12 tag returns) than other batches of green turtles. Reasons for this are unclear and could be viewed as both a positive or negative outcome. The average size of Batch 5 turtles was about 10 cm longer than others and they were released latest in the year. Their larger size may have made them more susceptible to capture (hence higher return rate); equally survivorship of these larger turtles may have been better, hence the higher recapture rate. Other factors such as seasonal variation in fishing effort, sea conditions, or Batch 5 turtles perhaps being in better condition than the others, need also to be considered.
 
The results at least provide some evidence that captive-reared turtles are able to survive for some time and move considerable distances after release into the sea.
 
 
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