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Returns of pond-reared juvenile green turtles tagged and released in Torres strait, Northern Australia
RETURNS OF POND-REARED JUVENILE GREEN TURTLES TAGGED A N D RELEASED IN TORRES STRAIT, NORTHERN AUSTRALIA
JOHN KOWARSKY~"8g. MIKAELCAPELLE~
Applied Ecology Pry Ltd, PO Box 26, Woden, ACT Australia, 2606
ABSTRACT
In 1974 a total of 1082 green Chelonia mydas and 53 hawksbill Eretmochelys imbricata turtles which had been pond-reared from egg or hatchling were tagged and released in Torres Strait. Twelve green and two hawksbill turtles were subsequently recaptured after periods free ranging from 12 to 400 days and after having moved minimum distances ranging from 70 to 570 krn. Examination of returns showed that ( i) one batch of green turtles had an apparently higher rate of recapture than other batches of green turtles released; (ii) most turtles were recaptured within thefirst year following release; and ( iii) there appeared to be a seasonal trend in direction of movement of tagged turtles. Factors which may have influenced the above findings are discussed, and the relevance of these data to establishing the feasibility of'head-starting' as a conservation measure, and further understanding wild turtle behaviour, is considered.
INTRODUCTION
Information from mark-release-recapture studies of pond-reared sea turtles may yield insight into (a) the effectiveness of 'head-starting' as a conservation measure (e.g. Schroeder 1966); and (b) the little known behaviour of turtles from when they enter the sea as hatchlings to when they later appear as sub-adults or adults at feeding or nesting grounds (see Hirth, 1971 for information relevant to green turtles). t Present address: Department of Biology, Capricornia Institute of Advanced Education, MS 76, Rockhampton, Q. 4700 Australia. :~ Present address: 39 Tenth Avenue, Maylands, W.A. 6051 Australia.
207 Biol. Conserv. (15) (1979)--© Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
208
JOHN KOWARSKY, MIKAEL CAPELLE
The most extensive data available for tagged pond-reared green turtles are those presented by Witham (1976) for 21 recaptured individual turtles from a total of 3106 tagged and released. These, and the results of other work (e.g. Burnett-Herkes, 1974; Balazs, 1976), have shown that pond-reared green turtles can survive for several years and move several thousand kilometres after release in the ocean. This paper presents an analysis of the return information associated with the tagging and release of over 1000 green, and 53 hawksbill turtles in Torres Strait, Northern Australia in 1974.
MATERIALS AND METHODS
Turtles were maintained from egg or newly emerged hatchling in captivity under similar conditions to those described by Carr & Main (1973). As far as we are aware, green turtle stock were obtained from Bramble Cay (latitude 909 'S, longitude 143 °52' E); hawksbill stock were probably taken from islands close to Yorke Island (lat. 9 °45' S, long. 143 °24' E) on which they nest. The exact ages of turtles were not known; all were over one year old, and it is likely that at least some were over two years. Curved carapace lengths as in Fig. 3A of Hughes (1974) were taken for all turtles except for 14 green turtles tagged during the second tagging period (Batch 5) on Murray Island (lat. 9°55 ' S, long, 144003 ' E). A 'Hasco' cattle ear tag was applied to the trailing edge of a foreflipper of each turtle. Turtles were released from Yorke Island, Murray Island and Darnley Island (lat. 9°35'S, long. 143°46'E). One of us (M.C.) was involved in the tagging operations on Yorke Island. Remaining records were supplied by Applied Ecology Pty Ltd.
RESULTS A N D DISCUSSION
Table 1 shows details of batches of turtles tagged in Torres Strait in 1974. The high TABLE 1 DETAILS OF BATCHES OF TURTLES TAGGED AND RELEASED IN TORRES STRAIT IN 1974
Batch Species
Period tagged and released 1974
Locality
Number Number released recaptured
Length at release
~ _ 2SE (cm)
Whole batch
1 Green 1 March 2 Hawksbill 1 March 3 Green 30 April-17 June 4 Green 13 May 5 Green 12 August~, October
Yorke Island Yorke Island Murray Island Darnley Island Murray Island
107 53 623 47 305
0 2 2 0 10
Those recaptured
22.7 _+0-6 24.9 _+0.9 24.2 24.0 + 0.2 25.4 23-4 + 0-6 34.1 ___0.7 34.8 + 2-5
RETURNS OF JUVENILE GREEN TURTLES
209
recapture rate of Batch 5 turtles compared with other batches of green turtles is striking. Differences included: (i) the average size of Batch 5 turtles was approximately 10 cm longer than others; (ii) they were released later in the year than others; and (iii) possibly as a consequence of (ii) above, the condition of Batch 5 turtles may have been better than others, because the more emaciated ones had already died (see Kowarsky, 1977 for comparison of weight-length relationships of living and dead green turtles in culture conditions). If there were a causal relationship between (i) above and the higher recapture rate of Batch 5 turtles for reasons such as greater catchability or survivorship of larger turtles, we might expect to find a bias in recapture rates in favour of larger turtles. Non-significant differences were found between mean carapace lengths (at release) and between cumulative length (at release) frequencies of the whole of Batch 5 compared with the 10 turtles of that Batch subsequently recaptured. Explanation in terms of (ii) above would have to consider seasonal variation in fishing effort, environmental conditions, and responses of turtles. If Batch 5 turtles were in better condition than the others, greater survivorship may have contributed to their higher recapture rate. Table 2 shows details of individual tagged turtles which were subsequently recaptured. The length information was considered unreliable, as many turtles showed apparent negative growth increments in carapace length. Figure 1 shows the release and recapture locations of tagged turtles released in Torres Strait. That 11 of the 12 recaptured green turtles were taken within one year of release suggests a substantial mortality rate of tagged individuals; however, other factors such as tag shedding, changes in the intensity of fishing effort and emigration of turtles from the area may have been important influences on the temporal pattern of tag returns. Accurate information of tag shedding rates in green turtles over long periods, as could be ascertained by a double-tagging strategy, does not yet appear to be available. However, the evidence of Simon & Parkes (1976) regarding the proportion of turtles nesting on Ascension Island having definite signs of healed tag holes compared with those still carrying tags from past seasons, and the evidence of Schultz (1976), who calculated a tag loss rate of 15-20 ~o, together strongly suggest that shedding rates of the conventional turtle tag applied to the foreflipper may be high. Witham (1976) postulated that Florida-released captive-reared turtles swam offshore until major ocean currents were reached after which time they alternately swam, fed and rested. His idea that the Florida-released turtles spent some time considerable distances offshore (and thus less available for recapture) is supported by the temporal pattern of returns of these turtles; only three of the 20 recaptured for the first time were caught within one year of release, 13 in the second year and the remaining four in the third year after release. The difference in this respect between the Florida- and Torres Strait-released turtles need not necessarily indicate a fundamental behavioural difference between the two groups. The extensive coastline of New Guinea in such close proximity to the release points could be expected to
210
JOHN KOWARSKY, MIKAEL CAPELLE
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RETURNS OF JUVENILE GREEN TURTLES
211
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JOHN KOWARSKY, MIKAEL CAPELLE
affect the time before recapture of Torres Strait releases, as it would effectively interrupt any northward, or even random, dispersal pattern. All recaptured turtles being taken close to the New Guinea coast would suggest that either (a) released turtles moved predominantly in a northerly direction, or (b), the region of recaptures was a reflection of an area of comparatively high fishing effort. There is some evidence (Kowarsky, 1978) that people living on an island in Torres Strait did not generally capture turtles below about 40 cm curved carapace length, but whether this was because smaller turtles were not present, or because the smaller turtles were present but not fished, we do not know. The distribution of turtle fishing effort along the New Guinea coast was not known. Most turtles were recaptured in a sector north to north-west of the release point; two green turtles were, however, taken in a sector east of release point. These latter two turtles were recaptured in December and January, while the other turtles were not taken in those two months or February. These data suggest that there may have been a change in direction of movement of turtles in December and January from a general movement north and westwards to a general movement eastwards. From 'December to March, surface water currents in Torres Strait are predominantly west to east, while from April to November the situation is the reverse (Wyrtki, 1960). That the movement of Torres Strait-released turtles was influenced by surface water currents is suggested by the correlation between the water current data and the geographic distribution of recapture points with time. Witham (1976) considered the dispersal of Florida-released turtles to their recapture points to be the result of ocean currents acting on them. The degree to which a turtle would be influenced by water currents would be determined by the proportion of time spent by that turtle at various levels in the water mass. Of importance here is the depth of the water, for if during the 'oceanic dispersal' phase envisaged by Witham (1976) turtles were in very deep water, the option of resting and/or feeding on the bottom would not be open to them. From the small quantity of information available about hatchling turtles at sea (Frick, 1976) and from observation of hatchlings in ponds, it seems that most time is spent at the water surface. The age or size at which they begin spending substantial time on the bottom in the wild remains part of the 'lost year' mystery. Witham (1976) suggested that turtles up to several years old may be able to find adequate food supplies in an oceanic existence by eating coelenterates, seaweed, and other drifting material.
CONCLUSIONS
The reported results provide evidence that pond-reared green turtles are able to survive for some time and move considerable distances after release into the sea. To our knowledge there have been no cases yet reported in the literature of pond-reared sea turtles of any species having been subsequently observed in nesting activity.
RETURNS OF JUVENILE GREEN TURTLES
213
However, several factors acting singly or in combination could be expected to reduce the probability of detecting nesting activity by pond-reared turtles tagged and released at one or two years of age, even if their mortality and behaviour were similar to their wild counterparts. One factor would be the problem of tag shedding. Secondly, the age at which green turtles nest is variously estimated between 4 and 13 years (Hirth, 1971); thus head-starting programmes involving yearling turtles may have to wait 12 years (or longer). Thirdly, the nesting beach selected by such turtles would not be known a priori. Fourthly, in the interval of years between release and reproductive activity it would be reasonable to expect a substantial proportion to die. Finally, the numbers of head-started turtles available for detection might be largely restricted to the females originally released. The relationship between the behaviour of released pond-reared green turtles and that of green turtle hatchlings (or wild-reared turtles of the same age/size) must remain conjectural. Until the presence or absence of some imprinting process which may occur during early life is confirmed, and the mechanisms (if present) understood, it will not be possible to evaluate the effect of pond-rearing upon normal development in this regard. Movement patterns of animals as determined by mark-recapture methods are generally open to various interpretations. If the released turtles were influenced by prevailing water currents, it would be reasonable to presume that the same would apply to hatchling turtles which could not be expected to maintain the same swimming speeds and would be more restricted to the surface water than larger turtles. Surface water current speeds in Torres Strait generally range from between 12 and 15 nautical miles per day, that is, between 0.93 and 1.16 km/hour (Wyrtki, 1960); these are of the same order of magnitude as the calculated swimming speeds of swimmer-tracked hatchling green turtles (Frick, 1976) (current speeds not measured). A striking feature of Frick's (1976) data was that within a few hours hatchlings released together could be several kilometres apart and on courses diverging as much as 90 °. If this trend continued over a period of time a 'diffusion' model, on which would be superimposed the effects of ocean currents, may be the most realistic representation of the dispersal pattern of young turtles.
ACKNOWLEDGEMENTS
The work described in this paper was undertaken by Applied Ecology Pty Ltd. This paper is published with the permission of the Company which does not, however, accept responsibility for any opinions expressed herein. We are grateful to Applied Ecology Pty Ltd for making the tag data available to us for analysis, and to Mr Sam Passi, their field officer on Murray Island, for diligently keeping tag records and providing us with further information. We thank Dr K. R. Allen for his continued interest and valuable advice through all stages of the analysis of data, and Dr Allen,
214
JOHN KOWARSKY, MIKAEL CAPELLE
T. Kowarsky, and J. Penn for their constructive criticism of the manuscript in preparation. Some assistance towards the cost of preparation of this manuscript was provided by the Capricornia Institute of Advanced Education.
REFERENCES BALAZS, G. H. (1976). Green turtle migrations in the Hawaiian Archipelago. Biol. Conserv., 9, 125~40. BURNETT-HERKES,J. (1974). Returns of green sea turtles (Chelonia mydas Linnaeus) tagged at Bermuda. Biol. Conserv., 6, 307-8. CARR,A. F. • MAIN, A. R. (1973). Report on an inquiry into ecological implications of a turtle farming project. Turtle farming project in Northern Australia, Canberra, Commonwealth of Australia. FRICK, J. (1976). Orientation and behaviour of hatchling green turtles (Chelonia mydas) in the sea. Anita. Behav., 24, 849-57. HIRTH, H. F. (1971). Synopsis of biological data on the green turtle, Chelonia mydas (Linnaeus) 1758. FAO Fish. Biol. Synopses, 85. HUGHES, G. R. (1974). The sea turtles of south-east Africa. 1. Status morphology and distributions. lnvestigational Report No. 35. Durban, Oceanographic Research Institute. KOWARSKY, J. (1977). Culture trials of young green turtles (Chelonia mydas) in Tortes Strait, Northern Australia. Aquaculture, l l , 197 215. KOWARSKY J. (1978). Observations of green turtles (Chelonia mydas) in north-eastern Australia during the 1975/1976 nesting season. Biol. Conserv., 13, 51-62. SCHROEDER, R. E. (1966). Buffalo of the sea. Sea Frontiers, 12, 176-83. SCHULTZ, J. P. (1976). Sea turtles nesting in Surinam. Zoologische Verhandelingen, No. 143, Rijksmuseum Van Natuurlijke Historie, Leiden. SIMON, M. H. & PARKES, A. S. (1976). The green sea turtle (Chelonia mydas): nesting on Ascension Island, 1973-1974. J. Zool., Lond., 179, 153-63. WITHAM, P. R. (1976). Et,idence for ocean current mediated dispersal in young green turtles, Chelonia mydas (Linnaeus). M. Lib. Stud. Thesis, University of Oklahoma Graduate College. (Note: Some of the information presented by Witham (1976) can be found in: Witham, R. & Futch, C. R. (1977). Early growth and oceanic survival of pen-reared sea turtles. Herpetologica, 33, 404-9. Wvgwgl, K. (1960). The surface circulation in the Coral and Tasman Seas. Technical Paper, No. 8, p. 1-44. Melbourne, CSIRO Division of Fisheries and Oceanography.
JOHN KOWARSKY~"8g. MIKAELCAPELLE~
Applied Ecology Pry Ltd, PO Box 26, Woden, ACT Australia, 2606
ABSTRACT
In 1974 a total of 1082 green Chelonia mydas and 53 hawksbill Eretmochelys imbricata turtles which had been pond-reared from egg or hatchling were tagged and released in Torres Strait. Twelve green and two hawksbill turtles were subsequently recaptured after periods free ranging from 12 to 400 days and after having moved minimum distances ranging from 70 to 570 krn. Examination of returns showed that ( i) one batch of green turtles had an apparently higher rate of recapture than other batches of green turtles released; (ii) most turtles were recaptured within thefirst year following release; and ( iii) there appeared to be a seasonal trend in direction of movement of tagged turtles. Factors which may have influenced the above findings are discussed, and the relevance of these data to establishing the feasibility of'head-starting' as a conservation measure, and further understanding wild turtle behaviour, is considered.
INTRODUCTION
Information from mark-release-recapture studies of pond-reared sea turtles may yield insight into (a) the effectiveness of 'head-starting' as a conservation measure (e.g. Schroeder 1966); and (b) the little known behaviour of turtles from when they enter the sea as hatchlings to when they later appear as sub-adults or adults at feeding or nesting grounds (see Hirth, 1971 for information relevant to green turtles). t Present address: Department of Biology, Capricornia Institute of Advanced Education, MS 76, Rockhampton, Q. 4700 Australia. :~ Present address: 39 Tenth Avenue, Maylands, W.A. 6051 Australia.
207 Biol. Conserv. (15) (1979)--© Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
208
JOHN KOWARSKY, MIKAEL CAPELLE
The most extensive data available for tagged pond-reared green turtles are those presented by Witham (1976) for 21 recaptured individual turtles from a total of 3106 tagged and released. These, and the results of other work (e.g. Burnett-Herkes, 1974; Balazs, 1976), have shown that pond-reared green turtles can survive for several years and move several thousand kilometres after release in the ocean. This paper presents an analysis of the return information associated with the tagging and release of over 1000 green, and 53 hawksbill turtles in Torres Strait, Northern Australia in 1974.
MATERIALS AND METHODS
Turtles were maintained from egg or newly emerged hatchling in captivity under similar conditions to those described by Carr & Main (1973). As far as we are aware, green turtle stock were obtained from Bramble Cay (latitude 909 'S, longitude 143 °52' E); hawksbill stock were probably taken from islands close to Yorke Island (lat. 9 °45' S, long. 143 °24' E) on which they nest. The exact ages of turtles were not known; all were over one year old, and it is likely that at least some were over two years. Curved carapace lengths as in Fig. 3A of Hughes (1974) were taken for all turtles except for 14 green turtles tagged during the second tagging period (Batch 5) on Murray Island (lat. 9°55 ' S, long, 144003 ' E). A 'Hasco' cattle ear tag was applied to the trailing edge of a foreflipper of each turtle. Turtles were released from Yorke Island, Murray Island and Darnley Island (lat. 9°35'S, long. 143°46'E). One of us (M.C.) was involved in the tagging operations on Yorke Island. Remaining records were supplied by Applied Ecology Pty Ltd.
RESULTS A N D DISCUSSION
Table 1 shows details of batches of turtles tagged in Torres Strait in 1974. The high TABLE 1 DETAILS OF BATCHES OF TURTLES TAGGED AND RELEASED IN TORRES STRAIT IN 1974
Batch Species
Period tagged and released 1974
Locality
Number Number released recaptured
Length at release
~ _ 2SE (cm)
Whole batch
1 Green 1 March 2 Hawksbill 1 March 3 Green 30 April-17 June 4 Green 13 May 5 Green 12 August~, October
Yorke Island Yorke Island Murray Island Darnley Island Murray Island
107 53 623 47 305
0 2 2 0 10
Those recaptured
22.7 _+0-6 24.9 _+0.9 24.2 24.0 + 0.2 25.4 23-4 + 0-6 34.1 ___0.7 34.8 + 2-5
RETURNS OF JUVENILE GREEN TURTLES
209
recapture rate of Batch 5 turtles compared with other batches of green turtles is striking. Differences included: (i) the average size of Batch 5 turtles was approximately 10 cm longer than others; (ii) they were released later in the year than others; and (iii) possibly as a consequence of (ii) above, the condition of Batch 5 turtles may have been better than others, because the more emaciated ones had already died (see Kowarsky, 1977 for comparison of weight-length relationships of living and dead green turtles in culture conditions). If there were a causal relationship between (i) above and the higher recapture rate of Batch 5 turtles for reasons such as greater catchability or survivorship of larger turtles, we might expect to find a bias in recapture rates in favour of larger turtles. Non-significant differences were found between mean carapace lengths (at release) and between cumulative length (at release) frequencies of the whole of Batch 5 compared with the 10 turtles of that Batch subsequently recaptured. Explanation in terms of (ii) above would have to consider seasonal variation in fishing effort, environmental conditions, and responses of turtles. If Batch 5 turtles were in better condition than the others, greater survivorship may have contributed to their higher recapture rate. Table 2 shows details of individual tagged turtles which were subsequently recaptured. The length information was considered unreliable, as many turtles showed apparent negative growth increments in carapace length. Figure 1 shows the release and recapture locations of tagged turtles released in Torres Strait. That 11 of the 12 recaptured green turtles were taken within one year of release suggests a substantial mortality rate of tagged individuals; however, other factors such as tag shedding, changes in the intensity of fishing effort and emigration of turtles from the area may have been important influences on the temporal pattern of tag returns. Accurate information of tag shedding rates in green turtles over long periods, as could be ascertained by a double-tagging strategy, does not yet appear to be available. However, the evidence of Simon & Parkes (1976) regarding the proportion of turtles nesting on Ascension Island having definite signs of healed tag holes compared with those still carrying tags from past seasons, and the evidence of Schultz (1976), who calculated a tag loss rate of 15-20 ~o, together strongly suggest that shedding rates of the conventional turtle tag applied to the foreflipper may be high. Witham (1976) postulated that Florida-released captive-reared turtles swam offshore until major ocean currents were reached after which time they alternately swam, fed and rested. His idea that the Florida-released turtles spent some time considerable distances offshore (and thus less available for recapture) is supported by the temporal pattern of returns of these turtles; only three of the 20 recaptured for the first time were caught within one year of release, 13 in the second year and the remaining four in the third year after release. The difference in this respect between the Florida- and Torres Strait-released turtles need not necessarily indicate a fundamental behavioural difference between the two groups. The extensive coastline of New Guinea in such close proximity to the release points could be expected to
210
JOHN KOWARSKY, MIKAEL CAPELLE
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~÷~*
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e~ Z
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RETURNS OF JUVENILE GREEN TURTLES
211
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JOHN KOWARSKY, MIKAEL CAPELLE
affect the time before recapture of Torres Strait releases, as it would effectively interrupt any northward, or even random, dispersal pattern. All recaptured turtles being taken close to the New Guinea coast would suggest that either (a) released turtles moved predominantly in a northerly direction, or (b), the region of recaptures was a reflection of an area of comparatively high fishing effort. There is some evidence (Kowarsky, 1978) that people living on an island in Torres Strait did not generally capture turtles below about 40 cm curved carapace length, but whether this was because smaller turtles were not present, or because the smaller turtles were present but not fished, we do not know. The distribution of turtle fishing effort along the New Guinea coast was not known. Most turtles were recaptured in a sector north to north-west of the release point; two green turtles were, however, taken in a sector east of release point. These latter two turtles were recaptured in December and January, while the other turtles were not taken in those two months or February. These data suggest that there may have been a change in direction of movement of turtles in December and January from a general movement north and westwards to a general movement eastwards. From 'December to March, surface water currents in Torres Strait are predominantly west to east, while from April to November the situation is the reverse (Wyrtki, 1960). That the movement of Torres Strait-released turtles was influenced by surface water currents is suggested by the correlation between the water current data and the geographic distribution of recapture points with time. Witham (1976) considered the dispersal of Florida-released turtles to their recapture points to be the result of ocean currents acting on them. The degree to which a turtle would be influenced by water currents would be determined by the proportion of time spent by that turtle at various levels in the water mass. Of importance here is the depth of the water, for if during the 'oceanic dispersal' phase envisaged by Witham (1976) turtles were in very deep water, the option of resting and/or feeding on the bottom would not be open to them. From the small quantity of information available about hatchling turtles at sea (Frick, 1976) and from observation of hatchlings in ponds, it seems that most time is spent at the water surface. The age or size at which they begin spending substantial time on the bottom in the wild remains part of the 'lost year' mystery. Witham (1976) suggested that turtles up to several years old may be able to find adequate food supplies in an oceanic existence by eating coelenterates, seaweed, and other drifting material.
CONCLUSIONS
The reported results provide evidence that pond-reared green turtles are able to survive for some time and move considerable distances after release into the sea. To our knowledge there have been no cases yet reported in the literature of pond-reared sea turtles of any species having been subsequently observed in nesting activity.
RETURNS OF JUVENILE GREEN TURTLES
213
However, several factors acting singly or in combination could be expected to reduce the probability of detecting nesting activity by pond-reared turtles tagged and released at one or two years of age, even if their mortality and behaviour were similar to their wild counterparts. One factor would be the problem of tag shedding. Secondly, the age at which green turtles nest is variously estimated between 4 and 13 years (Hirth, 1971); thus head-starting programmes involving yearling turtles may have to wait 12 years (or longer). Thirdly, the nesting beach selected by such turtles would not be known a priori. Fourthly, in the interval of years between release and reproductive activity it would be reasonable to expect a substantial proportion to die. Finally, the numbers of head-started turtles available for detection might be largely restricted to the females originally released. The relationship between the behaviour of released pond-reared green turtles and that of green turtle hatchlings (or wild-reared turtles of the same age/size) must remain conjectural. Until the presence or absence of some imprinting process which may occur during early life is confirmed, and the mechanisms (if present) understood, it will not be possible to evaluate the effect of pond-rearing upon normal development in this regard. Movement patterns of animals as determined by mark-recapture methods are generally open to various interpretations. If the released turtles were influenced by prevailing water currents, it would be reasonable to presume that the same would apply to hatchling turtles which could not be expected to maintain the same swimming speeds and would be more restricted to the surface water than larger turtles. Surface water current speeds in Torres Strait generally range from between 12 and 15 nautical miles per day, that is, between 0.93 and 1.16 km/hour (Wyrtki, 1960); these are of the same order of magnitude as the calculated swimming speeds of swimmer-tracked hatchling green turtles (Frick, 1976) (current speeds not measured). A striking feature of Frick's (1976) data was that within a few hours hatchlings released together could be several kilometres apart and on courses diverging as much as 90 °. If this trend continued over a period of time a 'diffusion' model, on which would be superimposed the effects of ocean currents, may be the most realistic representation of the dispersal pattern of young turtles.
ACKNOWLEDGEMENTS
The work described in this paper was undertaken by Applied Ecology Pty Ltd. This paper is published with the permission of the Company which does not, however, accept responsibility for any opinions expressed herein. We are grateful to Applied Ecology Pty Ltd for making the tag data available to us for analysis, and to Mr Sam Passi, their field officer on Murray Island, for diligently keeping tag records and providing us with further information. We thank Dr K. R. Allen for his continued interest and valuable advice through all stages of the analysis of data, and Dr Allen,
214
JOHN KOWARSKY, MIKAEL CAPELLE
T. Kowarsky, and J. Penn for their constructive criticism of the manuscript in preparation. Some assistance towards the cost of preparation of this manuscript was provided by the Capricornia Institute of Advanced Education.
REFERENCES BALAZS, G. H. (1976). Green turtle migrations in the Hawaiian Archipelago. Biol. Conserv., 9, 125~40. BURNETT-HERKES,J. (1974). Returns of green sea turtles (Chelonia mydas Linnaeus) tagged at Bermuda. Biol. Conserv., 6, 307-8. CARR,A. F. • MAIN, A. R. (1973). Report on an inquiry into ecological implications of a turtle farming project. Turtle farming project in Northern Australia, Canberra, Commonwealth of Australia. FRICK, J. (1976). Orientation and behaviour of hatchling green turtles (Chelonia mydas) in the sea. Anita. Behav., 24, 849-57. HIRTH, H. F. (1971). Synopsis of biological data on the green turtle, Chelonia mydas (Linnaeus) 1758. FAO Fish. Biol. Synopses, 85. HUGHES, G. R. (1974). The sea turtles of south-east Africa. 1. Status morphology and distributions. lnvestigational Report No. 35. Durban, Oceanographic Research Institute. KOWARSKY, J. (1977). Culture trials of young green turtles (Chelonia mydas) in Tortes Strait, Northern Australia. Aquaculture, l l , 197 215. KOWARSKY J. (1978). Observations of green turtles (Chelonia mydas) in north-eastern Australia during the 1975/1976 nesting season. Biol. Conserv., 13, 51-62. SCHROEDER, R. E. (1966). Buffalo of the sea. Sea Frontiers, 12, 176-83. SCHULTZ, J. P. (1976). Sea turtles nesting in Surinam. Zoologische Verhandelingen, No. 143, Rijksmuseum Van Natuurlijke Historie, Leiden. SIMON, M. H. & PARKES, A. S. (1976). The green sea turtle (Chelonia mydas): nesting on Ascension Island, 1973-1974. J. Zool., Lond., 179, 153-63. WITHAM, P. R. (1976). Et,idence for ocean current mediated dispersal in young green turtles, Chelonia mydas (Linnaeus). M. Lib. Stud. Thesis, University of Oklahoma Graduate College. (Note: Some of the information presented by Witham (1976) can be found in: Witham, R. & Futch, C. R. (1977). Early growth and oceanic survival of pen-reared sea turtles. Herpetologica, 33, 404-9. Wvgwgl, K. (1960). The surface circulation in the Coral and Tasman Seas. Technical Paper, No. 8, p. 1-44. Melbourne, CSIRO Division of Fisheries and Oceanography.