A study of the loggerhead sea turtle, Caretta caretta, in eastern Australia

Limpus, Colin James (1985). A study of the loggerhead sea turtle, Caretta caretta, in eastern Australia PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Limpus, Colin James
Thesis Title A study of the loggerhead sea turtle, Caretta caretta, in eastern Australia
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 1985
Thesis type PhD Thesis
Supervisor Jiro Kikkawa
Total pages 212
Collection year 1985
Language eng
Subjects 06 Biological Sciences
0608 Zoology
060801 Animal Behaviour
Formatted abstract

1.         Caretta caretta has the most circumscribed nesting distribution of sea turtles nesting in eastern Australia. 

2.         There are three principal areas for nesting: the mainland coast between Bundaberg and Round Hill Head, the islands of the Capricorn- Bunker Groups and the Swain Reef cays. Together these rookeries support the principal breeding aggregation for Caretta caretta in the south Pacific region. 

3.         Nesting density does not appear to have altered substantially since discovery about 140 years ago. 

4.         The average breeding female at Mon Repos measured 95.8 cm in CCL and weighed 100.7 kg. She laid 3.4 clutches per season at 13.9 d renesting intervals. The mean clutch count was 127 eggs which measured 4.04 cm in diameter and weighed 36.5 g. Yolkless eggs were rare. Clutches were laid to an average depth of 33.1 cm to the uppermost egg and 57.9 cm to the bottom of the egg chamber.

5.         Clutch count was positively correlated with CCL. 

6.         Females which arrived to lay early in the breeding season laid more clutches than those arriving later. The more clutches the female laid in the season the larger her average clutch count. 

7.         There was a consistent tendency for eggs from the early part of a clutch, in order of laying, to be larger than eggs from the later part. 

8.         Egg diameter was variable between females, between seasons and between dates of commencement of laying by the females within a season. 

9.         The distribution of clutch deposition along Mon Repos beach has not changed in 14 years. The highest density nesting, up to 94 clutches per 25m of waterline per season consistently occurred in sector 5 near the northern end of this 1.539km beach. 

10.      Adult mortality rate while ashore nesting was 0.04%. 

11.      Adult mortality in the interesting habitat at Mon Repos was not quantified but predation by tiger sharks and drowning in prawn trawls were recorded. 

12.      The females had a strong drive to return to the same small beach within the one season. A female was more likely to change its nesting beach following the laying of a partial clutch than following a successful nesting or following a nesting attempt in which no eggs were laid at all. 

13.      While 11%-16% of females made shifts between the 6 beaches within the 10km of the Bundaberg coast study area only 1% made shifts between beaches 80km apart within any one season. 

14.      These turtles did not check the suitability of a beach using a ‘false crawl’. 

15.      Hatchling emergence occurs between late December and early May.

16.      There was intra-and inter-seasonal variation in incubation to emergence period, the mean was 56.6 d and 54.2 d in 1978-1979 and 1979-1980 respectively. 

17.      Nocturnal emergence occurred for 96% of clutches. 

18.      Egg destruction by nesting turtles accounted for 0.43% of the seasonal egg production.  

19.      For those clutches producing hatchlings, the emergence success averaged 81.9% and 80.4% in the 1978-1979 and 1979-1980 seasons respectively. 

20.      Flooding and erosion of nests was the principal cause of total failure of some clutches at Mon Repos. However introduced foxes cause near total clutch destruction on most of the remaining mainland Caretta caretta nesting beaches.

21.      Total failure of clutches was recorded for 13.2% of clutches in an average storm surge season but varied from 8.4% to over 70% of a season’s clutch production in other seasons. 

22.      Hatchling mortality while crossing the beach was low at all beaches, being <1%. Crabs and birds were the principal predators. 

23.      Hatchlings at Mon Repos weighed 20.9g and had an SCL=4.33cm.  

24.      Frequency of occurrence of abnormal hatchlings and embryos was recorded. 

25.      The females nesting on the eastern Australian Caretta caretta rookeries were recaptured post nesting in feeding grounds that were scattered from Yamba in the south, to New Caledonia in the east, Trobriand Islands in the north and Groote Eylandt in the west. The feeding grounds ranged from 11km to 2618km away from the rookery. At least some of the females cross oceanic waters in their nesting migrations. 

26.      Different sized females appear to inhabit the different feeding regions. 

27.      Average relocation speeds during the post-nesting return to the feeding ground as high as 34.1km per day were recorded. 

28.      The adult females were found to be as precise in their repetitive return to a feeding ground as they were in their repetitive remigrations to a specific nesting beach. 

29.      The female did not necessarily nest at the closest suitable nesting beach to her feeding ground. 

30.      98.3% of all remigration recaptures occurred at the rookery of original tagging. The probability of recapturing a remigrant female from a given rookery decreased rapidly the further away from the rookery a search was made for remigrants. 

31.      The mean remigration interval uncorrected for tag loss was 2.98yr. Individual females displayed irregular periodicity with successive remigrations. When corrected for tag loss the mean remigration interval was 3.48yr. 

32.      Females from two feeding grounds (Moreton Bay and Capricornia reefs) remigrated with different mean remigration intervals. 

33.      Females nesting for their presumed first breeding season were significantly smaller than the known remigrants nesting in the same season. 

34.      Mature Caretta caretta were growing at a rate of about 1-3mm per year in CCL and the growth rate was decreasing with time. 

35.      A 1.2% decrease in seasonal fecundity was recorded for successive breeding seasons. 

36.      Tag loss within the season of application occurred for 1.3% and 0.5% of tags applied in the L2 and L3 tagging positions on the front flipper respectively. 

37.      Rates of tag loss were calculated for National Band and Tag Co. monel 1005 #49 tags applied to the L2 and L3 tagging positions and for remigration intervals up to 6yr. The number of remigrant turtles captured in a season was corrected for tag loss.

38.      The recruitment rate of priminesting females into the Mon Repos population was 44.20 ± 5.67% of the annual nesting population. 

39.      Tag loss rates approaching 100% for tags applied more than 6yr prior to a remigration may be masking a substantial number of long interval remigrations and the above recruitment rate will be overestimated. 

40.      Approximately a quarter of a million hatchlings of three species were marked by a double mutilation tagging such that each hatchling was coded for year and beach of birth. No recoveries of these marked turtles has occurred within the ten years that the project has run. 

41.      The distribution of known post-hatching turtle captures within the Australian region was summarised. Records were scarce. The few known Caretta caretta post-hatchlings from the east coast all occurred down stream from the south Queensland rookeries along the East Australian Current.

42.      The Caretta caretta feeding on the reef of the southern Great Barrier Reef were found to be long term residents of individual reefs. Their sex ratio was 0.41F: 1.mmM. They ranged in size from immatures as small as CCL= 67cm to adult males and females. Recruitment of new individuals to this population occurred at a rate of less than 17.4% annually. Most recruits were in the size range of the smaller immature turtles.

43.      68% of the females and 46% of the males were sexually mature. On the average, 24% of the mature females in this feeding ground were breeding in any one year. Non-annual remigration of sea turtles to a specific rookery does not result from their movement to other rookeries in intervening years but from actual non-annual breeding cycles. 

44.      These turtles did not necessarily mature at the minimum breeding size for the species. Immature turtles larger than average breeding size were recorded. Each individual appears to grow to approximately its full size before maturing and growth beyond maturity is minimal.  

45.      Growth rates were low and not significantly different between the sexes. It was estimated that the average immature Caretta caretta with CCL=75.0cm that recruits to the southern Great Barrier Reef feeding grounds will take a further 27yr to reach sexual maturity. 

46.      High density courtship activity was reported from only the areas adjacent to the northern end of Fraser Island. Very little courtship occurred immediately adjacent to the rookeries. 

47.      Some males engaged in courtship without necessarily migrating. They courted females migrating through the feeding ground. 

48.      From all available evidence it would appear that there is only a single deme (or at the most two) represented in the eastern Australian Caretta caretta.

49.       The criteria necessary for a rookery to be self sustaining are discussed. 

50.      Using a hatchling productivity for Mon Repos of 60% of the annual egg production, it was calculated that it would require a survivorship of 0.3% of the hatchlings entering the sea to maintain a stable population. This also assumed that the hatchlings return to the natal beach as adults. The validity of this assumption is discussed. 

51.      The general agreement between the results of nesting studies from several countries while encouraging must be viewed with caution because all have the potential for similar substantial errors in all these results as a consequence of high tag loss rates. In addition there are substantial discrepancies between the results of the nesting studies and the feeding ground study with respect to recruitment rate, annual breeding rate and reproductive longevity. 

52.      The validity of existing population models will only be adequately assessed after there has been a long=term study of these turtles that has addressed their potential for long life to maturity, a possible long reproductive life with associated very low adult mortality and the possible non return of the turtles to the specific beach of their birth. 

53.      Recommendations for the conservation management of eastern Australian Caretta caretta populations are made. 


Keyword Loggerhead turtle
The loggerhead sea turtle in eastern Australia

Document type: Thesis
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