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Marine Turtle Newsletter 10:3-5, © 1979

Notes on Growth Rates of Wild Turtles

Colin Limpus
National Parks & Wildlife Service, of Queensland, P.0. Box 190, Brisbane North Quay 4000, Australia

My growth data is derived from a study of wild turtle populations that has just entered its 5th consecutive year and based on two adjacent coral reefs of the southern Great Barrier Reef, i.e. Heron Island Reef and Wistari Reef (approx. 23.5'S). These reefs are the year round feeding grounds of large numbers of greens and loggerheads and a small number of hawksbills. Each spring there is an influx of migrant adult greens and loggerheads on to these reefs, aggregating for mating and subsequent nesting on nearby islands. These migrant turtles are also feeding on these reefs. While the immature turtles are definitely residents to the area, the residency status of many of the adults is uncertain. See Table 1 for size range and diets of these turtles.

Table 1. Size range and observed diet of turtles resident on the coral reefs of the southern Great Barrier Reef. Midline curved carapace length (CCL) is used as the standard measurement.

Recaptures of these turtles during our successive trips to the area have provided growth measurements on over 100 green turtles., several hundred loggerheads and 4 hawksbills. The intervals between captures vary from a few months up to 4.25 years.

The overall impressions gained from these growth measurements are:

1. Green turtles above 38cm inhabiting southern Great Barrier Reef feeding grounds grow slowly- usually between 0.5 and 2 cm per year. Maximum recorded is 3.24 cm/yr. Immature green turtles, 60 - 90 cm CCL gave a mean growth rate of 1.3 cm/yr (Table 2).
2. Mature male and female green turtles breeding in the southern Great Barrier Reef are growing very slowly - of the order of only a few millimeters per year (Tables 2 and 3).

Table 2. Measured growth rates of green turtles captured in southern Great Barrier Reef feeding grounds compared with growth rate of nesting females from Tortuguero and Heron Island.

Curved carapace length (cm)	Growth Rate (cm/yr)				Reference
	X	SD	range	n
immatures 40-50	-	-	0 to 1.54	4	Limpus & Walter (MS)
immatures 50-60	-	-	0.95	1
immatures 60-70	1.432	1.957	0 to 3.24	14
immatures 70-80	1.42	0.653	0.6 to 2.25	15
immatures 80-90	1.098	0.993	-0.6 to 2.86	11
Mature males (90-102)	0.14	0.1132	-0.3 to 2.6	12	Limpus (unpublished data)
Nesting females (Tortuguero)	0.4	-	-	-	Carr & Goodman (1970)
Nesting females (Heron Island)	<1 cm in 4 to 5yrs	-	-	-	Bustard (1974)

3. Based on these growth rates it would appear that a green turtle living in the southern Great Barrier Reef could not reach maturity in less than 30 years. If green turtles, before they commence breeding, grow to an adult size beyond which little growth occurs then an average-sized nesting green turtle from Heron Island (CCL = 107 cm) could be more than 50 years old before she commences laying.

Table 3. Some of the largest growth increments recorded from turtles in the southern Great Barrier Reef feeding grounds.

Loggerheads	Initial CCL (cm)	Growth increment (cm)	Interval between captures (yr)
sub-adult	76	5.5	4
sub-adult	80	6.5	4
sub-adult	80.5	3.5	4
sub-adult %	88	2.5	4
Adult %	90.5	1.0	3 10/12
Adult %	100.5	0	3 6/12
Adult %	99.5	1	4 (also observed nesting in another area)
Adult %	99	1.5	10 (also observed nesting in another area)
Greens (see also Table 2)
Sub adult	77	6	4 3/12
%	99	1	3 10/12
Hawksbills
%	81	6.5	4
Sub adult	67	2.5	1 5/12

4. Growth rates from the numerous loggerhead and the few hawksbill recaptures show these to be growing at a similar rate to that of green turtles of the same size.
5. Growth rates of wild turtles are very much less than those of captive reared turtles.

If the age of maturity for a sea turtle is 30 years plus, then a number of our management practices may need revision. For example it may take several decades before the effect of over harvesting of turtle eggs is seen on the numbers of nesting females at a rookery. Attempts to restock rookeries by releasing hatchlings should not be regarded as failures if turtles haven't returned to nest in less than 20 years. Similarly success of releasing programs may not be measurable in only a few years.

Bustard, R. 1972. Australian Sea Turtles. William Collins Sons and Co., London.

Carr, A. and D. Goodman. 1970. Ecological implications of size and growth in Chelonia. Copeia 1970: 783-786.

Limpus, C.J. and D.G. Walter. Manuscript. Growth rates of immature wild green turtles, Chelonia mydas(L.): a preliminary report.