The Relationship between Body Size and Predation Rates on Hatchlings of the Green Turtle (Chelonia mydas): Is Bigger Better?

 Emma Gyuris

Abstract

The results of preliminary investigations into the relationship between hatchling size and hatchling survivorship in coral reef habitats are presented in this chapter. I compared survivorship between three size classes of hatchlings and found that bigger hatchlings have significantly higher survivorship than smaller ones. However, increasing hatchling size much beyond the size class that is normally produced may result in diminishing returns in survivorship per unit of resource invested The implications of these preliminary findings for the biology of sea turtles and for their management are briefly explored

Introduction

Like most Testudines, sea turtles delay sexual maturity and lack parental care. Consequently, to ensure that the required lifetime reproductive success is achieved, they must trade-off between fecundity and offspring size or quality. Compared to other turtles, sea turtles produce large numbers of very small hatchlings. Predation is believed to be one of the main causes of early hatchling mortality (Walker & Parmenter 1990, Walker 1991, Wyneken & Salmon 1992), and therefore predation can be expected to greatly influence the position of the size-number compromise of sea turtles. I compared predation rates between several size classes of hatchlings using methods from Gyuris (1994). Results our preliminary experiments demonstrated that predation rates over coral reef habitats are lower on larger hatchlings than on smaller hatchlings

Methods

Predation rates on green turtle hatchlings were measured over the reef flat at Heron Island (22°26'S; 151^°55'E) in February 1995 following methods deoped by Gyuris (1994). Hatchlings making their way to the sea were captured using a fence erected on the northern beach. For a week each hatchling captured (from approximately 30 nests) was measured to establish the mean SCL (straight carapace length). Comparison of predation rates were made in two experiments. In the first experiment we compared predation rates between hatchlings with SCL one standard deviation greater than the mean and hatchlings with SCL one standard deviation smaller than the mean. In the second experiment we compared hatchlings that were reared in aquaria for 10-14 days and fed raw chopped prawns with hatchlings with mean SCL that were kept in aquaria for only 2-3 days.

Experiments were conducted over the reef flat adjacent the northern beach of Heron Island around low tide, after nightfall when hatchlings were emerging from nests to swim across the reef flat to deep water beyond the reef In each experiment a pair of leashed hatchlings (one small and one large hatchling) were simultaneously followed by observers across the reef flat. The points of entry into the sea for the hatchlings was approximately 10 m apart. To avoid locality and observer biases, both the observers and the entry point between respective sized hatchlings were rotated. bservation of each hatchling terminated when the hatchling was taken by a predator or at the end of the ten minute observation period, upon which surviving hatchlings were released. Ten parallel observations under similar tidal and weather conditions constituted a trial. Each trial provided a measure of the proportion of hatchlings predated in each size class. Proportions were then arcsine-transformed and predation rates between the different size classes compared using paired T-tests.

Results and Discussion

The mean SCL of hatchlings along the northern beach of Heron Island captured by the drift fence was 49.4 mm. In the first experiment, predation rates on hatchlings with SCL ≤ 47 mm and hatchlings with SCL ≥ 51 mm were compared. In the second experiment hatchlings with a mean SCL of 49.4 mm (range = 46.2- 53.2 mm) and hatchlings with a mean SCL 61.1 mm (range = 59.0-63.3 mm) were compared.

Both experiments demonstrated a significant difference between the smaller and larger size classes of hatchlings (Table I). The results of the two experiments can not be compared as they were conducted under different moon phases, a variable which is known to significantly affect predation rates (Gyuris 1994).

Table I: Mean predation rate for hatchlings in each of the four size classes.

The results obtained in these preliminary experiments, which investigated the relationship of hatchling size on hatchling survivorship, support the 'bigger is better' hypothesis. That is, larger hatchlings experience significantly lower mortality during reef crossing than smaller hatchlings. Similar results were obtained by Janzen (1993) investigating ecological significance of phenotypic variation amongst hatchlings of common snapping turtles, Chelydra serpentina. However, further investigations by Congdon et al. (1999) found no evidence for selection on body size in hatchling snapping turtles during dispersal from the nest or over longer time intervals in the freshwater habitat. They noted however that most terrestrial predators would not be limited by prey size within the range of body sizes commonly found amongst hatchling C. serpentina. The size of predators over shallow coral reef habitats vary greatly from fish that are too weak and/or small to consume turtle hatchlings to very large predators that would commonly consume prey many times the size of C. mydas hatchlings. Thus, in such a habitat larger sized hatchlings would have an advantage over smaller hatchlings.

The little information exists that about the spatial and temporal variation of aquatic predation of hatchling sea turtles (Frick 1976, Witherington & Salmon 1992) is inadequate for understanding the evolution of life history strategies of sea turtles. Sea turtles have a two phase life-cycle that involves a oceanic dispersal stage followed by the occupation of shallow water coastal and reefal habitats. Hatchlings are believed to spend as little time in the nearshore environment as possible because of the high predation rate there (Walker & Parmenter 1990, Walker 1991, Wyneken & Salmon 1992). The only species of marine turtle that lacks an oceanic dispersal stage is the flatback turtle, Natator depressus, and hatchlings and adults of this species share the same habitat. Relative to adult body size, flatback turtles produce fewer (half as many) hatchlings that are twice as large than other species and predation on these larger hatchlings is presumably lower than on the smaller hatchlings of the other species.

The results of these preliminary experiments demonstrate that survivorship of hatchling sea turtles is size dependent. The study was limited by being restricted to one habitat and one location, small sample sizes and limited range of environmental conditions under which the experiments were performed. An understanding of the evolutionary significance of predation on hatchlings in near- shore habitats requires further measurements of predation rates over a range of environmental conditions, over larger spatial and temporal scales, and an assessment of the effect that body size and other parameters of hatchling condition have on hatchling survivorship.

Conditions during incubation and emergence have a significant effect on the size and locomotive performance of hatchlings (Packard & Packard 1988). These phenotypic variables are expected to have a significant effect on predation rates as well on other factors influencing early hatchling survivorship. Therefore mangers of sea turtle hatcheries must consider not only the quantity but the quality of hatchlings released to the sea if their conservation goals are to be achieved effectively.

References

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