ENSO Regulation of Indo-Pacific Green Turtle Populations

Abstract

The green turtle, Chelonia mydas, is a large, long-lived, herbivorous marine reptile that grazes on the marine macrophytes (seagrass, algae, mangrove fruit). Although green turtles are resident in shallow coastal waters for feeding, they migrate up to 2,600km to breed at traditional nesting beaches. Female green turtles do not breed annually. It takes in excess of a year for a female green turtle to lay down her fat reserves and deposit the yolk stores in her ovaries in preparation for the breeding migration. The annual number of breeding green turtles has been recorded in a monitoring program encompassing 23 years at nesting beaches within two genetically independent stocks in eastern Australia (Heron Island within the southern Great Barrier Reef breeding stock; Raine Island within the northern Great Barrier Reef breeding stock). The number of females recorded annually at the nesting beaches varies widely across years — sometimes varying up to 3 orders of magnitude in successive years. Approximately synchronous fluctuations in annual breeding numbers occur at the eastern Australian nesting beaches.

An examination of the green turtles in their home feeding grounds prior to the commencement of the breeding migration shows that the inter-annual fluctuations in breeding numbers are not due to fluctuations in the number of adult turtles in the feeding areas. Rather, the annual fluctuations in breeding numbers are the result of fluctuations in the proportion of adult females that prepare to breed in a particular year.

There is a significant correlation between the SOI index two years before the breeding season and the number of females recorded at the nesting beach. In the extremes, massed nesting occurs two years following major El Niño events and extremely low nesting numbers occur two years after major anti El Niño events. It is now possible to predict within reasonable confidence limits the size of the annual nesting population at key eastern Australian green turtle rookeries based on the SOI two years before the commencement of the breeding season. No comparable relationship has been identified between ENSO and any of the other species of marine turtles.

Additional research suggests that there is a nutritional basis to this annual fluctuation in green turtle population parameters. Regional breadth of the phenomenon is demonstrated by the synchrony of function of green turtles that feed almost exclusively on algae on the coral reefs of the outer Great Barrier Reef with those that feed primarily on seagrass in inshore bays. However, there is no suitable database available to determine whether the fluctuations are driven by changes in quantity or quality of the food resource. Changes in water temperature alone do not appear to be sufficiently large to account for the effects.

Without the linkage to ENSO regulation of breeding numbers, it has not been possible to use short term nesting census data to determine green turtle population stability. If the changes in green turtle population dynamics as a result of ENSO events are nutritionally based, then green turtle populations have the potential to provide parameters that can be monitored and compared with the performance of seagrass and algal pastures in response to environmental change. This would improve our understanding of the functioning of marine grazing ecosystems. This could apply to both changes from natural events such as climate fluctuation as well as from anthropogenic induced changes.