Turtle tourism has become a popular year-round activity in Rarotonga, with both government and civil society groups working to ensure it is safer for turtles and people. In a two-part series, Gerald McCormack of the Cook Islands Natural Heritage Trust writes about the distinct lifecycle of the Green sea turtle, which has a distinct lifecycle compared to Hawksbill turtles, the two types of ‘resident’ turtles that local turtle tourism is built around.

Recently, Te Ipukarea Society (Cook Islands News, February 22) reported 90 Green and 19 Hawksbills. They highlighted one Green Turtle seen in Avaavaroa Passage since 2021 as “a good example of a ‘residential turtle’ that is quite happy spending its days in Vaimaanga”.

The presence of around 90 “residential” Green turtles is a dramatic change from the 1980s and 1990s when such turtles were very rarely seen.

What caused this dramatic change? Has there been a local population boom? Have these migratory turtles found a good home and stopped migrating? Can these “resident” turtles contribute to the survival of their endangered species?

Green turtles are listed on the International Union for Conservation of Nature (IUCN) Red List as “threatened” due to a global population decline, although they have increased in some areas. It is also listed in Appendix 1 of CITES, which gives it the highest level of protection, prohibiting all commercial trade of animals or their parts, with minor non-commercial exceptions.

Our Pacific neighbours, the Society Islands and American Samoa, have undertaken years of detailed research on their turtles. We will include their turtles with our own, as turtles of “our area” or “our region” to increase the data to discuss (1) post-breeding migrations, (2) local breeding activities, (3) the natural migratory lifecycle, and (4) migration disruption.

Post-breeding migrations

Historically, Green turtles bred in our area during the southern summer and disappeared for the winter. It was not known where they went until scientific studies started in the 1970s.

Within our region, Scilly atoll, 900 kilometres east of Palmerston in the Society Islands, was long recognised as a most important breeding site for Green turtles. We will refer to it as Scilly, although it is now known as Manuae, which can lead to confusion with our Manuae, east of Aitutaki.

For twenty years (1952-1971), Scilly residents sent a few hundred turtles a year to Tahiti to be butchered, and they ate about one a week themselves. The turtles were protected in 1971, and in April 1972, the 67 held in lagoon pens “awaiting transportation” were confiscated, flipper-tagged with metal tags and released on the 30^th of April. The initial results were interesting, with single recaptures in July and August in Fiji and Tonga, respectively.

Between 1973 and 1984, metal tags were attached to an additional 461 turtles, including 448 females. All turtles were adults, and most females had shell lengths ranging from 90 to 110 centimetres, with the smallest at 80cm. The upper shell of a turtle is technically called the carapace, and it is usually measured in a curve known as the Curved Carapace Length (CCL). We will refer to this as shell length.

As of 1995, thirteen Scilly-tagged turtles had been recaptured (and invariably consumed): five in Fiji (3000km westward), three in Vanuatu (4000km), and two in New Caledonia (4500km), which are areas with extensive meadows of seagrass, the nutritious favourite food of Green turtles.

The other recaptures, one in Wallis (2500km), one in Tonga (2000km), and one in the Cook Islands at Palmerston (900km), are areas with little or no seagrass and likely represent interrupted migrations to Fiji. No tagged turtles were recaptured in French Polynesia, which has very little seagrass and no evidence of turtles being present during the non-breeding season. (See illustration: Anon. 1974; Balazs et al. 1995, Siota 2011, Mckenzie et al. 2020)

Post-breeding migrations of tagged turtles from our region typically swim to the shallow seagrass meadows in Fiji. Bold names indicate the main Green Turtle rookeries. Gerald McCormack/25032819

In 1996-'97 it was found that 37 per cent of the turtles in the Suva market were of French Polynesian origin, offering further support for the mass westward post-breeding migration of our turtles. (Boyle 1998)

While flipper tags display only the start and end points of an adult turtle’s post-breeding migration, satellite tags reveal the entire track until transmission ceases. Satellite tagging began in 1993 on Rose Atoll, the easternmost island of American Samoa, located immediately west of our Exclusive Economic Zone (EEZ).

By 1995, seven had been sat-tagged, with six post-breeding migrations directly southwest to seagrass areas in Fiji. One swam in the opposite direction to near Raiatea, northwest of Tahiti, where the transmission stopped – there is always one! (Craig et al. 2004)

Those swimming to Fiji typically averaged 41-43km/day. Additionally, 46 turtles were flipper-tagged on Rose Atoll, with three returns: two to Fiji and one to Vanuatu. It has been estimated that 105 females breed on the atoll (Seminoff et al. 2015).

In the Cook Islands, we have had only one satellite-tagged turtle. Mama Onu left Palmerston atoll on the 8^th of February 2001 and swam directly to Fiji. She covered the 2150km in 52 days (41km/day) to settle around Vatulele Island, 30km south of Viti Levu. (See the map of Mama Onu’s journey)

Satellite tracking of Mama Onu from Palmerston to Fiji in 2001. She covered 2,150km in 52 days, averaging 41km/day. For comparison, humpback whales migrate at speeds of 70-140 km/day. Gerald McCormack/25032820

In 2010-2011, the first satellite tagging in French Polynesia occurred on Tetiaroa Atoll, 50km north of Tahiti. Two transmitters soon failed, but those on Vahinerii Myssy, Vaimiti and Maruia remained active until the three reached Fiji. They covered about 3300km in 90 (37km/day), 65 days (51km/day) and 51 (65km/day), respectively.

Of the above 27 tagged turtles, 26 undertook westward post-breeding migrations, mainly to Fiji or beyond, which is overwhelming evidence that Green turtles in our area are seasonal visitors, not year-round “residents”.

Why live in Fiji? The simple answer is its extensive seagrass meadows. Fiji likely has approximately 500km² of nearshore seagrass meadows, extending from the intertidal zone to depths of around 25m. A few meadows also have seaweed (macroalgae) beds, which turtles eat, and they also opportunistically eat benthic invertebrates and dead fish left by fishermen. Seaweeds have about half the nutritional value of seagrass (Piovano et al. 2020).

Elsewhere in the South Pacific, seagrass estimates are: Vanuatu 27km², Wallis-Futuna 24km², America Samoa 1km², Tonga 23km², Cook Islands 0km², and French Polynesia 29km² (McKenzie et al. 2021).

There is little doubt that the immense seagrass meadows of Fiji are turtle heaven for our turtles.

Local breeding activities

On Tetiaroa atoll near Tahiti, the NGO Te Mana o te Moana has been researching the Green turtle since 2004, with detailed breeding research since November 2007. The primary nesting season lasts three months, from November through January, with nesting extremes in July and May.

One interesting discovery was that the number of nests is very variable. In the booming 2017-18 season, 1316 nesting events were recorded, including 629 (48 per cent) active nests, which averaged 86 eggs/nest with 91 per cent hatching success. In contrast, during the 2018-19 season, there were only 88 active nests. (See nesting graph TT2017-'18 – a good season.) (Touron et al. 2018, Touron et al. 2019)

Although the methods and criteria for recording nesting events differed, we can loosely compare nesting on Tetiaroa and Penrhyn atoll, where Penrhyn-based marine zoologist Dr Michael White made extensive records. By coincidence, the 2017-18 season was a boomer on both atolls, with 1617 nesting events on Penrhyn and 1316 on Tetiaroa. There was a significant peak in activity on both atolls from October through February. The main difference in the breeding profiles was the higher level of off-season nesting activity in Penrhyn (White 2012). There is a need to attach satellite tags to some Penrhyn turtles to better understand their location during the non-breeding season.

Penrhyn and Tetiaroa nesting. The nests preseason varies greatly. Coincidentally, the 2017-18 season was spectacular on both islands. Both had a major peak from November to January, although Penrhyn had more breeding during non-peak months. Gerald McCormack/25032821

In addition to Penrhyn, Palmerston atoll has long been recognised as an important breeding site in the Cook Islands, and, recently, Manuae atoll has been recognised as a significant rookery.

Despite Palmerston’s importance, there has never been a systematic study over an entire breeding season. The only substantial record of active nests was a Ministry of Marine Resources (MMR) count of 37 on the 6^th of November, 2018. With nesting continuing for at least another two months, we can estimate that Palmerston may have had over 120 active nests that season, which could indicate approximately 25 females, based on 3-5 nests per female.

Manuae was surveyed by MMR in early November 2017 and found 57 active nests (Morejohn et al. 2017 and 2019). With the season continuing for a further two months, we can speculate that during the season, there could have been approximately 150 active nests on Manuae, involving around 35 females. No active nests were found during a mid-October 2024 survey (Steibl et al. in press).

One of the ironies of turtles from our region, which mainly live on seagrasses close to the islands of Fiji, is that Fiji has relatively few nesting turtles, with only a hundred or two females (Pivano et al. 2019). The cause of such low numbers is believed to be a long-standing tradition of overharvesting. After years of various harvesting regimes, Fiji fully protected its Green turtles under its Endangered and Protected Species Act 2002, with a few permitted exceptions.

Knowing how many active nests have been found on Rarotonga would be interesting. My enquiries to the Ministry of Marine Resources, Te Ipukarea Society and the National Environment Service revealed no records of Rarotonga nesting events. However, a few elders did recall some nesting about 60 years ago.

Despite the lack of published information, I can report on a few historic turtle nests on Rarotonga: three in 1982, one in 1985 and one around 2005.

Faimau Robati lived on motu Oneroa until 1982, when he was thirteen. He recalls three successful nests in 1982 on the seaward side of Oneroa and Motutapu and had noticed similar nesting in previous years (pers. comm. 2025-02).

In February 1985, students Rangi Tuiravakai and Vaiaia Henry found a nest on the beach near the present Tūpapa Health Clinic. They observed approximately 40 eggs and four hatchlings, with two dead and two alive. They took the live hatchlings home and fed them bread and grated coconut for about two weeks before releasing them (CINHT records).

Captain Tama (2025-01 pers. comm.) recalls seeing the remains of turtle eggs on a beach in central Tītīkāveka circa 2005. He concluded that dogs had destroyed it.

The historical rarity of nesting attempts on Rarotonga is likely due to human and dog interference, resulting in a very low success rate. As discussed in part two, this would result in a severe shortage of hatchlings to mature on the feeding ground and migrate here to breed.

• Part II next week