In my blog of 16 July, I described some work that we had started in an attempt to explain how small fragments of plastic, mainly blue and green, appeared on the ground in the Sooty Tern colony on Bird Island. Joanna Suares, the island’s conservation officer, continued the weekly monitoring of the fixed quadrats in the colony until 1 September and collected all the fragments from each quadrat. By this time juveniles were fledging and their parents were leaving the island.
Small plastic fragments appeared in the marked quadrats throughout the breeding season, although the number of fragments diminished towards the end as the chicks were preparing to leave, but we still don’t know how plastics get there! Since my first finding of small pieces of plastic in the colony in 1994 I have never seen a Sooty Tern holding a fragment in its bill. This year we dissected seven adults found dead on the island and 17 chicks found dead in the colony. None of these birds contained plastic fragments in their digestive tracts, but none contained any food. The chicks had presumably starved during a period of food shortage and six of the adults showed signs of being attacked by Frigatebirds, which chase Sooty Terns to make them regurgitate their last meal, which Frigatebirds then eat. We also examined small fish and squid that Sooty Terns regurgitated when we handled them to ring them – none of these food items contained plastic fragments. Clearly, there is more work to do here to discover how Sooty Terns import the pieces.
Although we have not yet fully analysed the data, some of the findings are informative. From our 25 2 metre x 2 metre fixed quadrats our weekly samples totalled 2138 plastic fragments. Of these 1870 (87 %) were smaller than 5 mm (fitting through a 5mm sieve). Only one fragment failed to get through a 10 mm sieve.
67 % of the total fragments were blue and 29 % turquoise/green, the remaining small numbers being white, red, purple, orange and yellow.
As on many of the world’s coastlines, Bird Island’s beautiful beaches receive their share of plastic waste. This is usually in the form of plastic bottles and other containers (usually intact), cigarette lighters, flip-flops, fishing buoys, nets and ropes and other larger items. The island’s conservation staff undertake beach clean-ups and tourists are encouraged to participate, with receptacles (big plastic barrels!) at the top of the beaches. But we haven’t found the kinds of small fragment that we collected in the colony on the beaches. As far as we know, no-one has taken samples of sea-surface plastics in Seychelles, so we have little idea what the composition of floating plastic is around the islands.
After Christine and I left Bird Island at the end of the Sooty Terns’ breeding season, we paid two visits to Police Bay, an exposed sandy beach towards the southern tip of Seychelles’ main island, Mahe. On 8 October, we found the high tide mark contaminated with a wide variety of plastic debris among the seaweed, but unlike Bird most was in large fragments with fewer intact bottles. The second visit, on 14 November, was very different, with no seaweed and fewer and mainly smaller plastic remains than on the earlier date. None of the samples contained translucent plastic, as from most bottles, and the two beach samples from Mahe contained more white plastic than the Bird Island samples. But fragments of broken plastic items were generally much larger than the fragments in the Sooty Tern colony (see picture).
A final statistic – although the fragments are small, we estimate from multiplying up from the weight of the samples collected weekly from the quadrats to the area of the entire Sooty Tern colony on Bird Island, that more than 35 kg of plastic fragments were brought into the colony area during the 2019 breeding season.
The hows and whys need further investigation, but so far we have not detected any adverse effects on the birds.