Lost and found: Patterns of marine litter accumulation on the remote Island of Santa Luzia, Cabo Verde

Santa Luzia, an uninhabited island in the archipelago of Cabo Verde, serves as a natural laboratory and important nesting site for loggerhead turtles Carettacaretta. The island constitutes an Integral Natural Reserve and a Marine Protected Area. We assessed marine litter accumulation on sandy beaches of the island and analysed their spatial patterns using two sampling methods: at a fine scale, sand samples from 1 × 1 m squares were collected, identifying debris larger than 1 mm; at a coarse scale, drone surveys were conducted to identify visible marine debris (>25 mm) in aerial images. We sampled six points on three beaches of the island: Achados (three points), Francisca (two points) and Palmo Tostão (one point). Then, we modelled the abundance of marine debris using topographical variables as explanatory factors, derived from digital surface models (DSM). Our findings reveal that the island is a significant repository for marine litter (>84% composed of plastics), with up to 917 plastic items per m2 in the sand samples and a maximum of 38 macro-debris items per m2 in the drone surveys. Plastic fragments dominate, followed by plastic pellets (at the fine-scale approach) and fishing materials (at the coarse-scale approach). We observed that north-facing, higher-elevation beaches accumulate more large marine litter, while slope and elevation affect their spatial distribution within the beach. Achados Beach faces severe marine debris pollution challenges, and the upcoming climate changes could exacerbate this problem.

Globally important refuge for the loggerhead sea turtle: Maio Island, Cabo Verde

One of the largest nesting colonies of the Vulnerable loggerhead sea turtle Caretta caretta is in Cabo Verde. Here we present the first comprehensive study of loggerhead turtle nesting on the island of Maio in Cabo Verde. During 2016–2019 we monitored 38 km of undeveloped sandy beaches that have minimal artificial lighting and where all nesting on Maio takes place. We counted 4,063 nests in 2016, 5,429 in 2017, 14,364 in 2018 and 7,937 in 2019. The estimated total number of females was 1,016, 1,357, 3,591 and 1,984 in each of these years, respectively. Our findings suggest there are more loggerhead turtles nesting in Cabo Verde than previously estimated, and that this could be the species’ largest nesting subpopulation (followed by Florida, USA and Oman). The inter-annual hatching success (the proportion of eggs producing hatchlings) was 29–38% for the whole island but varied between sites. Our study of 250 clutches showed that flooding affected 38–61% and predation by crabs 40–42%, with hatching success on different beaches in the range of 1–59%. Poaching of eggs was rare (< 2% of clutches), but dogs predated 68.4% of all clutches on the beach nearest the largest human settlement. We evaluated different nest management strategies at multiple sites and estimated productivity of hatchlings (the number of hatchlings that would reach the sea for each management strategy), finding that hatcheries are not always the best option for nest management. As the beaches on Maio are relatively undisturbed, and there is a high abundance and density of turtle nests, the island should be protected as a globally important site for the conservation of the loggerhead turtle, and of coastal biodiversity more broadly.

Conservation importance of previously undescribed abundance trends: increase in loggerhead turtle numbers nesting on an Atlantic island

For many species abundance data from across their entire range are incomplete, and therefore it is difficult to accurately assess their conservation status. Even for species that are large, charismatic and relatively easy to study, conservation assessments are often hampered by lack of data. Here we report a marked, previously undescribed, increase in numbers at a breeding colony of the loggerhead turtle Caretta caretta, a species that is Critically Endangered in several parts of its range, and place this report in the global context for this species. We present a 10-year (2008–2017) dataset of nesting activities for this species on the island of Sal, one of the Cape Verde islands in the Atlantic Ocean. Foot patrols recorded 21,938 nests during the study period. We estimate that the annual number of nests on Sal increased from 506 in 2008 to 7,771 in 2017. Taking into account that there are only two known loggerhead turtle rookeries (on Masirah Island, Oman, and in Florida, USA) with > 50,000 nests reported annually, and few with > 1,000 nests per year, our results suggest that Sal is one of the 10 largest loggerhead turtle rookeries globally. Our work highlights the conservation significance of reporting trends in abundance at breeding sites for marine turtles and other taxa.