Reef fish communities of Mid-Atlantic ridge islands: Expeditions in Ascension (2015), Saint Peter and Saint Paul Archipelago (2018-2019), and Saint Helena (2023)

Study Extent

The sampling dates for each location were as follows: SPSPA in October 2018 and 2019, Ascension Island in August 2015 and St. Helena Island in January 2023.

Sampling

Underwater visual surveys (UVS) were employed to sample fish populations within 40 m2 (20m length ×2m width ×2m height) strip transects during scuba diving excursions. During UVS operations, the diver meticulously unspooled a measuring tape while visually identifying, quantifying, and assessing the total length (LT, cm) of non-cryptic fish specimens exceeding 10 cm in size. Subsequently, while rewinding the tape, identical protocol was followed to document benthic-associated non-cryptic fish species measuring less than 10 cm and cryptic species). Fish size was estimated with varying degrees of precision based on their length: 1 cm precision for fishes with lengths less than 10 cm (LT), 5 cm precision for fishes measuring between 10 and 60 cm, and 10 cm precision for fishes exceeding 60 cm. Estimating fish populations involved the counting of solitary individuals, pairs, or small schools (comprising tens of individuals), while intermediate-sized schools (ranging from tens to hundreds of individuals) were estimated with a precision of 10 individuals. For large schools consisting of many hundreds of individuals, a precision of 50 individuals was applied during the counting process. The range of depths surveyed in Ascension was between 4 m and 13 m depending on the site depth. In St. Helena, the shallowest site was 6m, and the deepest site was 18m. In SPSPA, depth varied from 4 m to 36 m, with most of the sites between 4 and 18 m. In total, we performed 120 UVSs: 55 in SPSPA, 81 in Ascension, and 65 in St. Helena.

Method steps

1. The fish species recorded in the surveys were categorised into distinct trophic groups based on their feeding habits, including macrocarnivores, mobile invertebrate feeders, omnivores, planktivores, herbivores-detritivores, sessile invertebrate feeders, and territorial herbivores. To estimate fish biomass, we employed length–weight transformations and allometric conversions using the equation W =a * LTb , where the parameters an and b represent constants in the allometric growth equation. In cases where coefficient values were unavailable for a particular species, we substituted coefficients derived from closely related species (congeners).

Sampled areas included three oceanic islands located on the Mid-Atlantic Ridge: St. Peter and St. Paul's Archipelago (SPSPA, 0°55′1"N | 29°20′44" W), Ascension Island (7°56′26"S | 14°22′25" W), and St. Helena Island (15°57′56"S | 5°42′41" W).