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Nearshore marine biodiversity of Osa Peninsula, Costa Rica

Cite this dataset

Friedlander, Alan (2022). Nearshore marine biodiversity of Osa Peninsula, Costa Rica [Dataset]. Dryad.


Osa Peninsula in remote southwest Costa Rica harbors 2.5% of global terrestrial biodiversity in only 1,200 km2 and has the largest remaining tract of Pacific lowland wet forest in Mesoamerica. However, little is known about the marine ecosystems of this diverse region. Much of the coastline consists of soft sediment exposed to strong wave action. Three major hard bottom habitat types define this region, including: 1) coral reefs around Isla del Caño Biological Reserve, a no-take marine protected area (MPA) of 52 km2, 2) coastal rocky reefs and islets along the peninsula, including Corcovado National Park, and 3) submerged pinnacles just outside the Isla del Caño MPA. Average coral cover at Isla del Caño was 21%, composed primarily of Porites lobata and Pocillopora elegans. In contrast, coastal rocky reefs were dominated by turf algae (39.8%) and macroalgae (20.7%) with low coral cover (1.1%). Submerged pinnacles were dominated by crustose coralline algae (33.3%) and erect coralline algae (25.7%). Fish assemblage characteristics (species richness, abundance, biomass) were significantly higher at the pinnacles compared to the other habitats and was dominated by schooling species such as Haemulon steindachneri, and the herbivores Kyphosus ocyurus, and Acanthurus xanthopterus. Top predators, primarily Triaenodon obesus, Caranx sexfasciatus, and Lutjanus argentimaculatus, were also most abundant at these pinnacles and accounted for the largest differences in fish trophic structure among habitats. Despite Isla del Caño being fully protected from fishing, biomass was similar to fished areas along the coast and lower than the adjacent submerged pinnacles outside the reserve. Similarly, Corcovado National Park includes 20.3 km2 of no-take MPAs; however, there is limited enforcement, and we noted several instances of fishing within the park. The unique configuration of healthy offshore coral reefs and pinnacles connected to coastal habitats provides corridors for many species including large predators such as sharks and other marine megafauna, which warrants additional protection.


Underwater visual censuses

For algae, corals, and other sessile invertebrates we used a line-point intercept methodology along each transect, recording the species or taxa found every 20 cm on the measuring tape. Five 10-m long transects were conducted at each depth strata (~ 10 and 20 m). Mobile invertebrate densities were quantified in 50 x 50 cm quadrats, with 25 quadrats per depth stratum. All individuals were identified to the lowest possible taxa.

Within each depth strata fish transects were conducted along three 25-m long transects laid along isobaths within a homogeneous habitat, with 5 m separating each transect. At each survey site, a scuba diver counted and sized all fishes of total length (TL) ≥20 cm within a 4 m wide strip on an initial ‘‘swim out’’ while the transect line was laid (transect area = 100 m2), and all fishes of TL <20 cm were counted in a 2 m wide strip along the transect line on the way back (transect area = 50 m2). Total fish lengths were estimated to the nearest cm. All fishes were identified to the species level based on Robertson and Allen and categorized into four trophic groups: top predators (trophic level ≥ 4.0), herbivores, secondary consumers, and planktivores using FishBase ( Valid scientific names were verified according to Fricke et al.

The biomass of individual fishes was estimated using the allometric length-weight conversion: W = aTLb, where parameters a and b are species-specific constants, TL is total length in cm, and W is weight in grams. Length-weight fitting parameters were obtained from FishBase ( The sum of all individual weights and numerical densities was used to estimate biomass density by species. Species diversity was calculated using the Shannon-Wiener diversity index: H’ =  Σ (pi × ln pi), where pi is the proportion of all individuals counted that were of species i.

Usage notes

All data are flatfile Excel files.


National Geographic Society and Pristine Seas donors