DOI: 10.5061/dryad.np5hqbzws
title: Nearshore marine biodiversity of Osa Peninsula, Costa Rica

description: 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.
description: <strong>Underwater visual censuses</strong>
 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 m<sup>2</sup>), and all
 fishes of TL &lt;20 cm were counted in a 2 m wide strip along the
 transect line on the way back (transect area = 50
 m<sup>2</sup>). 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 (http://www.FishBase.org). 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
 (www.fishbase.org). 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 (<em>p<sub>i</sub></em> × ln
 <em>p<sub>i</sub></em>), where
 <em>p<sub>i</sub></em> is the proportion of all
 individuals counted that were of species<em> i</em>.
description: All data are flatfile Excel files.