**1. Introduction**

Coral reefs are widely recognized as one of the most diverse marine ecosystems, providing essential habitats for numerous species; although coral reefs cover only 0.02% of the ocean's surface, they are believed to harbor nearly 30% of all known marine species [1–4]. Reef ecosystems provide multiple ecosystem services [5–7], including coastal protection [8,9] and the provision of food and livelihoods through fishing activities [10]. However, in recent decades, coral reef ecosystems worldwide have been facing degradation due to multiple environmental and human stressors [11,12]. Currently, ~60% of the world's coral reefs are considered to be threatened [13]. In the Caribbean, by 2003, at least 80% of the coral cover registered in the 1970s was lost [14,15], while in the Atlantic, 75% of coral reefs are considered at risk [12]. This loss of coral cover has often produced a decline in the reef's

**Citation:** Castaño, D.; Morales-de-Anda, D.; Prato, J.; Cupul-Magaña, A.L.; Echeverry, J.P.; Santos-Martínez, A. Reef Structural Complexity Influences Fish Community Metrics on a Remote Oceanic Island: Serranilla Island, Seaflower Biosphere Reserve, Colombia. *Oceans* **2021**, *2*, 611–623. https://doi.org/10.3390/ oceans2030034

Academic Editor: Rupert Ormond

Received: 30 October 2020 Accepted: 18 August 2021 Published: 3 September 2021

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structural complexity, affecting associated organisms including reef fish [16]. The replacement of structurally complex coral species represents a more silent problem [17]; branching coral species have often been replaced by "weedy" species of coral, which display less rugosity and tridimensional structure, so that even if coral cover recovers, the system may possess lower structural complexity [18]. On Caribbean reefs, a considerable loss of species that provide high structural complexity, such as *Acropora palmata*, *A. cervicornis,* and *Montastrea cavernosa*, was evident from the 1970s, aggravated by mass coral mortality as the result of reef erosion caused by the sea urchin *Diadema antillarum* through an increasing prevalence of coral disease and by coral bleaching events [17,19]. Such declines in structural complexity on coral reefs can have negative consequences, including loss of ecological function, biodiversity, and ecosystem services [2,7,20]. However, it is essential to identify, beyond structural complexity, other features of reef ecosystems which are involved in the maintenance of essential functions and processes [21].

Herbivory is one of the key processes in coral reefs that can prevent the overgrowth of corals by algae through food web control over turf and macroalgae [11,22,23]. Herbivorous fishes are considered essential to reef ecosystems' health, especially after the radical decline in the Caribbean of other large herbivores, including sea turtles and manatees [24], and previously dense populations of *D. antillarum* [25]. Among herbivorous fish, parrotfishes (Family: Scaridae) are considered important in promoting the recovery of coral reefs after disturbances (i.e., bleaching events or storms) and in preventing phase shifts from coral to algal-dominated communities [16,26–28]. Notably, in the Caribbean, some Marine Protected Areas (MPAs) have been found to possess high parrotfish biomass, low macroalgae cover, and high coral cover, with abundant coral recruits [28,29]. However, parrotfish are high among the fisheries' target species, crucial for most Caribbean economies [23]. Fishing has been found to deplete many coral reef fish species, promoting reef degradation [30–32].

The Seaflower Biosphere Reserve (Seaflower BR) is one of the largest protected areas in the Caribbean, covering 180,000 km<sup>2</sup> and incorporating a large percentage of Colombia's coral reefs [33]. Although the Seaflower BR is an extensive MPA and experiences oceanic conditions that might be expected to support pristine coral reefs with high structural complexity and biodiversity, the archipelago islands nevertheless experience illegal fishing, including overfishing, together with other anthropogenic pressures [23]. Of the nine islands in the archipelago, Serranilla, northwest of the Seaflower BR, is furthest from the mainland and it was here that recent research efforts were focused during the "Seaflower Research Expedition 2017". We designed our study to evaluate the status of fish assemblages in relation to reef habitat structural complexity, paying particular attention to comparing the parrotfish species of Serranilla from that found at San Andrés.
