**4. Discussion**

Our study demonstrates that variation in reef fish diversity, biomass, and abundance in Serranilla can be partly explained by two benthic components, coral cover and rugosity, that contribute to habitat complexity in coral reefs [18,19]. Nevertheless, the mean values of parrotfish biomass and length at Serranilla were low, which could reflect the effect of anthropogenic pressure in the area, mainly through illegal fishing [30–32].

Structural complexity in coral reefs has been evaluated for its ecological relevance, enhancing abundance, biomass, and biodiversity [47,48]. Multiple studies have found that the complexity of healthy coral reefs favors coastal protection, reducing the effects of wave action and other environmental phenomena and maintaining shoreline equilibrium [9,49,50]. A relationship of fish assemblage with the structural complexity, such as we observed at Serranilla in the Seaflower BR, has also been observed at other locations in the Atlantic [51,52] and in other regions of the world [53,54].

Reef habitat structures provide microhabitats and refuge areas that benefit multiple organisms, including reef fish [55,56], with different rugosity levels favoring multiple fish groups [52]. Loss of habitat complexity can negatively affect multiple species [47,57], including reef fish [58], leading to species losses [11] and localized extinctions [59], including species of commercial interest such as parrotfish. Rogers et al. [52] predicted that a loss of complexity could cause a more than three-fold reduction in fishery productivity. Therefore, preserving complexity in coral reefs is crucial, particularly since structural complexity is currently in rapid decline in the Caribbean [17], with one of the largest protected areas in the Caribbean, the Seaflower BR, remaining highly vulnerable to anthropogenic pressure [33].

Coral reef degradation may also result from the loss of critical groups, such as the parrotfish. Their loss, through increased in macroalgae, decreased coral cover, and subsequently a diminished habitat complexity, may lead to a decline in associated organisms [22,60]. At Serranilla it was found that transects closer to Beacon Cay showed lower rugosity and diversity of reef fish. Although greater habitat complexity may have helped maintain higher abundance and biomass on Serranilla's reefs compared with those on San Andrés, direct anthropogenic pressure may still limit any positive effect, and lead to a lower abundance and diversity of reef fish, including parrotfish [22,46,47,52].

Anthropogenic pressure is among the most recurrent threats to coastal ecosystems, since human populations reside nearby and depend on coastal areas for commerce and survival [61,62]. Studies have found a relationship between reef fish assemblages and the distance from dense human populations [63], and found that sites closer to human populations tend to have lower abundance, biomass, and richness of reef fish than areas that are more remote, with an exponential increase in reef fish biomass as the distance from the significant human population increases [64,65].

Considering that Serranilla is far from any significant human population, we expected to find higher biomasses of reef fish, particularly of key coral reef groups such as the parrotfish. Beacon Cay, Serranilla, has only around 12 permanent inhabitants (equivalent to 9.2 inhabitants per 0.1 km2), while San Andrés has more than 65,000 (equivalent to 241 inhabitants per 0.1 km<sup>2</sup> [33]. Additionally, more than 730,000 tourists visit San Andrés each year [65]. However, our data showed that despite the high reef rugosity, most parrotfish at Serranilla had low biomass and abundance values, similar to those found at San Andrés.

Differences in the size and abundance of some parrotfish species between Serranilla and San Andrés seem likely due to a lower fishing pressure at Serranilla. Nevertheless, fishing pressure may have modified the parrotfish assemblages at Serranilla. On San Andrés, fishing is one of the most important commercial activities, and includes spearfishing targeting multiple parrotfish species, such as *S. coeruleus*, *S. coelestinus*, and *S. guacamaia* [66], that were rarely observed during this study. While permitted fishing activity is mainly artisanal, illegal vessels have been reported carrying tons of illegally caught parrotfish [67]. In addition, Bruckner [34] reported illegal fishing boats at Serranilla island during the "Global Reef Expedition" in 2011, and such boats were also observed during the present "Seaflower Research Expedition" in 2017. Fishers have in recent years expressed concern over the absence of large parrotfish in the lagoon as a result of overfishing [68].

Herbivorous fish biomass has been used as one of the indicators for evaluating reef health. In the report on Mesoamerican reefs by the Healthy Reefs Initiative, they recorded 270 kg ha−<sup>1</sup> of herbivorous fish on the healthier reefs compared with 83 kg ha−<sup>1</sup> on reefs in critical conditions [69]. In particular, a relationship has been reported between fishing pressure and parrotfish biomass and size, making parrotfish abundance a useful biological indicator of fishing pressure [63]. On a small island in the Caribbean, exploited reefs, with areas open to artisanal fisheries, were found to have a parrotfish biomass of around 320 kg ha<sup>−</sup>1, with smaller parrotfish contributing most of this biomass (~200 kg ha−1).

We found that Serranilla BR had larger parrotfish than San Andrés, though the biomass in Serranilla BR was nevertheless lower (143 kg·ha−1) than those values reported from other Caribbean MPAs with adequate protection and low local human populations, in which parrotfish biomass values can exceed 500 kg·ha−<sup>1</sup> [70]. In contrast, in areas with high local human population densities and inadequate protection, parrotfish biomass can be rapidly reduced to 50 kg·ha−<sup>1</sup> or less [22,29]. However, other factors may also influence herbivorous fish abundance; for example, on the Alacranes reef, an MPA in the southern part of the Gulf of Mexico, where human activities are restricted, and fishing pressure is low, the values of parrotfish biomass were nevertheless relatively low (~58–128 kg h−1), although the parrotfish present had larger sizes than those observed on Caribbean reefs [71].

As with many other species, small individuals have become a target of fisheries due to the loss of larger individuals and the decline of other commercial fish species more generally, leading to decreased sizes [67]. Parrotfish size is likely related to reef resilience [23,57], since larger individuals can remove more algae from the reef and have more effect on benthic communities than smaller ones [24,29]. In the present study, we found that 90% of the parrotfish species at Serranilla had smaller mean sizes than the typical size reported in FishBase [38]. Notably, the observed mean size of *S. viride* and *S. chrysopterum* were both below the size at first maturity (L50), as recorded in the Colombian Caribbean [72], suggesting a higher fishing pressure at Serranilla than expected.
