INTRODUCTION

The Corales de Profundidad Natural National Park (PNNCP) is the only fully submarine Marine Protected Area (MPA) in the Colombian Caribbean. It was created in 2013 to protect one of the three known deep coral formations in Colombia, which lies on the edge of the continental shelf (^{Reyes et al., 2005}). Because of its location, the PNNCP comprises seabed on both the continental shelf and the continental slope from 35 to 1240 m (Morales et al., 2017). Due in part to its recent creation, but also to the logistical implications of scientific research in the area, knowledge about biodiversity in the PNNCP is scarce, with only one publication available to date (^{Alonso et al., 2015a}). That paper describes the first observations with remote-technology about biodiversity associated to the MPA seabeds, particularly coral species. There are no published inventories of other biotic groups for the protected area, and the biodiversity it contains is practically unknown. As for fishes, Alonso et al. (2015a) only mention the black grouper Hyporthodus nigritus, captured at a depth of 200 m in 2012 within the Park (^{Bustos-Montes et al., 2013}). ^{Henao-Castro et al. (2016)} recorded the presence of the invasive lionfish between 34-140 m depth within the Park, and Henao-Castro et al. (2019) using baited remote underwater video systems (BRUVS) recorded 18 fish species between 40-80 m depth. There is no other published information about the group for the Park.

The first studies on biological diversity in the PNNCP were carried out by Invemar in cooperation with National Natural Parks of Colombia and Universidad de los Andes. Non-invasive methods based on remote photography and video have been used for the deepest areas (drift camera and remotely operated vehicles - ROV), and technical diving with gas mixtures down to depths of 100 m, in what is known as mesophotic ecosystems (^{Alonso et al., 2015b} and Alonso et al., 2017).

Mesophotic reefs have not been widely studied, despite the fact that they are considered ecosystems that exhibit diverse and highly endemic biological communities (^{Sinniger et al., 2016}). This is mainly due to technical difficulties. Mesophotic ecosystems lay at depths of between 30-150 m, too deep to be studied by conventional SCUBA diving and too shallow to be explored by submersible vehicles, which, added to the terrain conditions in these environments, has conditioned the use of representative sampling methods (^{Puglise et al., 2009}). However, because of the great interest that has emerged in understanding deep ecosystems, techniques and procedures have been adapted for their exploration, including diving with closed circuit equipment or rebreathers (CCR for Closed Circuit Rebreather) using hypoxic air mixtures. This diving technique has been used to explore down to 150 m in different parts of the world for two decades, with important results (^{Rocha et al., 2005}; ^{Pyle et al., 2008}).

In the Colombian Caribbean, exploration of the deep environments of the continental shelf and slope began two decades ago and has been carried out using different methods, providing important records of species in the different biotic groups and the discovery of new species for science (Invernar, 2010; ^{Alonso et al., 2015a}). These findings highlights the importance of continuing researching to know and understand the biological composition, ecological dynamics, and determinants of biodiversity arrangements in these environments. In this sense, this work is a first contribution to the knowledge of the ichthyofauna associated with the mesophotic ecosystems of the PNNCP, specifically in Bajo Frijol, where small patches of zooxanthellated scleractinian corals and sponges are found, with biotic communities similar in species composition to those found in the shallow coral reefs of the Rosario and San Bernardo archipelagos.

STUDY AREA

The Corales de Profundidad Natural National Park is located on the edge of the continental shelf opposite the departments of Bolívar, Sucre and Córdoba, along the following coordinates: 9°43'16.591" - 10°7'30.277" N y 76°0'16.254" - 76°17'41.091" O. The Park comprises an approximate area of 142.192 ha with seabeds at depths of between 34-1234 m (MINAMBIENTE Resolution 0339 of 2013; Morales et al., 2017; Figure 1). The PNNCP is located in the tectonic terrain called the Sinu Folded Belt, formed by the interaction between the Caribbean oceanic plate and the South American continental plate (^{Duque-Caro, 1979}). Due to its location, the PNNCP features seabeds on the continental shelf and on the continental slope, corresponding to 16 geomorphological units (Morales et al., 2017). Among the geomorphological units of the continental shelf are the banks, geoforms to which Bajo Frijol corresponds. Ellipsoidal in shape, the Bajo Frijol bank is an elevation of 116 m on the ground of the platform, with a length of 8.2 km and a width of 1.4 km, facing northeast-southwest at a depth of between 34 to 150 m,with a semi-leveled slope terrace (0.5°) at the top (Morales et al., 2017).

Figure 1 Location of the Bajo Frijol bank in the Corales de Profundidad Natural National Park, Colombian Caribbean. 

MATERIALS AND METHODS

In April, June, and November 2017, three field trips were made to PNNCP, in the sector known as Bajo Frijol (Figure 1). During these trips, we visited six sampling points where the Park administration conducts biological monitoring. The sampling points are located along the following coordinates: 10°1'29'' N, 76°3'51" O y 10°0'47" N, 76°4'5" O, at a depth of between 35-50 m. The exploration was carried out by three divers with O2ptima Rebreather closed circuit technical diving equipment, using Trimix gas mixtures (air, oxygen and helium).

Two divers recorded the ichthyofauna during 20 minutes of roving diving and the third diver was in charge of the photographic record and safety of the activity. Every species observed in the field were identified and listed, and abundance estimates in categories were made. Subsequently, the photographic material obtained in each dive was reviewed to complement the list of observed species. References of the area and nearby sectors were consulted. The specific nomenclature used follows the Eschmeyer's Catalog of Fishes (^{Fricke et al., 2018}). The phylogenetic order of the families follows the proposal by ^{Betancur-R. et al. (2017)} for bony fishes and that of ^{Page et al. (2013)} for cartilaginous fishes. The species in the families are in alphabetical order.

RESULTS AND DISCUSSION

A list is presented with 80 species of fishes recorded in Bajo Frijol (PNNCP) grouped in 29 families, of which 72 correspond to direct observations, 4 to review of photographic material obtained during the field trips, while the remaining 4 were found in the only unpublished report on biodiversity of mesophotic ecosystems of the area that exists to date (^{Sánchez et al., 2016}), and in the book of abstracts of the V Colombian Congress of Zoology (^{Henao-Castro et al., 2019}). The families with the highest species richness were Labridae with 11 species; Serranidae with 10; Balistidae with 6; Carangidae, Pomacanthidae and Pomacentridae with 5 species each. The other families were represented by no more than 4 species (Table 1).

Table 1 Fish species registered in Bajo Frijol in the Corales de Profundidad Natural National Park. Type of record: OD=direct observation; R1=^{Sánchez et al. (2016)}; R2=^{Henao-Castro et al. (2019)}; RF=Photographic record. Abundance: qualitative indicator of abundance based on observations made by two divers by means of the roving diver technique (20 min), which refers to the probability of seeing a species during a dive (adapted from ^{Humann and DeLoach, 2002}), where: AB=abundant (at least several sightings of many individuals - at least 50 - can be expected on nearly every dive); VC=very common (at least several sightings can be expected on almost every dive, but not necessarily many individuals); CM=common (sightings are frequent, but not necessarily expected on every dive); OC=occasional (sightings are not unusual, but are not expected on a regular basis); UN=unfrequent (sightings are unusual); RA=rare (sightings are exceptional). 

The fish community observed corresponds to the known biodiversity in this part of the continental shelf of the Colombian Caribbean. Most of the species recorded are included in the list of 513 species compiled by ^{Polanco-Fernández and Acero-Pizarro (2011)} for the Rosario and San Bernardo archipelagos, with the exception of Halichoeres cyanocephalus and Serranus annularis, which are new records for the ARCO ecoregion (Figure 2).

Figure 2 A) Halichoeres cyanocephalus recorded in the PNNCP at 50 m depth. B) Aggregations of up to eight lionfish were observed in Bajo Frijol between 35-50 m depth. C) The PNNCP provides refuge for threatened species such as the Nassau grouper Epinephelus striatus (CR) and the Yellowfin grouper Mycteroperca venenosa (VU). D) Gravel seabeds covered with algae, characteristic in Bajo Frijol between 35-50 m depth. Photos A, B and D by Deibis Seguro (PNNCP); photo C by L. Chasqui. 

The presence of the invasive lionfish Pterois volitans (VC = very common) was constant during the dives. Generally in each sighting several individuals (up to eight) of considerable size (up to 40 cm LT) aggregated on patches of sponges and corals were observed (Figure 2). In those places it was usual to observe reef fishes that are common prey for lionfish in the Colombian Caribbean according to Acero et al. (2019), such as Thalassoma bifasciatum, Chromis cyanea, Acanthurus coeruleus, Clepticus parrae, Scarus iseri, among others. The lionfish was first sighted in the Colombian continental Caribbean in 2009, and is mentioned for the first time in the PNNCP by ^{Sánchez and Andrade (2014)} in an unpublished report of the first observations made by divers in the area. One year later, Invemar obtained images of that species using a ROV at depths of around 100 m in Bajo Frijol and other sectors of the Park (Alonso et al., 2015). Subsequently, Henao-Castro et al. (2106) summarize the work conducted on that invasive species in the PNNCP, recording the presence of P. volitans in 12 sites with an average size of 40 cm LT, including mesophotic ecosystems and deep coral banks up to 140 meters depth.

^{Andradi-Brown et al. (2017)} reviewed information on lionfish abundance and size in shallow and deep (i.e. mesophotic) reefs with large geographic coverage, concluding that the species has extensively colonized mesophotic ecosystems in the Western Atlantic, exhibiting densities similar to those observed in shallow reefs. They propose that lionfish populations in mesophotic ecosystems could be responsible for the high densities of lionfish recruits observed in shallow reefs, despite the control by spear fishing made by recreational divers in those environments. Thus, mesophotic ecosystems would be acting as a refuge for lionfish populations, in the absence of control efforts due to the depth restrictions (i.e. 30 m depth) of recreational SCUBA diving.

Several threatened species were recorded in the PNNCP, such as the Queen triggerfish Balistes vetula (EN), the snappers Lutjanus analis (VU) and L. cyanopterus (VU), the Nassau grouper Epinephelus striatus (CR), the nurse shark Ginglymostoma cirratum (VU), the silky shark Carcharhinus falciformis (VU), the Rainbow parrotfish Scarus guacamaia (EN) and the groupers Mycteroperca bonaci (VU) and M. venenosa (VU) (Chasqui et al. 2017, Minambiente Resolution 1912 of 2017). These species are currently infrequent in the shallow reefs of the Rosario and San Bernardo archipelagos, which are the nearest coral areas, suggesting the importance of Bajo Frijol as a refuge for different fish species which suffer heavy fishing pressure in shallower coastal areas (Figure 2).

The role of mesophotic coral environments as a key refuge for the conservation of fishing species such as large snappers and groupers, among others, has been highlighted in studies of fish communities associated with mesophotic ecosystems in Puerto Rico (^{Bejarano et al., 2014}), where the presence and predominance of Lutjanus analis, L. bucanella, L. cyanopterus, L. jocu, Mycteroperca bonaci and M. venenosa at depths greater than 40 m was obvious. The importance of these deep-sea environments for the conservation of snappers and groupers threatened by fishing has also been recognized in Brazil (^{Feitoza et al., 2005}). ^{Gress et al. (2016)} reached the same conclusion working on mesophotic reefs in Cozumel, Mexico. ^{Lindfield et al. (2016)} highlight the relationship of this refuge effect in particular for target species of spear fishing on reefs, a fishing gear generally limited to shallow environments because of the restrictions imposed by open circuit SCUBA diving.

Uncommon species in shallow environments such as the Cherubfish Centropyge argi and the Yellowcheek wrasse Halichoeres cyanocephalus were observed in all dives and always showed high abundance (VC = very common). Both species were included by García-Sais (2010) as part of a small assemblage of species indicating mesophotic habitats in Puerto Rico, as they were recorded only at depths greater than 30 meters. The predominant coverage of the bottom at the study site may be determining that abundance, since both species are usually associated with sand and gravel seabeds (^{Robertson et al., 2015}), such as the cover found in Bajo Frijol, where most of the bottom between 40-60 m depth is dominated by gravel covered with macroalgae and rhodolith beds (Figure 2).

Because of their ecological role as a source of food and refuge for different fish species, deep-sea coral formations have been recognized as aggregation sites for commercially important species (^{Roberts and Hirshfield, 2003}). In Florida, for example, a close relationship has been found between species of fisheries interest and the conservation status of deep-sea coral colonies (^{Koenig, 2001}). The presence in Bajo Frijol of a significant number of fisheries interest fish species commonly associated with shallow coral reefs (Figure 2) indicates its importance as a wildlife reserve, despite not having developed coral formations. It also suggests the role of Bajo Frijol as a possible stepping-stone between the shallow coral reefs of the Rosario and San Bernardo archipelagos and the deep habitats in this MPA, including deep coral formations (^{Alonso et al., 2015b}).

Finally, it is important to continue making progress in the knowledge of the PNNCP's ichthyofauna in mesophotic and deep environments. This will allow us to expand the species list for the area and confirm the presence of species such as Canthidermis maculata, Stegastes planifrons and Canthigaster jamestyleri. The first is a rare species in the Colombian continental Caribbean, recently recorded for Colombia only in remote areas of the Seaflower Biosphere Reserve (^{Bolaños-Cubillos et al., 2015}); the second is a damselfish from shallow reef, whose known bathymetric range does not exceed 30 m (^{Robertson et al., 2015}). Both species are included here based on the unpublished report by ^{Sánchez et al. (2016)}. Canthigaster jamestyleri was recorded by ^{Henao-Castro et al. (2019)} as one of the three dominant species in their surveys; however, given that this species was not observed during this work and because of its marked similarity to C. rostrata, a very common species (VC) in the surveys reported here, it is suggested that this record needs to be corroborated.

This work is a contribution to the knowledge of fish biodiversity in the Corales de Profundidad Natural National Park, a recently declared Marine Protected Area about which very little we known. It is also one of the first works on biodiversity in mesophotic ecosystems in general for Colombia. Research on mesophotic ecosystems is a dynamic frontier of knowledge around the world, recently unlocked by the commercial availability of technologies for closed circuit diving, which enables us to reach depths of around 150 m with suitable diving times. Studies in mesophotic ecosystem over the last three decades have yielded an important crop of new species and greatly expanded our knowledge about coral reef biota in aspects such as ecology, physiology, and biogeography. This is a boom in which Colombia, as a country with an emerging economy, is called upon to actively participate.