Keywords

Introduction

The Gulf of Guinea Oceanic Islands (GGOI), Príncipe, São Tomé, and Annobón, are oceanic islands located in the Tropical Eastern Atlantic. The GGOI are part of the Cameroon Volcanic Line, an intraplate basalt in the ocean-continent boundary region, ranging over 1600 km (Burke 2001; Elsheikh et al. 2014; Belay et al. 2019). All three GGOI are ideally located for fish diversity: close enough to the African continent to host typical shelf region fish species, but also with a narrow platform separated by seas with over 1500 m depth that provide coastal habitat to several fishes. In addition, several currents contribute to coastal upwelling (Bakun 1978; Djakouré et al. 2017) and biological productivity (Binet 1997; Ukwe et al. 2006) for highly migratory offshore and deep-sea fish species. The GGOI are at the crossroads of three major currents: one incoming (from west to east), the Guinea Current (GC), and two outgoing (from east to west), the South Equatorial Current (SEC) and the Gabon-Congo Undercurrent (Djakouré et al. 2014). The GC, sourcing from the combination of the North Equatorial Countercurrent and the Canary Current, flows east along the western coast of Africa (from Sierra Leone to Nigeria) with slight seasonal flow variations in direction and velocity, salinity, and sea-surface temperature (Richardson and Reverdin 1987; Odekunle and Eludoyin 2008; Djakouré et al. 2014, 2017). The northern part of the SEC borders the GC and flows westward. The Equatorial Undercurrent (EUC) also flows eastward below the SEC (Djakouré et al. 2014; Herbert et al. 2016; Houndegnonto et al. 2021; see also Ceríaco et al. 2022a). A Guinea Undercurrent (GUC) is also present, flowing eastward, in deeper water along the coast. The GGOI are influenced by all these currents in the “southern alternance region,” dominated by strong seasonal contrasts and with influxes of equatorial upwelling (Le Lœuff and Cosel 1998). Water salinity, temperature, and turbidity are also influenced by major freshwater river discharges (Congo and Niger basins) and resulting plumes (Alory et al. 2021; Houndegnonto et al. 2021; Ceríaco et al. 2022a). The combination of these characteristics, along with the upwelling and high biological productivity, contribute to the occurrence of a spectacular fish diversity.

The marine ichthyofauna of the Gulf of Guinea, including the continental shelf, continental islands (Bioko), and oceanic islands, presents a remarkable level of endemism (approximately 20%), but many species are still poorly known and studied (Jones 1994). The endemism of reef fish species reaches about 65% in some of these areas, indicating their high degree of isolation (Jones 1994). By contrast, only three freshwater fish species (all non-indigenous) are recorded for the islands of Príncipe, São Tomé, and Annobón, all showing tolerance to salinity and capable of dispersal among the oceanic islands (Jones 1994). The GGOI are part of the Guinea Current Large Marine Ecosystem (GCLME), extending from Guinea Bissau to Angola (to the northern seasonal limit of the Benguela Current) and covering 16 countries’ Exclusive Economic Zones (Ukwe et al. 2006). Because of its bathymetry, chemistry, hydrography, and trophodynamics, the GCLME is among the most productive coastal and offshore waters in the world with rich fishery resources, an important reservoir of marine biological diversity, and excellent potential for tourism (Ukwe et al. 2003).

Most of the fish species recorded from the GGOI are also present in other parts of the Gulf of Guinea, with few species endemic to the islands, and several species with amphi-Atlantic or circum-global distributions. As of 2019, a total of 268 coastal fish species have been recorded in the GGOI (see “Species Diversity” section) with about 12% of the species (28) reported as endemic to the Gulf of Guinea, and a few of these only observed in São Tomé and Príncipe, such as Clepticus africanus Heiser, Moura and Robertson, 2000, and Scorpaena annobonae Eschmeyer, 1969 (Wirtz et al. 2007; Wirtz 2017). This low level of endemism is likely a consequence of the vagility of marine fishes as zooplankton and the proximity of the islands to the African continent (Krakstad et al. 2010).

The current chapter presents a brief overview of the marine and freshwater ecosystems present in the GGOI, the current knowledge of marine and freshwater fish species, biogeography and evolution, and finally conservation. An updated taxonomic checklist of marine (coastal, offshore, and deep-water) and freshwater species is presented, with revised inventories for coastal and reef fish species.

Brief History of Ichthyology Research

In 1871, the Portuguese naturalist Félix António Brito Capello (1828–1879) published the first list of fishes accessioned at the Lisbon Museum collection. This list, in three parts, includes specimens from the Portuguese islands of Madeira and Azores, and from its overseas territories, including São Tomé and Príncipe (Capello 1871a, b, 1872). After his death, António Roberto Pereira Guimarães (?–1889?) continued Capello’s analysis of the material housed at the Lisbon Museum and published two additional papers on the topic (Guimarães 1882, 1884). Later, the Portuguese zoologist Balthazar Osório (1855–1926) presented the first list focusing on fish species from São Tomé and Príncipe, mostly based on the specimens collected by the Portuguese naturalists Adolfo Möller (1842–1920) and Francisco Newton (1864–1909) (Osório 1891, 1892, 1893, 1894, 1895a, 1898, 1906), and from Annobón (Osório 1895b), with several descriptions of species and original information. After a gap of about five decades, Frade (1955) and Frade and Correia da Costa (1956, 1957) reported new records based on pelagic fisheries species (see also Almeida and Alves 2019). Later, from 1961 to 1987, several international scientific expeditions provided complementary information (Arnoult et al. 1966; Bayer et al. 1966; Blanc et al. 1968; Iwamoto 1970; see Afonso et al. 1999 and Ceríaco et al. 2022b for a detailed bibliography), with rare studies reporting new records (Thys van den Audenaerde and Tortonese 1974). Only during the late twentieth and early twenty-first centuries, several publications focused on São Tomé or Príncipe and published new species description and records (e.g., Afonso et al. 1999; Wirtz et al. 2007; Rocha et al. 2012; Almeida and Alves 2017, 2019; Iwamoto and Wirtz 2018). Complementarily, several reports and guides estimating species occurrence around the Gulf of Guinea Islands were published by FAO and others (e.g., Allen 1985; Carpenter and De Angelis 2016a–c; Almeida and Biscoito 2019; Sutton et al. 2020).

Marine and Freshwater Ecosystems

A marine ecosystem can be defined as the geographic area (of any size), comprised of communities of organisms and their environment, where biological and energy interactions are greater within than with adjacent ecosystems (Zhao and Costello 2020). This biological system is characterized by two factors: the biotic (e.g., plants, animals, microbes) and abiotic (e.g., sunlight, oxygen, dissolved nutrients, depth, temperature). These components influence the dynamics of natural communities at different spatial scales, from global to local. Marine ecosystems of the GGOI are underwater equivalent of tropical forests. Both natural systems are complex and three-dimensional. Furthermore, they have an impressive variety of habitats from the intertidal zone to the abyssal region (Laborel 1974).

The three oceanic islands (Príncipe, São Tomé, and Annobón) that make up this system have different geological ages and the steep underwater relief results in a relatively small, shallow platform (Cowburn 2018; Maia et al. 2018a). The underwater areas of the island seascapes are mainly dominated by volcanic rocky reefs with limited coral growth (Laborel 1974; Quimbayo et al. 2019). Ecological studies carried out in recent years have described a variety of marine habitats. For example, on Príncipe Island, Cowburn (2018) mapped four subtidal habitats and four coastal habitats along the island. In addition, a recent study investigated the role of four different reef microhabitats in shaping biological interactions of fishes (Canterle et al. 2020). Regarding São Tomé Island, the scenario is very similar to that of Príncipe. Maia et al. (2018a) characterized reef environments on this island based on the composition of the benthic community and found a diversity of habitats between the ranges of 10–30 m deep, including a new habitat in the deep reef north of the island (Morais and Maia 2017).

Major Aquatic Ecosystems in the Gulf of Guinea Oceanic Islands

Estuaries (Fig. 17.1, 1): An estuary is a coastal zone sheltered from extreme weather where oceans meet rivers, and nutrients and salts from the ocean mix with those from the river (Cameron and Pritchard 1963). As a result, estuaries are among the most productive places on Earth and support many life forms. Because they are located where rivers join the ocean, estuaries have traditionally supported many human communities and activities like fishing, shipping, and transportation. Some of the larger rivers flowing from the islands form brackish lagoons, usually bounded at the seaward edge by sand banks that only submerge during the highest tides (Cowburn 2018). These lagoons appear to be an important habitat for some resident fish species (e.g., Periophthalmus barbarus (Linnaeus, 1766)) and a nursery area for reef fish species (e.g., Lutjanus agennes Bleeker, 1863; Caranx hippos (Linnaeus, 1766)), crustaceans, molluscs, and other marine life, probably due to the concentration of nutrients in these areas.

Fig. 17.1
Eight photographs of major aquatic ecosystems in the Gulf of Guinea.It includes estuaries, mangrove forests,coral reefs, seagrass, open, deep-sea oceans, and rivers.

Gulf of Guinea oceanic island aquatic ecosystems: (1) Estuary; (2) Mangrove forest; (3–5) Coral reefs; (6) Coral reefs and seagrass; (7) Open and deep-sea ocean; (8) River. Photo credits: (1, 6–8) Hugulay Albuquerque Maia, (2) Luis MP Ceríaco, (3–5) Luiz Rocha

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Mangrove Forests (Fig. 17.1, 2): Mangroves are considered blue carbon ecosystems because they are more efficient at absorbing and storing large amounts of carbon compared to terrestrial ecosystems (Mcleod et al. 2011). Until 2010 this habitat was mentioned in the literature as present only on the island of São Tomé (Spalding et al. 2010) but more recently, small extensions on Príncipe Island have been identified. Haroun et al. (2018) provided updated information about the flora and fauna, and environmental, conservation and management issues related to mangroves present along the coasts of these islands.

Coral Reefs (Fig. 17.1, 3–6): Despite crystal-clear waters and optimum temperature for coral development, the GGOI do not present a homogeneous matrix of coral reefs, but instead exhibit more complex microhabitats spread in their rocky and biogenic reefs (Maia et al. 2018a). These habitats are composed of some key benthic organisms, including epilithic algal matrix, calcareous algae, coralline algae (that form small ~5 cm diameter globular structures over mobile substrates), macroalgae, hard corals, sponges, zoanthids and gorgonians (Laborel 1974; Maia et al. 2018a). Rocky reefs and solid shores occur where the volcanic basalt bedrock is exposed (Cowburn 2018). The seagrass Halodule wrightii Ascherson, 1868 was found along the coast of São Tomé and Príncipe Islands (Alexandre et al. 2017). No data are available for Annobón Island.

Open and Deep-Sea Ocean (Fig. 17.1, 7): Open ocean ecosystems vary widely as the depth of the ocean changes. At the surface of the ocean (the euphotic zone), the ecosystem receives plenty of light and oxygen and thus is fairly warm and supports many photosynthetic organisms. Many of the organisms that we associate with marine ecosystems, such as whales, dolphins, cephalopods, and sharks, live in the open ocean. As the depth of the ocean increases, it gets darker, colder, and less oxygen is available. Organisms living in deep-sea ecosystems within the dysphotic and aphotic zones have unusual adaptations that help them survive in these challenging environments. Some organisms have extremely large mouths that allow them to catch whatever nutrients fall from shallower ocean depths. Others get their energy via the chemosynthesis of chemicals from hydrothermal vents. Although the underwater geomorphology of the GGOI is known, it is thought that they harbor some of the least known tropical reefs in the world. Underwater forests in mesophotic reefs are known from the northwest of São Tomé Island (Morais and Maia 2017) that are dominated by black corals between 30 and 50 m depth.

Streams and Rivers (Fig. 17.1, 8): The hydrographic structure of the GGOI is radial, from the central mountains to the shore, resulting in numerous streams and small rivers (up to 27 km length) (e.g., in São Tomé: Ió Grande, Caué, Mussacavu, Quija, Rio do Ouro, Contador; in Príncipe: Rio Papagaio; in Annobón: A Bobo; see Ceríaco et al. 2022a) or crater-lakes (e.g., in São Tomé: Lagoa Amélia; in Annobón: Lago A Pot). The river network is well distributed around the islands, entering the sea by creating small estuarine habitats (12 mangroves in São Tomé and 3 in Príncipe) or small waterfalls or cascades. Several small inland lagoons are also distributed over the islands. These habitats host the fishes Eleotris vittata Duméril, 1861, Sicydium bustamantei Greeff, 1884, and Aplocheilichthys spilauchen (Duméril, 1861). Several aquatic invertebrates also inhabit freshwater habitats, including GGOI endemics such as the snail Neritina manoeli (Dohrn, 1866), or the crabs Potamonautes principe Cumberlidge, Clark and Baillie, 2002, Potamonautes saotome Cumberlidge and Daniels, 2018, and Potamonautes margaritarius (Milne-Edwards, 1869) (Cumberlidge et al. 2002; Allen et al. 2011; Cumberlidge and Daniels 2018).

Species Diversity

To compile an updated taxonomic checklist of the marine (coastal, offshore, and deep-water) and freshwater fish species occurring in the GGOI, we reviewed the bibliography. This included historical and recent inventories and taxonomic studies (Osório 1891, 1892, 1893, 1894, 1895a, b, 1898, 1906, 1917; Fowler 1936a, b; Frade 1955; Frade and Correia da Costa 1956, 1957; Arnoult et al. 1966; Bayer et al. 1966; Blanc et al. 1968; Thys van den Audenaerde and Tortonese 1974; Afonso et al. 1999; Pezold et al. 2006; Fricke 2007; Wirtz et al. 2007; Kovačić and Schliewen 2008; Schliewen and Kovačić 2008; Rocha et al. 2012; Félix et al. 2016; Reiner and Wirtz 2016; Vasco-Rodrigues et al. 2016; Wirtz and Iwamoto 2016; Almeida and Alves 2017, 2019; Fricke and Wirtz 2017; Tuya et al. 2017; Wirtz 2017; Iwamoto and Wirtz 2018) as well as general reports and species revisions on the ichthyofauna of the eastern Atlantic Ocean (Compagno 1984a, b, 2001; Allen 1985; Nakamura 1985; Whitehead 1985; Whitehead et al. 1988; Carpenter and Allen 1989; Heemstra and Randall 1993; Nakamura and Parin 1993; Nielsen et al. 1999; Krakstad et al. 2010; Ebert 2015; Carpenter and De Angelis 2016a–c; Last et al. 2016; Vasco-Rodrigues et al. 2018; Parenti and Randall 2020; Sutton et al. 2020). These later sources allowed us to include deep-sea fishes (mostly Holocephali and Elasmobranchii) and large pelagic species that likely occur in waters around the GGOI.

In addition, we also compiled a list of species that may occur in the waters of the GGOI, based on known occurrences in the Gulf of Guinea. Therefore, we searched for voucher records in databases listing international natural history museum specimens (e.g., FishNet2 2021; Froese and Pauly 2021; GBIF 2021; iDigBio 2021; OBIS 2021) or other published references. Classification, authority and date follow Fricke et al. (2021), and family arrangement follows Van der Laan et al. (2014). We also list several questionable and erroneous records, which were verified by us against specimens in natural history museum collections or published data. Common names are mostly those adopted by the Food and Agricultural Organization of the United Nations (Carpenter and De Angelis 2016a–c), FishBase (Froese and Pauly 2021) or provided by the original species descriptions.

The compiled full list contains 1045 species (Appendix). Of these, 553 species are confirmed to occur in the GGOI, including 515 Actinopteri distributed in 39 orders (141 families), 37 Elasmobranchii in six orders (17 families), and one Holocephali. The 450 potentially occurring species consist of 385 Actinopteri (30 orders, 109 families) and 65 Elasmobranchii (10 orders, 28 families). Additionally, 32 Actinopteri and five Elasmobranchii previously reported for these islands are here considered as erroneous and five records of Actinopteri are questionable.

Focusing exclusively on the 553 confirmed species, Elasmobranchii (elasmobranchs: sharks, rays, skates, and wedgefishes) accounts for 6.7% (37 species) of the diversity, Holocephali (chimaeras) for 0.2% (one species), and Actinopteri (Actinopterygians: bony or ray-finned fishes) for 93.1% (515 species). A total of 46 orders and 159 families were recorded, with the richest families being Gobiidae (25 species), Carangidae (23), Serranidae (22), Stomiidae (19), and Myctophidae (18—Table 17.1).

Table 17.1 Classification and diversity of the confirmed Gulf of Guinea oceanic islands fish fauna. Taxonomic arrangement follows Van der Laan et al. (2014)
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Elasmobranchii

Among the Elasmobranchii, 37 species of sharks and batoid fishes (wedgefishes and rays) are confirmed to occur in the GGOI. Sharks, belonging to three orders (Carcharhiniformes, Lamniformes, Orectolobiformes), account for 45.9% (17 species), while rays, belonging to two orders (Myliobatiformes, Torpediniformes), account for 51.4% (19 species) of Elasmobranchii diversity. Wedgefishes, order Rhinopristiformes are represented by a single species (2.7%). The most speciose orders are Myliobatiformes, with 17 species (46%), followed by Carcharhiniformes with 12 species (32.4%). A total of 17 families are listed, with Carcharhinidae, Dasyatidae, and Mobulidae presenting the highest number of species, with 21.6% (8), 18.9% (7), and 10.8% (4), respectively.

The first record of African wedgefish, Rhynchobatus luebberti Ehrenbaum, 1915, for São Tomé (Reiner and Wirtz 2016) deserves a special highlight. This species is Critically Endangered (CR—Kyne and Jabado 2019), and has a limited Eastern Tropical Atlantic range distribution, from Mauritania to Congo (Carpenter and De Angelis 2016a). The Scalloped hammerhead shark, Sphyrna lewini (Griffith and Smith, 1834), and Sand tiger shark, Carcharias taurus Rafinesque, 1810, records from the GGOI (with the exception of Annobón) are also noteworthy as both species are also assessed as CR (Rigby et al. 2021). The iconic Whale shark, Rhincodon typus Smith, 1828, assessed as Endangered (EN—Pierce and Norman 2016), was observed in 2015 around São Tomé and the Gulf of Guinea (Vasco-Rodrigues et al. 2016). The species presents a circumtropical distribution with high suitability habitat in the eastern Atlantic (around Gabon, Congo, and Equatorial Guinea) (Sequeira et al. 2014). Recently, a shark specimen captured by locals had several features attributable to Tiger shark, Galeocerdo cuvier (Péron and Lesueur, 1822), a Near Threatened species (NT—Ferreira and Simpfendorfer 2019). Despite the low resolution of the available image (see Fig. 17.2, 1), this is the first observation confirming the occurrence of this species around São Tomé, but the species had already been reported from Príncipe (Carpenter and de Angelis 2016a). In a recent study, Bernard et al. (2021) confirmed that Tiger shark populations from the Atlantic Ocean are genetically distinct from the Indo-Pacific Ocean populations showing that these long-distance dispersing populations are not interbreeding.

Fig. 17.2
Eight photographs of species of fish from the Gulf of Guinea.It includes Galeocerdo cuvier,Periophthalmus barbarus,Apletodon wirtzi, Acanthostracion notacanthus,Bathygobius burtoni, Rubropunctatus Delais,Paraconger caudilimbatus, and Scorpaenodes africanus.

Gulf of Guinea oceanic island fishes: (1) Tiger Shark Galeocerdo cuvier (Péron and Lesueur, 1822); (2) Atlantic Mudskipper Periophthalmus barbarus (Linnaeus, 1766); (3) São Tomé Clingfish Apletodon wirtzi Fricke, 2007; (4) Island Cowfish (juvenile) Acanthostracion notacanthus (Bleeker, 1863); (5) Small Goby Bathygobius burtoni (O’Shaughnessy, 1875); (6) Small Goby Gobius aff. rubropunctatus Delais, 1951; (7) Margintail Paraconger caudilimbatus (Poey, 1867); (8) African Speckled Scorpionfish Scorpaenodes africanus Pfaff, 1933. Photo credits: (1) Ivete Carneiro, (2) Luis MP Ceríaco, (3–8) João Luiz Gasparini

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Holocephali

The sole Holocephali (Chimaeriformes, Rhinochimaeridae), the Sicklefin Chimaera, Neoharriotta pinnata (Schnakenbeck, 1931), accounts for 0.2% of the confirmed species in the region and is considered a Near Threatened species. The Sicklefin Chimaera is known in the eastern Atlantic Ocean off the west African coast from Western Sahara to Namibia, including the Gulf of Guinea islands. The species is found at the edge of the shelf in depths ranging from 200 to 600 m (Carpenter and De Angelis 2016a).

Actinopteri

The Actinopteri is the most diverse fish class, with 515 confirmed species for the GGOI. A total of 141 families were recorded, with Gobiidae being the richest with 4.8% of the species (25), followed by Carangidae with 4.5% (23), Serranidae with 4.3% (22), Stomiidae with 3.7% (19), Myctophidae with 3.5% (18), Sparidae with 2.9% (15), and Haemulidae, Muraenidae, and Ophichthidae with 2.1% (11) each. All 132 remaining families are represented by fewer than ten species and account for the remaining 70% of the species. To be as exhaustive as possible, the current checklist integrates potential deep-sea and large migrant pelagic fish species based on several guides and reports. Due to the deep water around all three GGOI, several species (e.g., Opisthoproctus soleatus Vaillant, 1888; Scopelosaurus argenteus (Maul, 1954)) were collected by offshore scientific surveys or accidentally by industrial fishing vessels. In addition, several deep-sea fish species are already reported in the literature for the GGOI (e.g., Cyclothone spp., Ichthyococcus ovatus (Cocco, 1838), Vinciguerria nimbaria (Jordan and Williams, 1895)).

The Actinopteri fishes present a high variety of shapes, distributions, and behaviors. The extraordinary and unusual looking Atlantic mudskipper, P. barbarus, occurring along the West African coast, from Morocco to Angola and several offshore islands, is also present in the GGOI (Fig. 17.2, 2). Of special interest are: the São Tomé clingfish, Apletodon wirtzi Fricke, 2007, endemic to the GGOI and currently only known from its type locality, Bombom Islet, north of Príncipe Island (Fricke 2007; Fig. 17.2, 3); the Island cowfish, Acanthostracion notacanthus (Bleeker, 1863), which has a restricted distribution around several islands (São Tomé, Príncipe, Saint Helena, Ascension, and Azores) and two African coastal locations (Ghana and Angola) (Fig. 17.2, 4); one small goby, Bathygobius burtoni (O’Shaughnessy, 1875), an Endangered species and Gulf of Guinea endemic (Ghana to Cameroon, Bioko Island), which is confirmed from São Tomé and Príncipe islands (Carpenter et al. 2015—Fig. 17.2, 5); another small goby, Gobius aff. rubropunctatus Delais, 1951, from São Tomé and Príncipe islands that is a putative undescribed species (Wirtz et al. 2007—Fig. 17.2, 6); the Margintail, Paraconger caudilimbatus (Poey, 1867), an amphi-Atlantic species, only reported from São Tomé Island in the eastern Atlantic (Wirtz et al. 2007—Fig. 17.2, 7); and the African speckled scorpionfish, Scorpaenodes africanus Pfaff, 1933, with a fragmented distribution including Senegal, São Tomé, and Annobón (Eschmeyer 1969—Fig. 17.2, 8).

The American whitespotted filefish, Cantherhines macrocerus (Hollard, 1853), a typical western Atlantic species, also occurs in the eastern Atlantic Ocean. The species is suspected to have been transported to the Gulf of Guinea by oil platforms coming from Brazil or the Caribbean (Herrero-Barrencua et al. 2019). Nonetheless, natural dispersal observed in a western Atlantic congener (Cantherhines pullus (Ranzani, 1842)) into the Gulf of Guinea (Afonso et al. 1999) suggests a similar scenario for C. macrocerus is possible (Herrero-Barrencua et al. 2019). Two introduced freshwater species have been reported for the islands: the Mozambique tilapia, Oreochromis mossambicus (Peters, 1852), in São Tomé (Félix et al. 2016), and the Banded lampeye, A. spilauchen, in Príncipe (Cravo 2021). Both introduction dates are not determined, but the Mozambique tilapia is already widespread throughout the island (Félix et al. 2016).

Compared to the most recent studies regarding the fishes of the GGOI (Wirtz 2017; Iwamoto and Wirtz 2018), the present work includes several unique and new records: Cichlidae—O. mossambicus, introduced species; Exocoetidae—Hirundichthys affinis (Günther, 1866), new record for São Tomé; Gempylidae—Nealotus tripes Johnson, 1865; Monacanthidae—C. macrocerus; Polymixiidae—Polymixia nobilis Lowe, 1836; Serranidae—Anthias cyprinoides (Katayama & Amaoka, 1986), Serranus accraensis (Norman, 1931), Serranus drewesi Iwamoto, 2018, Serranus heterurus (Cadenat, 1937); Sparidae—Spicara melanurus (Valenciennes, 1830); and Stomiidae—Bathophilus nigerrimus Giglioli, 1882 (Krakstad et al. 2010; Félix et al. 2016; Almeida and Alves 2017, 2019; Iwamoto and Wirtz 2018; Vasco-Rodrigues et al. 2018; Herrero-Barrencua et al. 2019; Parenti and Randall 2020; Cravo 2021). Gobioides cf. africanus (Giltay, 1935), Gobiidae, reported by Cravo (2021) needs confirmation.

Biogeography and Evolution of Fishes in Gulf of Guinea Oceanic Islands

Oceanic island ecosystems in the Tropical Eastern Atlantic (TEA) include the Cape Verde archipelago and the islands of the Gulf of Guinea: Príncipe, São Tomé, and Annobón (Floeter et al. 2008). Despite its relatively old age, São Tomé has low marine endemism (e.g., 3% for fishes; Hachich et al. 2015) due to high oceanographic connectivity to the African coast (Wirtz 2003; Floeter et al. 2008). On the other hand, the regional endemism level of the TEA is high (30%; Floeter et al. 2008), a phenomenon presumably due to the geographic isolation of the TEA from the other Atlantic reef areas (e.g., ~3500 km from the Brazil and ~8696 km from the Caribbean; Floeter et al. 2008), as well as a history of recurrent isolation and connectivity with the Indo-Pacific at an evolutionary timescale (Cowman et al. 2017). Indeed, several species show a trans-Atlantic distribution (amphi-Atlantic) with most of the species belonging to families of pelagic-spawners with long pelagic larval durations (e.g., Muraenidae, Serranidae), but also smaller-sized genera (e.g., Abudefduf taurus; Müller and Troschel, 1848) and Centropyge aurantonotus Burgess, 1974—Floeter et al. 2008). Entire families are composed of amphi-Atlantic species (e.g., Diodontidae, Holocentridae, Priacanthidae, Synodontidae). In addition to the Benguela Current that limits the movements of tropical species from the Indian Ocean, cold waters from the northeastern Atlantic also limit the geographic range of tropical species (Floeter et al. 2008; Almada et al. 2013). Thus, the TEA and the southwestern Indian Ocean only share about 15 species (e.g., Lithognathus mormyrus (Linnaeus, 1758) and Gnatholepis thompsoni Jordan, 1904) or genera (e.g., Prionurus and Plectorhynchus—Rocha et al. 2005; Wirtz et al. 2007; Floeter et al. 2008).

Since the waters of the Gulf of Guinea have received limited scientific attention, with Annobón the least studied area of the GGOI (Osório 1895b; Blanc et al. 1968), the marine organisms desperately require further study (Floeter et al. 2008). Recent works dealing with the biogeography and evolution of some reef fish families include representatives from this region, as in the case of the genus Clepticus (Labridae). This recent study revealed that C. africanus, an endemic species from the Gulf of Guinea, is genetically closer to Clepticus brasiliensis Heiser, Moura and Robertson, 2000, from the Brazilian coast, than to the Caribbean Clepticus parrae (Bloch and Schneider, 1801) (Beldade et al. 2009). The biogeographic affinities of other endemics in the archipelago are largely unknown.

With 268 coastal fish species in the GGOI, the diversity is high when compared to other Atlantic islands (e.g., 140 coastal fish species from Saint Helena, 170 species from Azores, or 226 species from Madeira—Table 17.2). This is mostly due to the location of the GGOI, closer to the African shelf and surrounded by a vortex created by all the currents crossing the area. However, the total number is low when compared to the Cape Verde (325 costal fish species) and Canary (330) islands, probably a sampling artifact due to the dearth of surveys around the GGOI.

Table 17.2 Coastal fish species richness, number of endemics and % endemism of Atlantic islands
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Marine fish species are moving, and tropicalization is one of the observed processes in some eastern Atlantic islands where several TEA species are expanding their ranges (e.g., Muraena melanotis (Kaup, 1859), Holacanthus africanus Cadenat, 1951, and Cirrhitus atlanticus Osório, 1893—Brito et al. 2005; Falcón et al. 2018). We also highlight the occurrence of Epinephelus fasciatus (Forsskål, 1775), an Indo-Pacific species, potentially introduced by ballast water or in association with oil platforms (Brito et al. 2005; Falcón et al. 2018). Rocha et al. (2005) and confirmed recent connections in several taxa during warm interglacial periods (Peeters et al. 2004), such as the genus Gnatholepis that invaded the Atlantic from the Indian Ocean. Currently, the Agulhas Current in extreme conditions can force any tropical invaders from the Indian Ocean to move to the Atlantic Ocean through the ‘Agulhas leakage’ that forms water rings at the Agulhas retroflection (Lutjeharms and Van Ballegooyen 1988; Gordon 2003; Lutjeharms 2006; Beal et al. 2011). Invading fish species are likely moving with these rings through the Western Atlantic and South-central Atlantic before ending up in the TEA, rather than moving north with the Benguela Current, which is probably more lethal to tropical fish species (Rocha et al. 2005).

Some fish species have highly skewed distributions, with 84 genera occurring in the eastern Atlantic, but not in the western Atlantic (e.g., Thorogobius, Wheelerigobius—Floeter et al. 2008; Cowman et al. 2017). Other genera are amphi-Atlantic, but much more diverse in the eastern Atlantic (e.g., Diplodus, Scartella—Cowman et al. 2017) or with sister-species in the western Atlantic (e.g., Hypleurochilus aequipinnis (Günther, 1861)—Wirtz et al. 2007). Finally, several eastern Atlantic genera occur in the Indo-Pacific, but are not present in the western Atlantic (e.g., Coris, Lethrinus—Cowman et al. 2017). Cowman et al. (2017) observed that the Gulf of Guinea fish species assemblages are distinctive within the east Atlantic cluster (Cape Verde, Gulf of Guinea West, and Sahelian Upwelling).

Conservation

The GGOI, together with Cape Verde, have been considered important global hotspots for marine conservation (Roberts et al. 2002), with high levels of endemism (~30%, Floeter et al. 2008). Several factors likely contribute to this designation including:

  1. 1.

    The geographic location and connectivity with tropical western Atlantic via the Equatorial Counter Current (Wirtz et al. 2007; Floeter et al. 2008; Herrero-Barrencua et al. 2019).

  2. 2.

    The moderate isolation from the Continental slope (Floeter et al. 2008; Hachich et al. 2015; Cowman et al. 2017; Quimbayo et al. 2019).

  3. 3.

    The Benguela Current working as a shield and limiting Indian Ocean tropical fish species from moving northward (Floeter et al. 2008).

  4. 4.

    The northeastern Atlantic cold waters limiting the northern range of tropical fish species (Floeter et al. 2008; Almada et al. 2013).

  5. 5.

    The lowest fish biomass and highest density in reef assemblage (Quimbayo et al. 2019).

With 27 fish species reported, plus two uncertain identifications (Gobioides cf. africanus and Citharus cf. linguatula (Linnaeus, 1758)—Cravo 2021), mangroves and seagrasses are also essential habitats to the fish diversity by providing fisheries production (Félix et al. 2016; Alexandre et al. 2017; Cravo 2021).

Globally, mangroves are highly impacted by deforestation for onshore aquaculture (for fish and shellfish production), agriculture, and urban development (FAO 2007; Friess et al. 2019; Goldberg et al. 2020). With at least 35% of world area lost, mangroves, or inter-tidal forest communities, are one of the major tropical environments threatened by agriculture, overharvesting, changing hydrology, pollution, and coastal erosion (Valiela et al. 2001). Otero-Ferrer et al. (2020) emphasized that at the island scale, the protection of fish assemblages needs to consider the interconnected habitat network by including the seascapes boundaries where fundamental ecological functions might also occur. The GGOI exhibit lower biomass despite higher primary productivity, an unexpected observation likely caused by intense fishing activities (Maia et al. 2018a; Quimbayo et al. 2019).

In 2000, Annobón Island and the surrounding waters were designated as a Marine Nature Reserve at National level, limiting fishing to traditional subsistence practices and scientific research (UNEP-WCMC and IUCN 2021). In 2006, the São Tomé Obô and Príncipe Obô Natural Parks were established, covering 262 km2 and 45 km2 in the respectively islands (UNEP-WCMC and IUCN 2021). The natural park in São Tomé covers three out of 12 mangroves, including Malanza, by far the largest of such ecosystems in the GGOI (Afonso 2019). The natural park in Príncipe included a marine portion on the southwestern coast. Since 2012, the Ramsar site of Tinhosas islets (covering Tinhosa Grande, Tinhosa Pequena and Tinhosinha, south of Príncipe) and the island of Príncipe are a UNESCO World Biosphere Reserve (UNEP-WCMC and IUCN 2021).

Regarding fishing activities, Equatorial Guinea and São Tomé and Príncipe are part of the Fishery Committee for the Eastern Central Atlantic (CECAF) with the purpose of promoting the sustainable utilization of all living marine resources within the delimited area by proper management and development of the fisheries and fishing operations. Fisheries catches include small-scale artisanal, subsistence (fishing operations in remote communities with no access to market to supplement family needs, and portion taken home for consumption from artisanal catch), and foreign industrial (dominated by fleets from the European Union, Japan, Taiwan, and China) catches (Belhabib 2015; Maia et al. 2018b). Nevertheless, only limited data from fisheries surveys are reported (Belhabib 2015). Increasing numbers of fishers, destructive blast fishing practices, and pollution from industrial fishing vessels (oil-spills) are the main causes of negative fish catch changes over time (Maia et al. 2018b). The low biomass of medium and large fish species reflects the long-term fishing pressure on São Tomé Island, as does deeper reef habitats having higher species richness, abundance, and biomass (Maia et al. 2018a).

Within Elasmobranchii, 27 (73%) out of 37 species are considered threatened (Vulnerable—VU, Endangered—EN or Critically Endangered—CR) and one species (2.7%) is Data Deficient (DD), following IUCN categories (Appendix). Few countries impose catch limits and overfishing is a main threat to oceanic sharks, as are the loss and degradation of habitat and climate change (Pacoureau et al. 2021). Regarding Actinopteri, 19 (3.7%) out of 515 species are considered threatened (VU or EN) and 47 species (9.1%) are DD. Fifteen species are reported as endemic to the GGOI, of which seven are Gobiidae (small species with limited dispersive abilities) and four are freshwater/brackish species (Appendix). Annobón Island is the least studied of the GGOI with very few scientific surveys (Osório 1895b; Blanc et al. 1968). Nevertheless, and as expected, about 75% of its ichthyofauna is shared with the African coast. From the remaining species, some are endemic to Annobón Island (S. annobonae), only present around the GGOI (Eleotris annobonensis Blanc, Cadenat and Stauch, 1968), common to other islands system (Rypticus saponaceus (Bloch and Schneider, 1801)), or amphi-Atlantic (Uroconger syringinus Ginsburg, 1954). Therefore, the establishment of a network of Marine Protected Area in the GGOI is fundamental to reduce further negative impacts on the reef by commercial fisheries and to secure their sustainability.

Concluding Remarks

The present checklist includes coastal, deep-sea, pelagic marine and freshwater fish species. Nevertheless, further surveys are still needed. These future surveys and research projects should combine traditional and new approaches (e.g., environmental DNA) to understand and highlight the occurrence of discrete (pelagic) species, but also to better define the distribution of endemic species around all three GGOI. Annobón Island should be a region of primary focus because it is the least studied. Moreover, the creation of one or more Marine Protected Areas in co-management with fisheries will be fundamental to protect the unique GGOI fish biodiversity hotspot, not only for the endemic species, but also to maintain sustainable fisheries.