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Biodiversity and Conservation 7, 419±433 (1998) The in¯uence of the Benguela upwelling system on Namibia's marine biodiversity A. L. SAKKO P.O. Box 4426, Vineta, Swakopmund, Namibia Namibia's marine environment falls within the Benguela system, an eastern boundary current upwelling system in the south eastern Atlantic Ocean. Conditions within much of this environment change continuously as a consequence of the upwelling of nutrient-rich water into the surface zone. In addition, irregular anomalies in temperature, oxygen concentration and salinity occur, particularly in shelf waters. These ¯uctuations, which are inherent in the functioning of the Benguela system, tend to favour the persistence of few, generalist species, while at the same time high productivity supports large abundances. This trend is evident in all the major marine habitats o Namibia, where diversity is often lower than in comparable habitats in the southern Benguela system o the west coast of South Africa. Namibia's marine environment is considered `relatively pristine', although threats to biodiversity are posed by both natural and anthropogenic phenomena. Keywords: marine biodiversity; Benguela; upwelling; Namibia. Introduction Namibia's marine environment falls entirely within the boundaries of the Benguela system, which extends along the eastern edge of the southern Atlantic Ocean between Cape Agulhas (35°S) and the Angolan port of Namibe (15°S). The Benguela is one of the four major eastern boundary current upwelling systems of the world, all of which are characterized by the presence of cool surface waters and high biological productivity. Upwelling systems in general are extreme examples of unstable environments where the physical, chemical and biological characteristics change continuously as a result of the process of upwelling. This has consequences for biological diversity since food, although at times exceptionally abundant, is patchily distributed and unpredictable. Such environments commonly support low diversities of species while at the same time being among the most productive habitats in the world (Barnes and Hughes, 1988). In Namibian waters the intensity and perennial nature of upwelling result in a marine environment among the most productive in the world, supporting some of the highest concentrations of marine life found anywhere (Shannon, 1985, 1989). This paper aims to review current knowledge on biological diversity in Namibian waters, and to place it in an international context against the background of the ¯uctuating Benguela upwelling system, of which Namibian waters form a part. In addition, potential threats to marine biodiversity in Namibia are discussed in the light of current biodiversity conservation theory which emphasizes the long-term retention of natural communities under conditions which allow for sustained resource use as well as for continuing evolution. The Namibian marine environment The Namibian coast is approximately 1500 km long (Fig. 1), and is hyper-arid desert along its entire length. The majority of the shore consists of sandy beaches with occasional rocky 0960-3115 Ó 1998 Chapman & Hall 420 Figure 1. Map of the Namibian coast showing relevant geographical localities. Sakko Marine biodiversity of Namibia 421 outcrops which are exposed to heavy wave action. The continental shelf o Namibia is generally narrow and is one of the deepest in the world, with an average shelf edge depth of 350 m (Shannon, 1985). Shelf sediments are mostly biogenic in origin and occur in textural zones parallel to the coast (Rogers and Bremner, 1991). The marine environment of Namibia falls within the Benguela system and, although the system is continuous, there is an unusually intense cell of upwelling o LuÈderitz which eectively divides it into two parts (Shannon, 1989). The southern Benguela system thus extends as far northwards as LuÈderitz, while the rest of the Namibian coast as far as the Kunene River mouth falls within the northern Benguela system. The driving physical process in the Benguela system is coastal, wind-induced upwelling. Prevailing south to southwesterly winds, which occur all year round o Namibia, tend to move nearshore surface water northwards and oshore, while cool, central water from a depth of about 300 m wells up to take its place (Shannon, 1989). The deeper water is rich in dissolved nutrients which, when present in the photic zone, facilitate rapid growth of phytoplankton (Chapman and Shannon, 1985). The high productivity of these microscopic plants (Estrada and Marrase, 1987) supports abundant marine life. The most intense upwelling regions o Namibia are found where the continental shelf is narrowest and the wind strongest, e.g. o Cape Frio, Palgrave Point and LuÈderitz (Fig. 1). The most extensive and intense centre of upwelling in the entire Benguela system is near LuÈderitz (Shannon, 1989). Associated with high productivity in the Namibian surface waters is the death, sinking and decay of large numbers of microscopic organisms. Shelf sediments o the Namibian coast comprise extensive areas of diatomaceous muds which support little or no marine life, but which have high concentrations of organic matter and sulphur (Rogers and Bremner, 1991). Decaying organic matter also consumes oxygen, so that bottom waters over much of the Namibian continental shelf, extending out to a depth of 100 to 150 m or more, have low oxygen concentrations (Chapman and Shannon, 1985). Water low in oxygen (as low as 0.25 ml l)1) is common o central Namibia, where it may extend as much as 90 km oshore. Upwelling in the Benguela system is potentially of great signi®cance for the biological diversity of Namibia's marine environment. Continuous physical, chemical and biological changes give rise to a three-dimensional mosaic of environmental conditions which varies continuously in time and space. This results in marine habitats of variety and variability, as well as imparting an inherent unpredictability to the system as a whole. Diversity of plankton The phytoplankton assemblage found in Namibian waters is dominated by diatoms, which have a high nutrient requirement and are adapted to turbulent conditions (Shannon and Pillar, 1986). These organisms are best able to compete for available nutrients and light, and to respond with rapid growth and reproduction such that dense `blooms' develop after upwelling events. Kruger (1980) records 184 diatom species in Namibian waters, with dino¯agellates and tintinnids represented by 158 and 95 species respectively. Dino¯agellates may become dominant during more quiescent, post-upwelling conditions because they can grow more eciently than diatoms at low nutrient concentrations. The phytoplankton species and the proportional representation of groups are similar in Namibian waters to those of the Mediterranean Sea and southwest Indian Ocean (Kruger, 1980), 422 Sakko making Namibian waters ¯oristically undistinguished. Hart and Currie (1960) list 44 species of diatoms which are abundant in, or typical of, the Benguela system; many of these occur commonly in 14 dierent marine regions. There is thus a low degree of endemism, with only four species of diatoms (Delphineis karstenii, Fragilaria granulata, Chaetoceros strictus and C. tetras) being restricted to the Benguela system, and with no exclusively Namibian endemics. In Namibian waters the zooplankton is characterised by a relatively low species diversity and high abundance (Shannon and Pillar, 1986). The most abundant and diverse group is the copepods, which may be grazers, predators or omnivores, and which are important prey for many other organisms including juvenile ®sh. A total of 243 copepod species has been recorded in Namibian waters, representing approximately 12% of the world's known species (Carola, 1994). The southern African distribution of 20 of these species is thus far con®ned exclusively to Namibian waters, although they have been recorded in other oceanic regions. Other important zooplankton groups include single-celled protozoans, hydrozoans, chaetognaths (10 spp), crustaceans such as euphausiids (14 spp), amphipods and isopods, and chordates, including tunicates (Shannon and Pillar, 1986; Pillar and Hutchings, 1989). The eggs and larvae of many of the plankton species, as well as of most benthic and intertidal organisms and of many species of pelagic and demersal ®sh, also contribute substantially to the abundance of zooplankton in Namibian waters. Diversity of algae Marine algae are the most important ®xers of organic carbon in the littoral zone. The community structure of Namibia's littoral algae indicates the existence of two distinct biogeographical regions. The communities found south of LuÈderitz have similar zonations and key species to those on the northwestern coast of South Africa, both areas falling within the Namaqua biogeographical province as de®ned by Emanuel et al. (1992). Communities of central and northern Namibia are more sand-aected and low-growing (turf-like) in appearance, and have dierent dominant species. They fall in the more northerly Namib biogeographical province (Bolton and Anderson, 1997). A total of 205 marine algal species has been collected in Namibian waters (Lawson et al., 1990), which is approximately half the number of species recorded on the west coast of South Africa (Stegenga et al., 1997). The majority (80%) of Namibian species also occur on the South African west coast, and there are no Namibian endemics. Only 40 northern Namibian species have also been recorded in Angola, indicating a major ¯oristic change in the vicinity of the Kunene River (Engledow et al., 1992). This is also the region of the northernmost front of the Benguela system. There is a gradual decrease in species diversity from the southwestern Cape of South Africa northwards through Namibia. This phenomenon is apparent in all three major groups of seaweeds (Fig. 2). Reasons for this progressive decrease are probably related to availability of suitable habitat (Engledow and Bolton, 1994). Fauna of the major habitats Littoral habitat Namibia's intertidal habitat falls within the Namaqua and Namib biogeographic provinces as de®ned by Emanuel et al. (1992). The fauna of both provinces are typically temperate and have strong anities with the fauna of more southerly littoral habitats in Marine biodiversity of Namibia 423 Figure 2. Number of marine algal species in six regions of southwestern Africa. Reproduced with permission from Engledow et al. (1992). the Benguela system (Penrith and Kensley, 1970; Stephenson and Stephenson, 1972; Kensley and Penrith, 1980; Bally, 1983). In general, however, Namibian sandy beaches and rocky shores support a low species diversity (Figs 3 and 4) and a moderate to high biomass of organisms compared with similar habitats on the west coast of South Africa. The number of macrofaunal invertebrate species recorded on Namibian sandy beaches is commonly between 10 and 20 (McLachlan, 1985; Tarr et al., 1985; Donn and Cockroft, 1989) while rocky shore species generally total between 30 and 40 (Fig. 4; Bustamante et al., 1993). There is a clear trend of decreasing species diversity from southern to northern Namibia (Figs 3 and 4). The endemic disc lamp shell Discinisca tenuis (Brachiopoda) occurs in dense beds at the low water mark. 424 Sakko Figure 3. Number of invertebrate species recorded on sandy beaches at 100 km intervals around the west coast of Namibia and South Africa. `Upwelling endemics' refers to species with a southern African distribution con®ned to the Benguela upwelling system. (Adapted from Branch and Bustamante, University of Cape Town, unpubl. data.) Figure 4. Number of invertebrate species recorded in rocky intertidal communities at 100 km intervals around the west coast of Namibia and South Africa. `Upwelling endemics' refers to species with a southern African distribution con®ned to the Benguela upwelling system. (Adapted from Branch and Bustamante, University of Cape Town, unpubl. data.) Marine biodiversity of Namibia 425 Benthic rocky subtidal communities in southern Namibia are similar in structure to those o the South African west coast, although they are considered to have a low diversity of faunal species (K. Grobler, Ministry of Fisheries, pers. comm.). O central and northern Namibia subtidal benthic habitats are largely muddy and have been poorly studied. The far northern region of the Namib province coincides with the area of transition between temperate and tropical biogeographical provinces. The front between the cool waters of the Benguela system and the warm Angolan Current waters moves seasonally in the vicinity of the Kunene River mouth (Shannon et al., 1987). Several intertidal species with tropical anities have been recorded on rocky shores and sandy beaches in this area (Tarr et al., 1985; B. Currie, Ministry of Fisheries, pers. comm.), and 12 of these are recorded regularly. Populations of these species appear to become established during and after the intrusions of warm Angolan water associated with seasonal relaxation of upwelling in the northern Benguela system, and with the irregular occurrence of the anomalous warm water events known as Benguela-NinÄos (Shannon, 1989). Biodiversity in this northern area of Namibia's littoral habitat is thus dynamic. Some 90 species of bony ®sh and 30 species of cartilaginous ®sh have been recorded in the littoral waters of Namibia (Bianchi et al., 1993). Among them are those caught by recreational anglers from the shore and from ski-boats. Although the diversity of angling ®sh species is low (approximately 10 spp of bony ®sh and 8 spp of cartilaginous ®sh), the Namibian coast is well-known for its angling opportunities (e.g. Lenssen et al., 1991). Favourite angling species include kob Argyrosomus inodorus, westcoast steenbras Lithognathus aureti which is thought to be endemic to the Benguela system, and galjoen Dichistius capensis. South of LuÈderitz, kob and westcoast steenbras are caught sporadically from the shore, while hottentot Pachymetopon blochii and white stumpnose Rhabdosargus globiceps form part of the regular catch. In the north of the country some warm-water angling species occur. The southernmost occurrence of these species varies seasonally, and depends mainly on the strength of upwelling and the intrusion of warm Angolan waters. Shelf habitat In Namibian waters 10 species of bony ®sh are pelagic speci®cally in shelf waters, while another 14 species are pelagic in the shallow coastal waters only, and 13 additional species are pelagic in habitats ranging from shelf to deep ocean (Bianchi et al., 1993). A further 21 species are benthopelagic (occurring in the water column just above the sea bed) in shallow waters, while 16 additional species are benthopelagic in habitats ranging from shallow waters to deep ocean. Twelve species of cartilaginous ®sh have been recorded as pelagic in shelf as well as in deep ocean waters (Bianchi et al., 1993). Many of the pelagic bony ®sh species o Namibia are either Perciformes, such as mullets, horse mackerels Trachurus capensis, chub mackerel Scomber japonicus and geelbek Atractoscion aequidens, or Clupeiformes. The latter include round herring Etrumeus whiteheadi, pilchard Sardinops ocellatus and anchovy Engraulis capensis. The clupeiform species, as well as juvenile horse mackerels, occur in the surface waters and are specialized planktivores, adapted to make use of the substantial planktonic production characteristic of shelf waters in the Benguela system. Demersal species on the continental shelf include bony ®sh (59 spp), sharks (18 spp), skates and rays (12 spp), cephalopods (6 spp) and crustaceans (10 spp) (data from the research vessel `Dr Fridtjof Nansen' database, Nansis). The number of species recorded 426 Sakko during surveys varies, but diversity generally is low in comparison with demersal species recorded in other upwelling systems, such as o northwest Africa (Roel et al., 1985). Distinct faunal assemblages can be identi®ed on the Namibian continental shelf (Lleonart and Roel, 1984; Macpherson and Roel, 1987), and these tend to change along both a depth and a latitudinal gradient (Mas-Riera et al., 1990). The demersal fauna of the central shelf area is dominated by Cape hake Merluccius capensis and, at times, pelagic goby Suogobius bibarbatus. There is a low species diversity in this region, due possibly to the presence of extensive oxygen-de®cient shelf waters. South of LuÈderitz, where oxygen levels are consistently higher (Chapman and Shannon, 1985), the species diversity is greater, with dominant ®sh species including hake M. capensis and M. paradoxus, kingklip Genypterus capensis, Cape john dory Zeus capensis and Cape gurnard Chelidonichthys capensis. The invertebrates are represented mainly by squid and cuttle®sh (Bianchi et al., 1993). Several species reach their northernmost distribution limits in this area, their main populations occurring to the south o South Africa. The faunal assemblage in the northern shelf region is dominated by Cape hake and Cape horse mackerel, but also includes a number of species that reach their southernmost limits of occurrence in this area. These are species typical of Angolan waters, such as large-eye dentex Dentex macrophthalmus, thinlip split®n Synagrops microlepis and long®n bone®sh Pterothrissus belloci. The southernmost occurrences of these species vary depending on the strength of upwelling. Other vertebrate fauna of the Namibian shelf habitat include sea turtles, seabirds, cetaceans and seals (Bianchi et al., 1993). Of the eight species of sea turtles worldwide, ®ve occur in Namibian waters. They are all considered endangered and are protected under international agreement. Some 60 species of seabirds have been recorded o Namibia, and 12 of these breed along the coast. These include the endangered jackass penguin Spheniscus demersus and the rare, near-endemic Damara tern Sterna balaenarum and bank cormorant Phalacrocorax neglectus. Several species are economically important for their guano. These species, including Cape gannets Morus capensis and cormorants Phalacrocorax carbo, P. capensis, P. neglectus and P. coronatus, breed on oshore islands and manmade platforms, and are protected under Namibian law (Berruti, 1989). Baleen whales are represented by eight species in Namibian waters, while 23 species of dolphins and toothed whales have been recorded (see also M. Grin, this issue). This represents approximately 70% of all cetacean species occurring in the south Atlantic (Best and Ross, 1989; Ross and Best, 1989). Some of these are cosmopolitan in distribution, while others show a preference for cool-temperate conditions. Heaviside's dolphin Cephalorhynchus heavisidii is the only cetacean that is endemic to southern Africa, occurring in coastal waters associated with the Benguela system. A single species of seal, the Cape fur seal Arctocephalus pusillus, breeds on the Namibian mainland and on oshore islands. This subspecies, A. p. pusillus, is considered near-endemic to the Benguela system, and has been exploited for approximately two centuries o southern Africa. The population was virtually extirpated at the end of the 19th century (David, 1989) and is currently harvested in Namibia using a system of annual quotas. Slope habitat The slope between continental mass and oceanic ¯oor o Namibia is characterized by waters which have in the region of 2 ml l)1 dissolved oxygen (Chapman and Shannon, 1985). This is higher than for shelf waters, although the substratum is still typically muddy. Marine biodiversity of Namibia 427 Most ®sh in this habitat are considered benthic or demersal, although many of them undertake daily migrations to shallower waters. A small group of species is associated speci®cally with the slope habitat (e.g. members of the Chimaeridae and Rhinochimaeridae). Most species, however, have distributions which extend to shallower waters on the shelf, or to deeper waters on the ocean ¯oor. Typical slope dwellers include species of light®sh (Photichthyidae), the rough-snout grenadier Trachyrincus scabrus, squaliform sharks Deania calcea and Centrophorus squamosus, and striped red shrimps Aristeus varidens (Bianchi et al., 1993). Other species of note include Cape hake, deep-water hake, alfonsino Beryx splendens and orange roughy Hoplostethus atlanticus, all of which are exploited commercially. Several species of lantern®sh (Myctophidae) are also commonly found in this habitat. Abyssal habitat No research has been done speci®cally on the biodiversity of oceanic or abyssal habitats in Namibian waters. Truly oceanic ®sh species are few, and are perhaps typi®ed by the perciform families Scombridae (tunas), Xiphiidae (sword®sh), and Istiophoridae (sail®sh and marlins). All of these are highly mobile, epipelagic or mesopelagic species which consume pelagic prey. Bianchi et al. (1993) record a total of 57 species of bony ®sh with ranges extending deeper than 1000 m. In addition, seven species are exclusively bathypelagic, occurring only at depths greater than 1000 m. The majority of these deep-sea species are benthopelagic, being found on or near the bottom. Amongst the cartilaginous ®sh, 21 species have been recorded at depths greater than 1000 m, the majority being Rajiformes (skates and guitar®sh) or squaliform sharks. No species are exclusively bathypelagic. Species diversity of ®sh at these depths is typically lower than in shallower habitats, and this is accompanied by a decrease in biomass (Angel, 1995). An estimated 500 species of demersal ®sh are found at depths greater than 1000 m in the Atlantic as a whole, indicating the relatively low diversity recorded in Namibian waters. The deep-sea benthic communities o Namibia are largely unknown. The deep-sea red crab Chaceon maritae is an abundant demersal species o the shelf edge of Namibia. It occurs on muddy substrata up to a depth of 950 m. The average crab density over the entire Namibian ®shing ground is estimated at 98.4 ha)1 (Beyers, 1994), but densities can be as high as 227.5 ha)1. These organisms are doubtless important prey items for numerous other deep-ocean species, and form the basis of a lucrative ®shery. Rex et al. (1993) collected samples of macrobenthic organisms from ten deep-ocean Atlantic basins. Species richness in sediments of the Cape and Angola Basins, o the coast of Namibia, was intermediate between the high diversity of tropical regions and the low diversity of areas in the higher latitudes. However, the paucity of information about biodiversity in this habitat hampers interpretation of the few existing studies. Overview of Namibian marine biodiversity Biodiversity in the Namibian marine environment shows several pertinent trends. In most habitats there are no endemic species. A few species are endemic to the Benguela system, of which Namibian waters form a part. Species richness in most habitats is considered to be relatively low. This is evident amongst sandy shore, rocky shore and marine benthic invertebrate communities, littoral algae, phytoplankton, ®sh of the littoral and pelagic habitats, as well as demersal ®sh in the shelf and slope habitats. In all these cases diversity 428 Sakko is lower than in comparable habitats in the southern Benguela system o the west coast of South Africa. In most cases the low diversity is accompanied by high biomass in those species that are represented. There is some evidence in support of a latitudinal gradient in patterns of global species richness (Clarke, 1992; Angel, 1993; Lewis and Beardmore, 1995), with highest diversity recorded in equatorial regions and lowest diversity towards the poles. Namibian marine diversity provides an anomaly in this gradient since, in general, species richness is substantially lower than in the more southerly marine habitats o South Africa. In addition, there is a clear trend of decreasing species richness from the south to the north of the marine system o Namibia. It is possible that features characteristic of the Benguela system (e.g. unstable and unpredictable environment, high productivity) are more important than latitudinal gradient when predicting species diversity in Namibia's marine environment (see Sanders, 1968). Upwelling systems in general are extreme cases of unstable environments, where continuous variation prevents the ®ne-tuning of genotypes to local conditions. Food availability is variable and generalist feeders are favoured (McNaughton and Wolf, 1970; Brown, 1984). Such systems predictably support a low diversity of species, while at the same time being among the most productive habitats in the world (Barnes and Hughes, 1988). Signi®cantly, the Namibian marine environment (and particularly the northern Benguela system) supports low numbers of species even in comparison to other upwelling systems, such as the southern Benguela system and the west African upwelling system. This is possibly partly due to the intense and perennial upwelling o this coast, and to the irregular anomalies in temperature, salinity and oxygen concentration which lead to extreme instability and unpredictability of environmental factors. Potential threats to Namibian marine biodiversity Natural threats The functioning of the Benguela system typically is based on continuous environmental changes, on time scales from hours to decades. Fluctuations are thus a feature of the Benguela system. However, exceptional conditions, such as those recorded during Benguela-NinÄos and during the upwelling of anoxic water from the deep ocean ¯oor onto the continental shelf, can cause substantial mortality of marine organisms. During a BenguelaNinÄo event a deep layer of warm, saline water of equatorial and Angolan origin intrudes into northern Benguela waters and occupies approximately the upper 100 m of the water column (Boyd and Thomas, 1984; Shannon et al., 1986; Shannon, 1989; O'Toole and Bartholomae, 1995). Considerable impact on the marine biota has been recorded (Stander and de Decker, 1969; Kruger and Boyd, 1984; Boyd et al., 1985; Le Clus, 1985), and includes decreases in plankton abundance, ®sh mortalities and movements, and poor spawning and recruitment in commercially exploited ®sh species. Similarly, anomalous environmental conditions associated with the presence of lowoxygen water have been reported in the past (Copenhagen, 1953; Stander and de Decker, 1969; O'Toole and Bartholomae, 1995). Documented eects of such conditions on marine biota include mortalities of ®sh and seals, and movements of ®sh to less-aected areas. Following the 1994 intrusion of anoxic water onto the shelf o northern and central Namibia, biomass of commercially exploited pelagic species such as pilchard, anchovy and horse mackerel was markedly reduced in inshore waters (O'Toole and Bartholomae, 1995). Marine biodiversity of Namibia 429 Natural ¯uctuations in environmental conditions in the Benguela system could thus be seen as a potential threat to marine species o Namibia. However, these ¯uctuations are inherent in the functioning of the system, a system which has been in existence (certainly o northern Namibia) for more than two million years (Shannon, 1985). Clearly, species that persist have evolved mechanisms for coping with the inherent variability. Mortalities in response to environmental ¯uctuations should therefore be seen as signi®cant only on a local scale. Anthropogenic threats Pollution of ship and shore origin. Namibia's coastal zone is sparsely populated and the inland desert is not suitable for agricultural development. The marine environment is thus free from the level of pollution commonly associated with large urban communities, and is considered `relatively pristine' (Moldan, 1989). In addition, most vessels using shipping lanes along the southwestern African coast remain outside Namibian waters. There are harbour facilities and ®sh-processing factories at LuÈderitz and Walvis Bay. However, monitoring of water quality in Walvis Bay has, to date, not indicated pollution levels which have anything more than a local impact (Department of Water Aairs, Windhoek). Diamond mining. Extensive areas of the marine environment in southern Namibia are currently set aside for diamond mining activities (approximately 300 km between 28°S and 26°S). Public access to these areas is restricted and mining activities are conducted in all the marine habitats from coastal land to deep sea (about 120 m deep). All operations involve the removal of sediment in search of diamonds, and the re-deposition of this sediment, mostly in a suspended form in the water column. On a local level these activities are highly destructive to biodiversity since substratum morphology is altered and entire communities may be disturbed or totally eradicated. In addition, the eects of numerous small mining concerns along the coast may be cumulative in terms of decreasing the supply of eggs and larvae which are essential for the recovery of disturbed areas. Strict control and monitoring of mining activities is necessary to prevent large-scale loss of biodiversity due to this activity. Introduced exotic species. The invasive alien Mediterranean mussel Mytilus galloprovincialis, which was introduced to South Africa in the late 1970s (Hockey and Van Erkom Schurink, 1992), has become well-established on southern Namibian rocky shores (B. Currie, pers. comm.), where it has displaced the indigenous intertidal mussels Aulacomya ater and Choromytilus meridionalis. Thus far the presence of M. galloprovincialis does not appear to have had any ecosystem eects. The intertidal distributions of indigenous species have, however, altered on a local level. Fishing. The high productivity of the waters o Namibia supports large stocks of commercially valuable species. Three main resource groups have formed 90% of the total catches since the major ®sheries commenced in the mid 1900s (Bianchi et al., 1993). These are the pelagic species (pilchard, anchovy and juvenile horse mackerel) which are caught by purse seine, the adult horse mackerel and chub mackerel which are caught with midwater trawls, and the demersal species (two hake species, kingklip, Cape monk Lophius vomerinus, and west coast sole Austroglossus microlepis) which are caught with bottom trawls (Crawford et al., 1987). In addition, there are experimental ®sheries on orange 430 Sakko roughy and alfonsino, both ®sh of the shelf slope and deep ocean. Important ®sheries also exist on the deep-sea crab and the Cape rock lobster Jasus lalandii. In 1995, Namibia's ®sheries landed a combined total of more than half a million metric tonnes of marine organisms, with a value of nearly N$1500 million (MFMR, 1996). This represented 7% of the GDP. Healthy ®sh stocks are clearly vital to the Namibian economy. Most commercially exploited species are currently nowhere near as abundant as they have been in the past, and there are few of these species that have not, at some time, experienced population crashes (Crawford et al., 1987). Unfavourable environmental conditions have usually accompanied these reductions in numbers, and there is evidence of cyclical booms and crashes in pilchard and anchovy populations which predate the commencement of commercial ®sheries in the area (Crawford and Shelton, 1981; Shackleton, 1987, 1988). However, injudicious exploitation of already declining populations has doubtless exacerbated the situation. Such crashes can result in a reduction in the genetic diversity (heterozygosity) of the remaining populations, and a consequent potential decrease in the ability of populations to adapt to changing environmental conditions (Lewis and Beardmore, 1995). The crashes of Namibian pilchard stocks several times in the past three decades have no doubt lead to reduced heterozygosity. In addition, records of increased growth rates and reduced age at maturity in pilchard (Le Clus et al., 1987; Boyer et al., 1997) and horse mackerel (Anon., 1996) indicate the probable occurrence of directional genetic selection (Gaudian and Medley, 1995). Although there have been no recorded extinctions of commercially exploited species in Namibian waters, it is important that the country's marine resources are managed such that breeding populations are conserved and genetic bottlenecks are avoided. Conclusions The Namibian marine environment is remarkable in several ways. It is part of one of four major upwelling systems in the world and is exceptionally productive, supporting abundant marine life. However, the ¯ora and fauna are characterised by low species richness and paucity of endemics in all the major marine habitats, a situation which re¯ects the position of Namibia's marine waters within the Benguela upwelling system. The marine habitats are relatively pristine, and the Namib Desert, lying just inland of the coast, makes increased urban and agricultural pressures unlikely in the future. And lastly, the fact that many organisms in the Namibian marine environment are adapted to survival in the inherently variable and unpredictable Benguela system perhaps makes Namibia's biological diversity somewhat resilient to the vicissitudes of human activities. Acknowledgements I wish to thank the sta of the Namibian Ministry of Fisheries and Marine Resources, Directorate of Resource Management, for their assistance and guidance. My particular thanks go to C. Bartholomae, D. Boyer, I. Cordes, B. Currie, K. Grobler, J. Holtzhauzen, C. Kirchner-Frankel and M. O'Toole, as well as to three referees. Thanks also to Mrs N. Coetzee for help in locating references. Marine biodiversity of Namibia 431 References Angel, M.V. (1993) Biodiversity of the pelagic ocean. Conserv. Biol. 7, 771. Angel, M.V. (1995) Biodiversity of the deep ocean. Biological Diversity in Tropical Marine Environments: Reviews Contributing to the Conservation of Aquatic Biodiversity and its Sustainable Use by Tropical Developing Countries. London: Overseas Development Administration of the United Kingdom of Great Britain and Northern Ireland. Anon. (1996) Survey of the oshore and inshore horse mackerel stocks of Namibia. Surveys of the Fish Resources of Namibia. Preliminary report, cruise 3/96 of the research vessel Dr Fridtjof Nansen. Bally, R. (1983) Intertidal zonation on sandy beaches of the west coast of South Africa. Cahiers Biol. Marine (Paris) 24, 85±103. Barnes, R.S.K. and Hughes, R.N. (1988) An Introduction to Marine Biology, 2nd edn. London: Blackwell Scienti®c Publications. Berruti, A. (1989) Resident seabirds. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 257±73. Cape Town: Vlaeberg Publishers. Best, P.B. and Ross, G.J.B. (1989) Whales and whaling. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 315±38. Cape Town: Vlaeberg Publishers. Beyers, C.J. de B. (1994) Population size and density of the deep-sea red crab Chaceon maritae (Manning and Holthuis) o Namibia determined from tag-recapture. S. Afr. J. Mar. Sci. 14, 1± 9. Bianchi, G., Carpenter, K.E., Roux, J-P., Molloy, F.J., Boyer, D. and Boyer, H. (1993) The Living Marine Resources of Namibia. Rome: FAO. Bolton, J.J. and Anderson, R.S. (1997) Marine vegetation. In Vegetation of Southern Africa (R.M. Cowling, D.M. Richardson and S.M. Pierce, eds) pp. 348±70. Cambridge: Cambridge University Press. Boyd, A.J., Hewitson, J.D., Kruger, I. and Le Clus, F. (1985) Temperature and salinity trends o Namibia from August 1982 to August 1984 and their relation to plankton abundance and the reproductive success of pelagic ®sh. ICSEAF Collection of Scienti®c Papers 1, 53±8. Boyd, A.J. and Thomas, R.M. (1984) A southward intrusion of equatorial water o northern and central Namibia in March 1984. Trop. Ocean±Atmosphere Newslett. 27, 16±7. Boyer, D.C., Goosen, A.C., Boyer, H.J. and Coetzee, J.C. (1997) Analysis of the 1990 and 1991 Namibian pelagic ®shing seasons. Madoqua (in press). Brown, J.H. (1984) On the relationship between abundance and distribution of species. Amr. Natur. 124, 255±79. Bustamante, R.H., Branch, G.M., Velasquez, C.R. and Branch, M. (1993) Intertidal survey of the rocky shores at the Elizabeth Bay area (Sperrgebiet, Namibia). Consultancy report. Windhoek: Consolidated Diamond Mines. Carola, M. (1994) Checklist of the marine planktonic copepoda of southern Africa and their worldwide geographic distribution. S. Afr. J. Mar. Sci. 14, 225±53. Chapman, P. and Shannon, L.V. (1985) The Benguela ecosystem. Part II. Chemistry and related processes. Oceanogr. Mar. Biol. Ann. Rev. 23, 183±251. Clarke, A. (1992) Is there a latitudinal diversity cline in the sea? Trends Ecol. Evol. 7, 286±7. Copenhagen, W.J. (1953) The periodic mortality of ®sh in the Walvis region. A phenomenon within the Benguela Current. Investig. Rep. Div. Sea Fish. S. Afr. 14, 1±35. Crawford, R.J.M. and Shelton, P.A. (1981) Population trends for some southern African seabirds related to ®sh availability. In Proceedings of the Symposium on Birds of the Sea and Shore, 1979 (J. Cooper, ed.) pp. 15±41. Cape Town: African Seabird Group. Crawford, R.J.M., Shannon, L.V. and Pollock, D.E. (1987) The Benguela ecosystem. Part IV. The major ®sh and invertebrate resources. Oceanogr. Mar. Biol. Ann. Rev. 25, 353±505. 432 Sakko David, J.H.M. (1989) Seals. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 288±302. Cape Town: Vlaeberg Publishers. Donn, T.E. Jr. and Cockroft, A.C. (1989) Macrofaunal community structure and zonation of two sandy beaches on the central Namib coast, South West Africa/Namibia. Madoqua 16, 129±35. Emanuel, B.P., Bustamante, R.H., Branch, G.M., Eekhout, S. and Odendaal, F.J. (1992) A zoogeographic and functional approach to the selection of marine reserves on the west coast of South Africa. S. Afr. J. Mar. Sci. 12, 341±54. Engledow, H.E. and Bolton, J.J. (1994) Seaweed alpha-diversity within the lower eulittoral zone in Namibia: the eects of wave action, sand inundation, mussels and limpets. Bot. Mar. 37, 267± 76. Engledow, H.E., Bolton, J.J. and Stegenga, H. (1992) The biogeography of the seaweed ¯ora of Namibia. In Proceedings of the First Workshop on Sustainable Seaweed Resource Development in sub-Saharan Africa (K.E. Mshigeni, J.J. Bolton, A.T. Critchley and G. Kiangi, eds) pp. 117±30. Windhoek: K.E. Mshigeni, University of Namibia. Estrada, M. and Marrase, C. (1987) Phytoplankton biomass and productivity o the Namibian coast. S. Afr. J. Mar. Sci. 5, 347±56. Gaudian, G. and Medley, P.A.H. (1995) Coastal marine biological diversity. Biological Diversity in Tropical Marine Environments: Reviews Contributing to the Conservation of Aquatic Biodiversity and its Sustainable Use by Tropical Developing Countries. London: Overseas Development Administration of the United Kingdom of Great Britain and Northern Ireland. Grin, M. (1998) The species diversity, distribution and conservation of Namibian mammals. Biodiv. Conserv. 7, 483. Hart, T.J. and Currie, R.I. (1960) The Benguela current. Discovery Rep. 31, 123±298. Hockey, P.A.R. and Van Erkom Schurink, C. (1992) The invasive biology of the mussel Mytilus galloprovincionalis in southern Africa. Trans. Roy. Soc. S. Afr. 48, 123±39. Kensley, B. and Penrith, M-L. (1980) The constitution of the intertidal fauna of rocky shores of South West Africa. Part III. The north coast from False Cape Frio to the Kunene River. Cimbebasia Ser A 5, 201±14. Kruger, I. (1980) A checklist of South West African marine phytoplankton, with some phytogeographical relations. Fisheries Bull. S. Afr. 13, 31±53. Kruger, I. and Boyd, A.J. (1984) Investigation into the hydrology and plankton at the surface waters o southwestern Africa in ICSEAF Divisions 1.3, 1.4 and 1.5 in 1982/83. ICSEAF Collection of Scienti®c Papers 11, 109±34. Lawson, G.W., Simons, R.H. and Isaac, W.E. (1990) The marine algal ¯ora of Namibia. Bull. Brit. Mus. Nat. Hist. (Bot.) 20, 153±68. Le Clus, F. (1985) Eects of a warm water intrusion on the anchovy ®shery in Namibia. ICSEAF Collection of Scienti®c Papers 12, 99±101. Le Clus, F., Hewitson, J.D., Melo, Y.C., Cooper, R.M. and Malan, P.E. (1987) The multi-species pelagic ®shery o Namibia 1982±1986 and stock assessments for pilchard and anchovy. ICSEAF Collection of Scienti®c Papers 14, 7±15. Lenssen, J., Tarr, P. and Berry, H. (1991) An assessment of visitor statistics and line®shing along the Sandwich shoreline, Namib-Naukluft Park, Namibia. Madoqua 18, 33±6. Lewis, R.I. and Beardmore, J.A. (1995) Genetics and biodiversity in tropical freshwater, estuarine, coastal and deep sea environments. Biological Diversity in Tropical Marine Environments: Reviews Contributing to the Conservation of Aquatic Biodiversity and Sustainable Use by Tropical Developing Countries. London: Overseas Development Administration of the United Kingdom of Great Britain and Northern Ireland. Lleonart, J. and Roel, B. (1984) Analisis de las comunidades de peces y crustaceos demersales de la costa de Namibia (Atlantico Sudoriental). Investigacion Pesqueras (Barcelona) 48, 187±206. Macpherson, E. and Roel, B.A. (1987) Trophic relationships in the demersal ®sh community o Namibia. S. Afr. J. Mar. Sci. 5, 585±96. Marine biodiversity of Namibia 433 Mas-Riera, J., Lombarte, A., Gordoa, A. and Macpherson, E. (1990) In¯uence of Benguela upwelling on the structure of demersal ®sh populations o Namibia. Mar. Biol. 104, 175±82. McLachlan, A. (1985) The ecology of two sandy beaches near Walvis Bay. Madoqua 14, 155±63. McNaughton, S.J. and Wolf, L.L. (1970) Dominance and the niche in ecological systems. Science 167, 131±9. MFMR (1996) Economic performance by ®shery. Internal memorandum. Windhoek: Ministry of Fisheries and Marine Resources. Moldan, A.G.S. (1989) Marine pollution. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 41±9. Cape Town: Vlaeberg Publishers. O'Toole, M.J. and Bartholomae, C. (1995) An overview of marine environmental conditions o Namibia during 1994±1995. Proceedings of Annual Research Meeting, February 1995. Windhoek: Ministry of Fisheries and Marine Resources. Penrith, M.J. and Kensley, B.F. (1970) The constitution of the intertidal fauna of rocky shores of South West Africa 1: LuÈderitzbucht. Cimbebasia A(1) 9, 191±239. Pillar, S.C. and Hutchings, L. (1989) Plankton. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 28±40. Cape Town: Vlaeberg Publishers. Rex, M.A., Stuart, C.T., Hessler, R.R., Allen, J.A., Sanders, H.L. and Wilson, G.K.F. (1993) Global-scale latitudinal patterns of species diversity in the deep-sea benthos. Nature 365, 636±9. Roel, B.A., Rucabado, J., Lloris, D. and Lleonart, J. (1985) Las comunidades de peces demersales del a¯oramiento de Africa occidental (Sahara y Namibia). In Simposio internacional sobre las areas de a¯oramiento mais importantes del oueste africano (Cabo Blanco y Benguela) (C. Bas, R. Margalef, and P. Rubies, eds) pp. 691±700. Barcelona: Instituto Investigaciones Pesqueras. Rogers, J. and Bremner, J.M. (1991) The Benguela ecosystem. Part VII. Marine-geological aspects. Oceanogr. Mar. Biol. Ann. Rev. 29, 1±85. Ross, G.J.B. and Best, P.B. (1989) Smaller whales and dolphins. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 303±14. Cape Town: Vlaeberg Publishers. Sanders, H.L. (1968) Marine benthic diversity: a comparative study. Am. Natur. 102, 243±82. Shackleton, L.Y. (1987) A comparative study of fossil ®sh scales from three upwelling regions. S. Afr. J. Mar. Sci. 5, 79±84. Shackleton, L.Y. (1988) Fossil pilchard and anchovy scales ± indicators of past ®sh populations o Namibia. Proceedings of the International Symposium on Long Term Changes in Fish Populations, 1986, pp. 55±68. Vigo. Shannon, L.V. (1985) The Benguela ecosystem. Part I. Evolution of the Benguela, physical features and processes. Oceanogr. Mar. Biol. Ann. Rev. 23, 105±82. Shannon, L.V. (1989) The physical environment. In Oceans of Life O Southern Africa (A.I.L. Payne and R.J.M. Crawford, eds) pp. 12±27. Cape Town: Vlaeberg Publishers. Shannon, L.V. and Pillar, S.C. (1986) The Benguela ecosystem. Part III. Plankton. Oceanogr. Mar. Biol. Ann. Rev. 24, 65±170. Shannon, L.V., Boyd, A.J., Brundrit, G.B. and Taunton-Clark, J. (1986) On the existence of an ElNinÄo type phenomenon in the Benguela System. J. Mar. Res. 44, 495±520. Shannon, L.V., Agenbag, J.J. and Buys, M.E.L. (1987) Large- and meso-scale features of the Angola-Benguela front. S. Afr. J. Mar. Sci. 5, 11±34. Stander, G.H. and De Decker, A.H.B. (1969) Some physical and biological aspects of an oceanographic anomaly o South West Africa in 1963. Investig. Rep. Marine Res. Lab. S. W. Afr. 13, 1±35. Stegenga, H., Bolton, J.J. and Anderson, R.J. (1997) Seaweeds of the South African west coast. Contrib. Bolus Herb. (in press). Stephenson, T.A. and Stephenson, A. (1972) Life Between Tidemarks on Rocky Shores. San Francisco: W.H. Freeman and Co. Tarr, J.G., Griths, C.L. and Bally, R. (1985) The ecology of three sandy beaches on the Skeleton Coast of South West Africa. Madoqua 14, 295±304.