Download The influence of the Benguela upwelling system on Namibia`s

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 e€ectively 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 o€shore, 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 o€shore.
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 eciently 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),
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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 di€erent 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-a€ected and low-growing
(turf-like) in appearance, and have di€erent 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 anities 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.
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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 anities 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
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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 Su‚ogobius 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 o€shore 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. Grin, 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 o€shore 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
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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 e€ects of such conditions on marine
biota include mortalities of ®sh and seals, and movements of ®sh to less-a€ected 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 A€airs,
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 e€ects 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 e€ects. 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
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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
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