Nile tilapia invades the Lake Malawi catchment

Abstract

The Lake Malawi/Nyasa catchment contains over 835 endemic cichlid fish species. This unique biodiversity has made it widely recognised as one of the world's most significant freshwater ecosystems. Here we report the first occurrence records of two invasive tilapiines, Oreochromis niloticus and Oreochromis leucostictus, inside the Lake Malawi catchment. The introductions took place during initiatives to develop aquaculture and new capture fisheries. Oreochromis niloticus is an important competitor and predator of native species, has potential to hybridise with indigenous Oreochromis species, and has been widely implicated in biodiversity loss globally. It was a key contributor to the destruction of the Lake Victoria indigenous Oreochromis fishery. In light of apparent risks to unique biodiversity, and in the absence of robust evidence that introductions will bring enhanced socio-economic benefits over indigenous species, it is advisable that efforts be made to eradicate invasive species. The precautionary principle holds that future fisheries and aquaculture development in the region should be based exclusively on non-invasive indigenous species.

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Nile tilapia invades the Lake Malawi catchment
MJ Gennera, E Connella, A Shechongeb, A Smithc, J Swanstroma, S Mzighanid, A Mwijaged,
BP Ngatungad & GF Turnerb
a School of Biological Sciences, University of Bristol, Woodland Road, BS8 1UG, Bristol, UK
b School of Biological Sciences, Bangor University, Deiniol Road, LL57 2UW, Bangor,
Gwynedd, UK
c Department of Biological Sciences, University of Hull, Cottingham Road, Kingston-upon-
Hull, HU6 7RX, UK
d Tanzania Fisheries Research Institute (TAFIRI), PO Box 9750, Dar-es-Salaam, Tanzania
Published online: 12 Nov 2013.
To cite this article: MJ Genner, E Connell, A Shechonge, A Smith, J Swanstrom, S Mzighani, A Mwijage, BP Ngatunga & GF
Turner (2013) Nile tilapia invades the Lake Malawi catchment, African Journal of Aquatic Science, 38:sup1, 85-90
To link to this article: http://dx.doi.org/10.2989/16085914.2013.842157
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African Journal of Aquatic Science 2013, 38(Suppl.): 85–90
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AFRICAN JOURNAL OF
AQUATIC SCIENCE
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http://dx.doi.org/10.2989/16085914.2013.842157
African Journal of Aquatic Science is co-published by NISC (Pty) Ltd and Taylor & Francis
Nile tilapia invades the Lake Malawi catchment
MJ Genner1*, E Connell1, A Shechonge2, A Smith3, J Swanstrom1, S Mzighani4, A Mwijage4, BP Ngatunga4 and GF Turner2
1 School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, UK
2 School of Biological Sciences, Bangor University, Deiniol Road, Bangor, Gwynedd, LL57 2UW, UK
3 Department of Biological Sciences, University of Hull, Cottingham Road, Kingston-upon-Hull, HU6 7RX, UK
4 Tanzania Fisheries Research Institute (TAFIRI), PO Box 9750, Dar-es-Salaam, Tanzania
* Corresponding author, e-mail: m.genner@bristol.ac.uk
The Lake Malawi/Nyasa catchment contains over 835 endemic cichlid fish species. This unique biodiversity has
made it widely recognised as one of the world’s most significant freshwater ecosystems. Here we report the first
occurrence records of two invasive tilapiines, Oreochromis niloticus and Oreochromis leucostictus, inside the
Lake Malawi catchment. The introductions took place during initiatives to develop aquaculture and new capture
fisheries. Oreochromis niloticus is an important competitor and predator of native species, has potential to
hybridise with indigenous Oreochromis species, and has been widely implicated in biodiversity loss globally. It
was a key contributor to the destruction of the Lake Victoria indigenous Oreochromis fishery. In light of apparent
risks to unique biodiversity, and in the absence of robust evidence that introductions will bring enhanced
socio-economic benefits over indigenous species, it is advisable that efforts be made to eradicate invasive species.
The precautionary principle holds that future fisheries and aquaculture development in the region should be based
exclusively on non-invasive indigenous species.
Keywords: alien species, aquaculture development, fisheries development, hybridisation, species loss
Species introductions and loss of regional endemic species
are leading to a global homogenisation of freshwater
biodiversity (Rahel 2002). Invasive freshwater species are
often the culprits driving biodiversity loss, either directly
through biotic interactions, or indirectly by affecting availa-
bility of essential resources, facilitating the spread of
infectious disease, or through hybridisation with native
taxa (Simoes Vitule et al. 2009). Primary drivers for the
spread of invasive species in freshwaters are translocations
aimed at the establishment or improvement of fisheries or
aquaculture (Gozlan et al. 2010). While such initiatives are
sometimes successful from economic or social perspectives
(Gozlan 2008), their benefits can be short-lived. Invasions
are typically irreversible, and their consequences for indige-
nous biodiversity and ecosystem functioning are often
unclear until many years later (Barel et al. 1985).
The Great Lakes of East Africa, Malawi, Tanganyika and
Victoria, are home to the largest freshwater fish adaptive
radiations on Earth. Spectacular endemic biodiversity
of cichlid fishes has been recorded from each lake. Some
835 species are recorded from Lake Malawi (Konings
2007), while it has been estimated that Lake Tanganyika
has ~250 species (Turner et al. 2001), and Lake Victoria is
likely to have had ~500 species (Genner et al. 2004). The
Lake Victoria cichlid flock suffered a large-scale extinction
following the introduction of Nile perch (Lates niloticus) and
Nile tilapia (Oreochromis niloticus) in the 1950s to enhance
fisheries yields (Barel et al. 1985). This led to significant
ecological change, characterised by a loss of endemic
haplochromine and tilapiine cichlids (Ogutu-Ohwayo 1990,
Witte et al. 1992), that coincided with catchment-level
deforestation, increased eutrophication, spread of invasive
water hyacinth and increased water turbidity (Kaufman
1992). The resulting socio-economic changes were also
substantial (Balirwa 2007). Over recent years there has been
a resurgence of a handful of haplochromine species (Witte et
al. 2000, 2013), but indigenous tilapiine species remain rare
(e.g. Oreochromis variabilis) or are absent (e.g. Oreochromis
esculentus) from the main lake body and can be reliably
found only in satellite lakes (Goudswaard et al. 2002,
Maithya et al. 2012). The scale of the loss of biodiversity
provides a warning that invasive species should be consid-
ered a real threat to indigenous fish communities across all
African freshwaters.
Tropical inland aquaculture is one of the major growth
areas of fish production, responsible for 78% of growth
between 2006 and 2011 (FAO 2012). Key species include
tilapiine cichlids of genus Oreochromis, particularly O.
niloticus (Diana 2009), endemic to the Nile catchment
and west Africa, but now distributed globally, including
the tropical and subtropical Americas, Africa, Asia and
Australasia (Zambrano et al. 2006). The species is also a
prolific invader and has been recognised to be a major threat
to endemic biodiversity across its introduced range, acting
as a significant dominant competitor over native species
(Weyl 2008) and, in Africa, hybridising with indigenous
Oreochromis species (D’Amato et al. 2000, Angienda et al.
2011, Firmat et al. 2013). This species is recognised as a
IntroducƟ on
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Genner, Connell, Shechonge, Smith, Swanstrom, Mzighani, Mwijage, Ngatunga and Turner
86
specific threat to endemic biodiversity in Lake Malawi (Weyl
et al. 2010), and the country of Malawi has discouraged
introductions of this and other species by law because of the
risk to its indigenous fishes (Government of Malawi 2010,
Lind 2012). To date, only two fish species are known to be
have become established after introduction into the Lake
Malawi catchment, a deliberate introduction of rainbow trout
Oncorhynchus mykiss into high-altitude streams of the Nyika
plateau (Snoeks 2004), and the accidental introduction of
lungfish Protopterus annectens into seasonal wetlands on
the margins of Lake Malawi around Salima (Tweddle 1989,
Snoeks 2004). The impacts of both introductions appear to
have been geographically and ecologically trivial, perhaps
due to the narrow ecological niches of both species.
Here, we provide evidence of two recent introductions of
Oreochromis into the Lake Malawi (‘Nyasa’ in Tanzania)
catchment, including the prolific invader O. niloticus, and
emphasise new threats to the unique biodiversity and
genetic resources of the catchment.
Methods
The tilapiine fauna of natural water bodies and aquaculture
ponds in the southern Tanzanian section of the Lake Malawi
catchment were surveyed during July and November 2011
and in September 2012 (Figure 1). These were fished with
either multi-mesh gillnets or seine nets, as appropriate.
Oreochromis specimens encountered were photographed
and fin clips were preserved in absolute ethanol for genetic
analysis. Individuals were classified in the field using
morphological characters.
To confirm field identification, we used the protein-coding
mitochondrial gene NADH-2 from individuals field-identified
as non-indigenous taxa. This gene has previously been used
to resolve partially the phylogenetic relationships of East
African Oreochromis (Klett and Meyer 2002). Specifically,
we sequenced specimens identified as O. niloticus from
Lake Itamba (9°21'04" S, 33°50'39" E), a crater lake to
the north of Lake Malawi (Figure 1). We also sequenced
specimens identified as O. niloticus and O. leucostictus from
government aquaculture ponds near Songea (10°37'22" S,
35°38'10" E), to the east of Lake Malawi (Figure 1). In
addition, we sequenced representatives of Oreochromis
species indigenous to the Lake Malawi catchment, including
O. shiranus, O. squamipinnis and O. chungruruensis.
Total genomic DNA was extracted from tissue using the
Wizard genomic DNA purification kit (Promega), following the
manufacturer’s protocol. Polymerase chain reaction (PCR)
employed the primers ND2Met (5’ CAT ACC CCA AAC ATG
TTG GT 3’) and ‘ND2Trp’ (5’ GTS GST TTT CAC TCC CGC
TTA 3’) (Kocher et al. 1989, Schliewen and Klee 2004). PCR
was carried out in 25 μl volumes, including 12.5 μl of MyTaq
(Bioline), 0.5 μl of each primer, 10.5 μl of H2O, and 1 μl of
DNA. PCR conditions were as follows: 1 min at 95 °C, then
34 cycles of 95 °C for 30 s, 50 °C for 30 s and 72 °C for
1 min, followed by 72 °C for 5 min. Sequencing of the PCR
products was outsourced to Macrogen (Korea). Sequences
were checked for quality and polymorphisms confirmed using
ChromasLite (Technelysium [Pty] Ltd).
These sequences were aligned with additional
wild-caught reference samples of Oreochromis (Klett and
Meyer 2002, Cnaani et al. 2008) using ClustalW in DAMBE
(Xia 2013). The most appropriate model of sequences
evolution was determined using jModeltest (Darriba et al.
2012). Phylogenetic reconstructions were undertaken using
BEAST v1.7.4 (Drummond et al. 2012) with 10 million
generations, sampling every 1 000 trees. The first 20% of
trees were disregarded as burn-in, before a maximum clade
credibility tree was calculated in TreeAnnotator (Drummond
et al. 2012), and viewed in FigTree (http://tree.bio.ed.ac.
uk/software/figtree/). New sequences have GenBank
Accession numbers (KF772214–KF772228).
Results
The sequence alignment file included 33 sequences and
978 bp. The most appropriate model of sequence evolution
was identified as the GTR++I model. Phylogenetic
reconstructions using these data demonstrated that field-
identified representatives of O. niloticus from the Lake
Malawi catchment formed a monophyletic clade with O.
niloticus from its African indigenous range (O. niloticus
vulcani; Lake Turkana) and the introduced range (Lake
Victoria) (Figure 2). Field-identified representatives of
O. leucostictus from the Lake Malawi catchment formed
a monophyletic clade with O. leucostictus individuals
from Lake Victoria. Both these groups were distinct from
representatives of the indigenous Lake Malawi Oreochromis
fauna, including O. shiranus, O. karongae, O. squamipinnis
and O. chungruruensis.
Discussion
Evidence from field sampling and phylogenetic reconstruc-
tions shows that O. niloticus and O. leucostictus have
been translocated into the Lake Malawi catchment.
Oreochromis niloticus is present in Lake Itamba, one of a
chain of maar crater lakes in the Rungwe volcanic range.
Information obtained from the regional office of the Tanzania
Department of Fisheries indicates that the introduction
took place in 2010 as an attempt to initiate a new fishery in
the lake, and that the fish were sourced from Morogoro in
central Tanzania. Territorial male fish were observed in Lake
Itamba during November 2011, suggesting that the popula-
tion is capable of reproducing, and subsequent rapid popula-
tion expansion is plausible (see Weyl 2008). Additionally,
the government records indicate that several introductions
have taken place in other crater lakes in the region, namely
North American bass (Micropterus sp.) into Lake Kyungululu
in 1998, O. niloticus and Tilapia rendalli into Lake Massoko/
Kisiba in 1993 and 2010, respectively, O. niloticus and T.
rendalli into Lake Ngosi in 2002 and 2005, respectively,
and T. rendalli into Lake Ikapu in 1998, Lake Ilamba in
1998 and Lake Kingili in 1998. During our sampling in July
2011, November 2011 and January 2013 North American
bass were not observed in Lake Kyungululu, O. niloticus
was not observed in Lake Massoko, and T. rendalli was not
observed in either Lake Ikapu or Lake Ilamba. Lake Ngosi
was not sampled. Tilapia rendalli was, however, found
in Lakes Kingili and Kyungululu, in the latter together with
T. sparrmanii. Given that T. rendalli is native to the Lake
Malawi catchment, whether these populations are a direct
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African Journal of Aquatic Science 2013, 38(Suppl.): 85–90 87
result of introductions is unclear, but evidence suggests
that no Tilapia species were present in Lake Kyungululu
when it was sampled by Fülleborn in about 1923 (Ahl 1924,
Trewavas 1976). Thus, all the evidence points towards a
systematic campaign to introduce alien fish to these crater
lakes in the Lake Malawi catchment over the last 20 years.
This appears to have happened without investigation of the
indigenous biodiversity or the productivity of the lakes, or
the socio-economic needs for alien fish resources. Similar
concerns have been expressed for at least one other
African crater lake, Lake Ejagham, Cameroon, where the
predatory catfish Parauchenoglanis cf. balayi has recently
been introduced (Dunz and Schliewen 2010, Martin 2012).
Our recent research has confirmed the presence of
large-bodied indigenous tilapiines in Lakes Kyungululu (O.
chungrurensis), Itamba (O. shiranus, O. karongae), Ilamba
(O. shiranus, O. squamipinnis), Massoko (O. squamipinnis),
Kingiri (O. shiranus) and Ikapu (O. sp. ‘golden chambo’).
Thus, ecological niches that would be occupied by invasive
tilapiines are possibly already filled. Moreover, the naturally
low abundance of fishes may be related to low biolog-
ical productivity, with the small closed catchments giving
low potential for introduction of allochthonous carbon and
nutrients, and the permanent stratification that is character-
istic of tropical maar lakes (Cohen 2003) restricting nutrient
cycling.
Oreochromis niloticus and O. leucostictus were both
confirmed present in government fish ponds near Songea
in September 2012, on the basis of their morphology and
mtDNA sequences. While O. niloticus is commonplace
in aquaculture and grows to large body sizes (maximum
64 cm total length; Fishbase, accessed 1 May 2013), O.
leucostictus is small-bodied (maximum 32 cm total length;
Fishbase, accessed 1 May 2013). Several specimens
observed in the ponds were morphologically interme-
diate between the species, suggesting that they may have
been from a well-mixed hybrid stock of the two invasive
species. It has long been known that these species readily
hybridise (Trewavas 1983, Nyingi and Agnese 2007). We
are uncertain of the dates of the introductions of these taxa,
but information from the local government officials suggested
the fish were sourced from a hatchery in Morogoro, central
Tanzania. Precisely why these species were chosen over
indigenous O. shiranus is unclear, given that the latter has
been used in aquaculture initiatives for over 50 years (Ambali
et al. 1999). We are unaware of any quantitative trials that
have directly compared the productivity of these species.
Colonisation risk and potential consequences
The risk of natural escape of O. niloticus from Lake Itamba is
low, as the lake has no natural outflow at present (Delalande
2008). The lake contains several native species, including
putative endemic Astatotilapia, and indigenous Oreochromis
(MJG, BPN and GFT pers. obs.). The consequences of this
introduction will be geographically isolated, unless further
translocations of O. niloticus take place with individuals
sourced from this lake. By contrast, the risk of escape from
the fish ponds near Songea is considerably higher. The
ponds are situated immediately adjacent to a flowing stream,
which is a tributary of the Ruhuhu River, with pond-water
overflows directed into the stream. Colonisation of the
Ruhuhu will inevitably lead to colonisation of Lake Malawi
(a)
100 km
TANZANIA
MOZAMBIQUE
MOZAMBIQUE
ZAMBIA
MALAWI
Tanzania
Mozambique
Zambia
Malawi
Lake Malawi
34° E 36° E
34° E 36° E
12° S
15° S
AFRICA
(b)
Lake Malawi catchment
Figure 1: Catchment of Lake Malawi, with coloured circles indicating sites where invasive Oreochromis have been recorded: (a) Lake Itamba,
where O. niloticus (inset) was collected during July and November 2011; (b) aquaculture ponds near Songea where O. niloticus (inset left) and
O. leucostictus (inset right) were collected in September 2012
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Genner, Connell, Shechonge, Smith, Swanstrom, Mzighani, Mwijage, Ngatunga and Turner
88
and inflowing rivers, potentially with significant consequences
for the native fauna of Lake Malawi. Further surveys of
the Ruhuhu River catchment are required to determine
whether O. niloticus and/or O. leucostictus have success-
fully established populations outside the aquaculture ponds.
Efforts should be made to reduce future risk of colonisation
through the removal of these non-indigenous species from
aquaculture ponds, and their replacement with native large-
bodied species such as Oreochromis shiranus and Tilapia
rendalli.
The Lake Malawi catchment contains a mosaic of
habitats, many suitable for O. leucostictus and O. niloticus.
Oreochromis leucostictus is a small species, character-
istic of macrophyte-rich shorelines or peripheral lagoons.
Such habitats are currently occupied by generalist species
including O. shiranus and Astatotilapia calliptera, plus
a handful of representatives of the indigenous radiating
haplochromine flock, including Trematocranus placodon and
Lethrinops lethrinus (Konings 2007). In Lake Victoria, O.
leucostictus has been confined to such marginal habitats,
and has generally been considered a relatively benign
species in relation to O. niloticus. By contrast, it seems
likely that O. niloticus will occupy reedy shorelines, the open
waters of peripheral lakes and open inshore bays, habitats
traditionally occupied by the indigenous Oreochromis of the
‘chambo’ group (O. karongae, O. lidole and O. squamipinnis)
and by many endemic haplochromines. One mechanism by
which it may outcompete other species is through aggres-
sive competition for shelter from shared predators (Martin et
al. 2010).
In Lake Nicaragua, the introduction of non-native
Oreochromis led to 80% reductions in the abundance of
0.02
1
1
1
1
0.59
1
1
1
1
0.99
0.85
0.99
0.85
0.79
0.49
1
1
1
1
0.87
0.95
1
Isolate 10-09-12/247 (Songea)
Isolate 10-09-12/248 (Songea)
Oreochromis shiranus
Oreochromis karongae
Oreochromis squamipinnis
Oreochromis chungrurensis
Oreochromis mossambicus
Oreochromis mossambicus
Oreochromis variabilis
Oreochromis amphimelas
Oreochromis urolepis
Oreochromis schwebischi
Oreochromis tanganicae
Oreochromis mweruensis
Oreochromis macrochir
Oreochromis andersonii
Isolate 10-09-12/246 (Songea)
Isolate 10-09-12/249 (Songea)
Oreochromis leucostictus (Lake Victoria)
Oreochromis esculentus
Isolate 10-09-12/266 (Songea)
Isolate 10-09-12/267 (Songea)
Isolate 10-09-12/268 (Songea)
Isolate 10-09-12/265 (Songea)
Isolate 19-07-11/277B (Lake Itamba)
Isolate 19-07-11/277A (Lake Itamba)
Oreochromis niloticus (Lake Victoria)
Oreochromis niloticus vulcani(Lake Turkana)
Isolate 21-11-11/1F3B (Lake Itamba)
Isolate 21-11-11/1F3A (Lake Itamba)
Oreochromis aureus
Tilapia sparrmanii
Tilapia rendalli
Indigenous
Lake Malawi
Oreochromis
leucostictus
Oreochromis niloticus
Figure 2: Bayesian phylogenetic reconstruction of Oreochromis, based on the mitochondrial NADH-2 gene, including specimens
morphologically identified as O. niloticus and O. leucostictus populations from the Lake Malawi catchment. The phylogeny includes 18 of the
32 valid Oreochromis species (Fishbase, accessed 1 May 2013). Numbers on branches indicate proportional posterior probability support
Downloaded by [University of Bristol] at 12:54 12 November 2013

African Journal of Aquatic Science 2013, 38(Suppl.): 85–90 89
native cichlid species within a decade following their first
recorded presence (McKaye et al. 1995, Canonico et al.
2005). In Lake Victoria, the shift from a traditional fishery
based on O. esculentus and O. variabilis, to one based
on O. niloticus and O. leucostictus also happened rapidly
(Barel et al. 1985). Thus, there is a risk that the introduction
of O. niloticus could hasten the decline of the established
‘chambo’ fishery of Lake Malawi. There is also a consid-
erable risk of hybridisation with native taxa occurring.
Introduced O. niloticus has been identified as hybridising with
O. esculentus in the Lake Victoria catchment (Angienda et al.
2011), and with O. mossambicus in southern Africa (D’Amato
et al. 2000). This implies that hybridisation is plausible with
all four Oreochromis species found in Lake Malawi, perhaps
leading to a homogenisation or loss of genetic diversity.
Notably, however, O. niloticus is also present in Lake
Tanganyika (Kullander and Roberts 2011), without any
reported adverse effects on the indigenous fauna. This
directly contrasts with the observations from Lake Victoria
(Ogutu-Ohwayo 1990). It is plausible that O. niloticus has
been denied a foothold in Lake Tanganyika because of
the ecological differences between the lakes. Lake Victoria
is shallow, turbid and ecologically similar to water bodies
in the Nile catchment where O. niloticus naturally occurs,
such as Lakes Edward, Albert and George. By contrast,
Lake Tanganyika is mainly characterised by deeper and
clear water, and thus here O. niloticus may be restricted
to the inflowing rivers (e.g. Kullander and Roberts 2011).
The Lake Malawi catchment lies ecologically intermediate
between the two lakes, having both large areas of deep,
clear water and of shallow turbid water.
Policy implications
The first known introductions of non-native species to Lake
Victoria occurred in the 1950s, but it took 30 years for the
effects of these to be widely noticed by the scientific com-
munity. The recognition only took place after the collapse
of indigenous artisanal fisheries, and their replacement with
initially low-value and unpopular invasive species (Barel et
al. 1985, Balon and Bruton 1986). The effects on the Lake
Victoria fish community were unknown, and continued
monitoring has shown the ecosystem has still not fully
stabilised (Witte et al. 2000). Given the uncertainty of the
effects of introduced species on the African lake biodiver-
sity, plus the importance of existing fisheries to food security
in the riparian nations, the precautionary principle should be
adopted and future introductions should be avoided.
Future introductions are likely to come from attempts to
enhance capture fisheries and to establish new aquacul-
ture enterprises, thus policies of zoned development are
most appropriate if the negative effects of these are to be
avoided (Lind et al. 2012). Such policies ensure that, where
new fisheries and aquaculture facilities are established,
they are stocked with indigenous fishes from the region,
and not with alien species. This has the benefits of
eliminating risk of alien introduction, maintaining endemic
biodiversity, and ensuring that there are established
regional markets for the species, so that the fish produced
will not have intrinsically low value due to unfamiliarity.
Moreover, zoned development means that the fish species
chosen are more likely to be adapted to the regional climate
and natural pathogens, which may partially explain why
attempts to initiate new fisheries and aquaculture schemes
can be unsuccessful.
A final consideration is that tilapiine-based aquaculture is
expanding rapidly across Africa, Asia and the Neotropics,
and thus the conservation of genetic resources is likely to
be of future long-term benefit for the development of new
aquaculture strains and the maintenance of global food
security (Lind et al. 2012). There is a clear need for policy-
makers from the riparian countries of Lake Malawi to address
these issues from a multilateral perspective, to protect the
indigenous biodiversity and fish stocks of the region.
Acknowledgements — For assistance with logistics, we thank
the staff of TAFIRI who have facilitated field work, including
Jonathan Kihedu and Baraka Sekadende. We are grateful to Jacob
Mwaibako, District Fisheries Office, Government of Tanzania, for
information on the timing of the species introductions. The work
was funded by a Royal Society-Leverhulme Trust Africa Award to
MJG, BPN and GFT, and a NERC DTG studentship for JS.
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Received 10 July 2013, revised 29 August 2013, accepted 30 August 2013
Edited by OLF Weyl
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