Risk assessment for Nile tilapia Oreochromis niloticus in South Africa

Taxonomy

Species: Oreochromis niloticus niloticus (Linnaeus, 1758) Family: Cichlidae (Cichlids) Subfamily: Pseudocrenilabrinae Order: Perciformes (perch-like fishes) Class: Actinopterygii (ray-finned fishes) Taxonomic Code: 1705905102 Oreochromis niloticus has seven sub-species, the most widely distributed of which is O. niloticus niloticus. Other sub-species are O. n. eduardianus, O. n. filoa, O. n. baringoensis, O. n. sugutae, O. n. cancellatus and O. n. vulcani (Trewavas, 1983). Common names: Nile tilapia, Tilapia du Nil, Tilapia del Nilo.

Originating environment.

The native range of Nile tilapia (Figure 1) is tropical and subtropical Africa in West Africa (Figure 2). The species naturally occurs in the Nile basin (including lake Albert, Edward and Tana), Jebel Marra, Lake Kivu, Lake Tanganyika, Awash River, various Ethiopian lakes, Omo River system, Lake Turkana, Suguta River and Lake Baringo (FishBase 2014). In West Africa, natural distribution covers the basins of the Senegal, Gambia, Volta, Niger, Benue and Chad. This species has been widely distributed for aquaculture and sport fisheries and has established populations in many countries where introduced (Figure 2, Picker & Griffiths 2011).

Figure 2: Global native and introduced range of Nile tilapia (from Picker and Griffiths, 2011).

A review of ecological tolerances of Nile tilapia was conducted by Zengeya et al (2013a). Nile tilapia is tolerant to a wide range of temperatures (8–42oC) but their natural temperature range is 13.5-33.0oC and is tolerant to brackish water of salinity ranges from 20–30 g L-1 (Phillippart & Ruwet 1997). Optimal growth is attained at temperatures between 31 and 36 oC. The species can persist in degraded habitats and have been able to establish populations in many countries where they have been introduced. The primary limitation to their establishment appears to be low temperature and there are no records of establishment from environments where temperatures are below 14oC and summer temperatures must exceed 20 oC to facilitate spawning (Trewavas 1983). Reproductive strategy and potential, dispersal abilities Although variable, sexual maturity can be attained as early as 5-6 months (FAO 2014). They are summer spawners and reproduction is limited to temperatures exceeding 20oC. Males excavate nests and defend territories that are visited by females. Nile tilapia are maternal mouthbrooders and females brood eggs and fry for a period of 1-2 weeks (Trewavas 1983; de Moor & Bruton 1988; FishBase 2014; FAO 2014). In culture conditions, the egg number is consistent with the body weight of the female. A 100 g female will produce about 100 eggs per spawn, while a female weighing 600-1000 g can produce 1000 to 1500 eggs (FAO 2014). In the wild, fecundity varies with fish condition, genetic and dietary differences and in the rift valley lakes mean absolute fecundity ranges between 1992 ± 233 and 3723 ± 147 ripe eggs per female (irrespective of size class) (Balirwa 1998). After spawning, the male remains in his territory, guarding the nest, and is able to fertilize eggs from a succession of females. Nile tilapia can live longer than 10 years and reach a weight exceeding 5 kg (Picker & Griffiths 2011). As a result of this reproductive strategy, Nile Tilapia are able to rapidly colonise and dominate fish faunas in invaded ecosystems (Weyl 2008). The rapid invasion of the 116 km2 Lake Chicamba, Mozambique (19°08′S; 33°08′E), from a small (<0.3 km2) upstream reservoir illustrates the significant invasion threat that small point-sources of this species pose to southern African freshwater systems. Nile tilapia were absent from the lake until a flood in 1996 but within a year of detection, the species was found throughout the lake and contributed 5.2 t month−1 top the artisanal fishery (Weyl 2008). Other examples include the invasion of Lake Victoria, Lake Kariba and the Limpopo system (see Tweddle & Wise 2007; Zengeya et al. 2013). Invasive tendencies of the species elsewhere and taxonomic predisposition

While only its congener Mozambique tilapia Oreochromis mossambicus is listed among the world’s 100 worst invaders (Lowe et al. 2000), Nile tilapia are considered highly invasive and feral populations exist in tropical and

subtropical regions worldwide (Canonico et al. 2005). The great success of Nile tilapia as an invasive species has been attributed to aggressive spawning behaviour; high levels of parental care; the ability to spawn multiple broods throughout the year and its broad diet (Canonico et al. 2005). In areas where this species has become established, ecological effects include decreased abundance and extinction of native species resulting from habitat and trophic overlaps and competition for spawning sites (See reviews by Canonico et al. 2005; Tweddle & Wise 2007), habitat destruction and water quality changes (Figueredo & Giani 2005) and hybridization with other Oreochromis species (Firmat et al. 2013).

History of domestic propagation or cultivation of the species, introductions and the extent of naturalization;

Nile tilapia is the main farmed tilapia species mainly due to its superior growth rates and their global production exceeds 2 million tons (FAO 2014). As a result of the success of Nile tilapia in stock enhancements and aquaculture, they are one of the ten most introduced species in the world (Garciá - Berthou et al. 2005). Introductions are summarised in www.fishbase.org (Fishbase 2014) which lists the occurrence of Nile tilapia in 102 countries. Introductions outside of its native range include: Albania, Bangladesh, Belgium, Bolivia, Botswana, Brazil, Cambodia, Central African Republic, China, Colombia, Comoros, Democratic Republic of Congo, Costa Rica, Cuba, Cyprus, Czech Republic, Dominican Republic, Ecuador, El Salvador, Eritrea, Fiji, Gabon, Galapagos Islands, Greece, Grenada, Guatemala, Guyana, Haiti, Honduras, Hong Kong, India, Indonesia, Iran, Italy, Jamaica, Japan, Kiribati, Korea, Laos, Liberia, Madagascar, Malaysia, Mauritius, Mexico, Mozambique, Nepal, Netherlands, Nicaragua, Pakistan, Panama, peru, Philippines, Puerto Rico, Reunion, Saint Lucia, Sao Tome, Saudi Arabia, Sierra Leone, Singapore, Slovakia, South Africa, Sri Lanka, St Vincent, Syria, Taiwan, Tanzania, Thailand, Trinidad and Tobago, Tunisia, Turkey, United Kingdom, USA, Vietnam and Zimbabwe.

In southern Africa, established populations are documented from Zambia, Botswana, Zimbabwe, Mozambique, Angola, the Democratic Republic of Congo and Tanzania (Picker & Griffiths 2011). In South Africa, Nile tilapia was introduced into South Africa for aquaculture in 1955 and is thought to be confined to the Limpopo River system and small coastal river systems in the Kwa-Zulu Natal Province, although their current status in the latter is uncertain (de Moor & Bruton 1988; van Rensburg et al. 2011; Zengeya et al. 2013b).

Dietary requirements

Nile tilapia is an omnivorous grazer that feeds on phytoplankton, periphyton, aquatic plants, small invertebrates, benthic fauna, detritus and bacterial films associated with detritus (see summary in Zengeya et al. 2011; FAO 2014). Nile tilapia can filter feed by entrapping suspended particles, including phytoplankton and bacteria, on mucous in the buccal cavity, although its main source of nutrition is obtained by surface grazing on periphyton mats. The species is also known to exhibit opportunistic feeding strategies and Nile tilapia have been shown to feed at any trophic level, including small insect stages, micro-crustaceans and fish (McKaye et al. 1995; Njiru et al. 2004; Zengeya et al. 2011).

Ecosystem impacts

Nile tilapia is a popular aquaculture species that is also highly invasive (Weyl 2008) and has had devastating

impacts through competition or hybridisation with native congenerics (Canonico et al. 2005; Tweddle & Wise

2007). In Nicaragua, the escape of Nile Tilapia from aquaculture and its subsequent establishment resulted in a

decline in native cichlid catches of more than 50 % (McKaye et al. 1995). In Lake Alaotra, Madagascar, the

progressive introductions of different species including Nile tilapia induced a drastic decline of native fish

(Lévêque 1997) and in changes in phytoplankton communities in Brazil (Figuerdo & Giani 2005).

Ability to hybridize with native species The most devastating impact of Nile tilapia introductions in Africa is via hybridisation. Nile tilapia are able to hybridise with several other Oreochromis species. In the Lake Victoria basin, hybridisation has been reported

between O. niloticus and the endemic Oreochromis variabilis (Welcomme 1967) and Oreochromis esculentus (Mwanja et al. 2001). In southern Africa, Nile tilapia introductions have resulted in extensive hybridisation and introgression with native O. mossambicus in the Limpopo River system (D’Amato et al. 2007; Firmat et al. 2013), with Oreochromis andersonii and Oreochromis macrochir in the Kafue River in Zambia (Deines et al. 2014) and in Lake Kariba they have almost replaced the native O. mortimeri in Lake Kariba, Zimbabwe (Tweddle 2010). In South Africa, studies on the impact of O. niloticus introductions into hybridisation and introgression with native O. mossambicus has been fairly well studied (Moralee et al. 2000; D’Amato et al. 2007; Firmat et al. 2014). Hybridisation is recognised as a primary threat to O. mossambicus and they are consequently IUCN redlisted as ‘Near Threatened’ (Cambray & Swartz 2007). Suitability of receiving environment in South Africa

i) Climate match Nile tilapia is a tropical species that prefers to live in shallow water. The lower and upper lethal temperatures for Nile tilapia are 11-12 °C and 42 °C, respectively, while the preferred temperature ranges from 31 to 36 °C (FAO 2014). Zengeya et al. (2013a) present the results from an ecological niche model for this species. Their results provided evidence that Nile tilapia are able to survive in conditions incongruent with their native range and demonstrated that the species was able to survive in a large proportion of river systems in southern Africa. They also showed that Nile tilapia exhibited a broad invasive potential over most of southern Africa. In South Africa, Zengeya et al. (2013a) demonstrated a broad invasive potential over entire Limpopo River basin and the coastal rivers along the Indian Ocean. The models, however, predicted low suitability for most of the Orange River basins and west-flowing rivers in the south Atlantic coast. A qualitative ecological risk assessment for the Limpopo River for determining the risk of establishment and spread of the invasive Nile tilapia within the central sub-catchment of the Limpopo River basin in northern South Africa was developed by Zengeya et al. (2013b). The assessment used known physiological tolerance limits of Nile tilapia in relation to minimum water temperature, presence or absence of dams, seasonality of river flows, and the presence of indigenous fish species of concern to identify river systems that would be suitable for establishment. They found that: river sections along the Limpopo main river channel and the immediate reaches of its associated tributaries east of the Limpopo/Lephalala river confluence along the Botswana–South Africa–Zimbabwe border were highly vulnerable; rivers in the upper Bushveld catchment (Upper Limpopo, Mogalakwena, Lephalala, Mokolo, Matlabas and Crocodile rivers) were at medium ecological risk; while headwater streams were considered to be of low ecological risk. The decrease in vulnerability between lowveld and highveld river sections was mainly a function of low water temperatures (8–12 °C)

associated with increasing altitude. Because environmental tolerances of Nile tilapia are similar to those of Mozambique tilapia, the climatic suitability map developed by Weyl & Keevey (2013) is likely to be a close match for O. niloticus. Based on criteria of temperature, altitude, rainfall and water conductivity, Weyl & Keevey (2013) showed that low lying areas on the east coast and the low veldt were highly suitable for tilapia fisheries. These areas should also be considered at risk of Nile tilapia invasion.

Figure 3: GIS map showing areas suitable for trout (in green), based on conductivity, altitude, mean annual rainfall, mean annual air temperature and mean winter air temperature.

(ii) Habitat Freshwater, tolerant of brackish water. Favours shallow vegetated standing waters of lake shores and large rivers. Intolerant of temperatures below 14oC and requires temperatures of > 20 °C to spawn. In South Africa this

will limit the species to subtropical areas.

(iii) Presence of natural enemies, predators and competitors This species is highly competitive and although preyed upon by piscivorous fishes it has been able to invade a large variety of ecosystems.

(iv) Presence of potentially reproductive compatible species.

Nile tilapia are able to hybridise with indigenous congenerics. In South Africa, this pertains to the distribution of Mozambique tilapia which is considered near threatened due to hybridisation with Nile tilapia in its native range.

Figure 4: Map of current distribution of Mozambique tilapia across South Africa (from Picker and Griffiths 2011). In its natural range, this species is potentially threatened through hybridisation with Nile tilapia.

Potential as a vector for other introduced organisms While generally considered quite resistant to disease and parasitisation (Bittencourt et al. 2014), Nile tilapia are affected by several common fish diseases including: Motile Aeromonas Septicaemia (MAS), Vibriosis,

Columnaris, Edwardsiellosis, Streptococcosis infection, Saprolegniosis (FAO 2014) and several parasites including Ichthyophthirius multifilis, Trichodina centrostrigeata, Paratrichodina africana, Trichodina nobilis (Protozoa) and Cichlidogyrus tilapiae (Monogenoidea) (Bittencourt 2014). Seventeen different species of parasites were recovered in parasitological studies on juvenile and adult Nile tilapia. Most of the parasites that were recovered are known to cause diseases in other fishes that are used for aquaculture. Caution should be taken to ensure that they are not transferred to new bodies of water along with Nile tilapia that is intended for aquaculture (Natividad et al. 1986). Listing of the Species: Nile tilapia, including other tilapia species and excluding indigenous species O. mossambicus and O. placidus

are listed in category 3 in the list of Invasive Freshwater Fish Species. The species is listed in category 2 for

permitted aquaculture facilities and live transportation and the sale of Nile tilapia is prohibited except from

accredited hatcheries. Also, Import of live tilapia is prohibited except by hatcheries with a permit.

Risk Assessment Framework Considerations

Synopsis of information Main Reference

(1) A risk assessment must consider-

(a) information regarding the relevant species, including-

(i) the taxonomy of the species, including its class, order, family, scientific name (if known), genus, scientific synonyms and common names of the species;

Species: Oreochromis niloticus niloticus (Linnaeus, 1758) Family: Cichlidae (Cichlids), Subfamily: Pseudocrenilabrinae Order: Perciformes (perch-like fishes) Class: Actinopterygii (ray-finned fishes) Common names: Nile tilapia, Tilapia du Nil, Tilapia del Nilo.

http://www.fishbase.org/summary/Oreochromis-niloticus.html; Trewavas 1983

(ii) the originating environment of the species, including climate, extent of geographic range and trends;

The native range of Nile tilapia is are tropical and subtropical Africa in West Africa. Naturally occurs in the Nile basin (including lake Albert, Edward and Tana), Jebel Marra, Lake Kivu, Lake Tanganyika, Awash River, various Ethiopian lakes, Omo River system, Lake Turkana, Suguta River and Lake Baringo (FishBase 2014). Also basins of the Senegal, Gambia, Volta, Niger, Benue and Chad. Nile tilapia tolerates a wide range of temperatures (8–42oC) but their natural temperature range is 13.5-33.0oC and is tolerant of brackish water to salinity ranges from 20–30 g L-1. Summer temperatures must exceed 20oC to facilitate spawning.

Trewavas 1983; Tweddle & Wise 2007; Picker & Griffiths 2011; Zengeya et al. 2013a

(iii) persistence attributes of the species, including reproductive potential, mode of reproduction, dispersal mechanisms and undesirable traits;

Maternal mouthbrooders. Mature from 5-6 months old and are capable of multiple broods in a spawning season. Absolute fecundity ranges between 1992 ± 233 and 3723 ± 147 ripe eggs per female (irrespective of size class). Nile tilapia can live longer than 10 years. Has proven dispersal abilities and the ability to invade from even small point sources. Demonstrable ability to rapidly colonise and dominate fish faunas in invaded

FAO 2014; Canonico et al. 2005; Tweddle & Wise 2007; Weyl 2008

ecosystems.

(iv) invasive tendencies of the

species elsewhere and

taxonomic predisposition;

Considered highly invasive and feral populations exist in tropical and subtropical regions worldwide. Success is attributed to aggressive spawning behaviour; high levels of parental care; the ability to spawn multiple broods throughout the year and its broad diet. Ecological effects include: decreased abundance and extinction of native species resulting from habitat and trophic overlaps and competition for spawning sites; habitat destruction and water quality changes and hybridization with other Oreochromis species.

de Moor & Bruton

1988; Canonico et

al. 2005; Figueredo

& Giani, 2005;

Tweddle & Wise

2007; Firmat et al.

2013

(v) the history of domestic

propagation or cultivation of the

species, introductions and the

extent of naturalization;

Primary species of tilapia that is farmed with global production in excess of 2 million tons.

Introductions outside of its native range include: Albania, Bangladesh, Belgium, Bolivia, Botswana, Brazil, Cambodia, Central African Republic, China, Colombia, Comoros, Democratic Republic of Congo, Costa Rica, Cuba, Cyprus, Czech Republic, Dominican Republic, Ecuador, El Salvador, Eritrea, Fiji, Gabon, Galapagos Islands, Greece, Grenada, Guatemala, Guyana, Haiti, Honduras, Hong Kong, India, Indonesia, Iran, Italy, Jamaica, Japan, Kiribati, Korea, Laos, Liberia, Madagascar, Malaysia, Mauritius, Mexico, Mozambique, Nepal, Netherlands, Nicaragua, Pakistan, Panama, peru, Philippines, Puerto Rico, Reunion, Saint Lucia, Sao Tome, Saudi Arabia, Sierra Leone, Singapore, Slovakia, South Africa, Sri Lanka, St Vincent, Syria, Taiwan, Tanzania, Thailand, Trinidad and Tobago, Tunisia, Turkey, United Kingdom, USA, Vietnam and Zimbabwe.

In southern Africa, established populations are documented in Zimbabwe, Mozambique, Zambia and Tanzania. In South Africa, Nile tilapia is thought to be confined to the Limpopo River system and small coastal river systems in the Kwa-Zulu Natal Province.

de Moor & Bruton

1988; FAO

2014;Zengeya et al.

2013b

(vi) nutritional or dietary

requirements of the species

and, where applicable, whether

it has a specialist or generalist

diet;

Omnivorous grazer that feeds on phytoplankton, periphyton, aquatic plants, small invertebrates, benthic fauna, detritus and bacterial films associated with detritus. Also known to exhibit opportunistic feeding strategies and can feed at any trophic level, including on small insect stages, micro-crustaceans and fish.

Njiru et al. 2004;

Zengeya et al.

2011, FAO 2014

(vii) the ability of the species to

create significant change in the

ecosystem; and

Has had devastating impacts through competition or hybridisation with native congenerics. Threatens indigenous Mozambique tilapia in southern Africa (including South Africa); has resulted in native fish declines in Nicaragua, Madagascar, Zimbabwe and Zambia and

Tweddle & Wise

2007; Firmat et al.

2013

(viii) the potential to hybridize with

other species and to produce

fertile hybrids; and

In South Africa, Nile tilapia introductions have resulted in extensive hybridisation and introgression with native Mozambique tilapia Oreochromis mossambicus in the Limpopo River system.

D’Amato et al. 2007; Moralee et al. 2000; Firmat et al. 2014

(b) information regarding the restricted

activity in respect of which the permit

is sought, including-

(i) the nature of the restricted

activity;

Will not be addressed by SAIAB

(ii) the reason for the restricted

activity;

Will not be addressed by SAIAB

(iii) the location where the restricted

activity is to be carried out;

Will not be addressed by SAIAB

(iv) the number and, where

applicable, the gender of the

specimens of the species

involved; and

Will not be addressed by SAIAB

(v) the intended destination of the

specimens, if they are to

be translocated; and

Will not be addressed by SAIAB

(c) information regarding the receiving

environment, including-

(i) climate match; Nile tilapia is a tropical species that prefers to live in shallow water. The lower and upper lethal temperatures for Nile tilapia are 11-12 °C and 42 °C, respectively, while the preferred temperature ranges from 31 to 36 °C. Niche models indicate that Nile tilapia exhibited a broad invasive potential over most of southern Africa. In South Africa, demonstrated a broad invasive potential over entire Limpopo River basin and the coastal rivers along the Indian Ocean.

Zengeya et al.

2013a & b

(ii) habitat; Rivers, lakes, dams and canals in tropical and

subtropical areas.

(iii) the presence of natural

enemies, predators and

competitors; and

None. This species is highly competitive and although preyed upon by piscivorous fishes it has been able to invade a large variety of ecosystems, including those already invaded by other alien fishes such as Nile perch Lates niloticus and largemouth bass Micropterus salmoides.

Tweddle & Wise

2007

(iv) the presence of potentially

reproductive compatible

species.

Mozambique tilapia, Oreochromis mossambicus occur

naturally in the Limpopo and east flowing South African

rivers down to the Bushman’s River in the Eastern

Cape.

Picker & Griffiths

2011

(2) A risk assessment carried out in

terms of subregulation (1) must

identify-

(a) the probability that the species will

naturalize in the area in which the

restricted activity is to be carried

out or in any other area elsewhere

Will not be addressed by SAIAB

in the Republic;

(b) the possible impact of the species

on the biodiversity and sustainable

use of natural resources of-

Will not be addressed by SAIAB

(i) the area in which the restricted

activity is to be carried out; and

Will not be addressed by SAIAB

(ii) in any other area elsewhere in

the Republic;

Will not be addressed by SAIAB

(c) the risks and potential impacts on

biodiversity by the species to which

the application relates;

Will not be addressed by SAIAB

(d) the risks of the specimen serving

as a vector through which

specimens of other alien species

may be introduced;

Comment will be provided based on available literature.

(e) the risks of the method by which a

specimen is to be introduced or the

restricted activity carried out

serving as a pathway through

which specimens of other alien

species may be introduced; and

Will not be addressed by SAIAB

(f) any measures proposed in order to

manage the risks.

Will not be addressed by SAIAB

(3) Based on the information in sub

regulations (1) and 2), a risk

assessment must consider-

(a) the likelihood of the risks being

realized;

Will not be addressed by SAIAB

(b) the severity of the risks and

consequences of the realization of

the risks for other species, habitats

and ecosystems;

Will not be addressed by SAIAB

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Other resources

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Bwanika, GN, Chapman LJ, Kizito Y & Balirwa J. 2006. Cascading effects of introduced Nile perch (Lates niloticus) on the foraging ecology of Nile tilapia (Oreochromis niloticus). Ecology of Freshwater Fish 15: 470–481.

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