Salut Olaf, C'est une chose sur laquelle je m'interrogeais. Nous savons que le tilapia est capable de tirer profit d'un environnement perturbé dans lequel on trouve des écosystèmes avec des niches soit vacantes, ou occupées par des espèces qui furent dans le passé de rivières, mais essaient actuellement de s'adapter à la vie en lac (dans le Chicamba, il y a bien sûr aussi Micropterus salmoides (?), une vraie beauté si je me souviens bien...). Nous observons par contre une capacité d'invasion bien moindre dans les écosystèmes non modifiés avec une faune indigène intacte... tout au moins dans les zones de forêts humides qui sont de vieux écosystèmes avec une biodiversité très importante et avec des niches alimentaires très étroites, ce qui rend la situation assez compliquée pour des espèces non-prédatrices (par exemple, nous observons une expansion du Clarias dans des écosystèmes vierges, mais ils mangent plus qu'ils n'entrent en compétition avec les espèces indigènes.) Néanmoins, je suis d'accord avec les découvertes de votre travail et suis d'accord qu'introduire le niloticus dans des zones où il n'est pas ne vaut pas la peine de courir le risque... quel que soit le risque. Même si quelques fermiers qui pourraient en retirer un bénéfice, nous en retirerions pas grand chose au niveau global, que ce soit pour l'approvisionnement en poisson ou la croissance économique nationale. Nous ferions mieux de conserver les derniers écosystèmes vierges qu'il nous reste. Cordialement Randy Début du message réexpédié :

De : "Brummett, Randall \(WorldFish\)" <r.brummett@CGIAR.ORG> Date : 11 novembre 2008 12:48:18 HNEC À : "Olaf Weyl" <o.weyl@ru.ac.za>, <Sarnissa-african- aquaculture@lists.stir.ac.uk> Objet : Rép : [Sarnissa-african-aquaculture] Introduction of niloticus

Hey Olaf,

This is something that I have been wondering about. We know that the niloticus can take advantage of disrupted habitats in which you see ecosystems with niches either vacant or currently occupied by species that were once riverine but are now trying to adapt to life in a lake (also in Chicamba, of course, are largemouth bass...some real beauties as I recall...), but we have found much less invasiveness in pristine, unmodified ecosystems with an intact indigenous fauna...at least in rainforests which are old ecosystems with very high biodiversity and very narrow feeding niches making it difficult for non-predatory fishes to invade (i.e., we are seeing an expansion of Clarias into pristine ecosystems, but they are eating rather than competing with indigenous species.

Nevertheless, I concur with the findings of your report and agree that introducing niloticus into places where it isn't is not worth the risk...whatever that risk might be. From the point of view of some few farmers who might gain, overall we get little in terms of global fish supply or national economic growth. We are better off conserving those very few intact ecosystems that we have left.

Cheers,

Randy

-----Original Message----- From: sarnissa-african-aquaculture-bounces@lists.stir.ac.uk [mailto:sarnissa-african-aquaculture-bounces@lists.stir.ac.uk] On Behalf Of Olaf Weyl Sent: 11 November 2008 12:18 To: Sarnissa-african-aquaculture@lists.stir.ac.uk Subject: Re: [Sarnissa-african-aquaculture] Introduction of niloticus

Dear All, I would like to call attention to a recent paper were I showed just how fast O. niloticus can invade a 120 km2 fishery:

Weyl OLF (2008) Rapid invasion of a subtropical lake fishery in central Mozambique by Nile tilapia, Oreochromis niloticus (Pisces: Cichlidae). Aquatic Conservation Marine and Freshwater Ecosystems. 18: 839-851. DOI:10.1002/aqc.897

Abstract 1. Lake Chicamba, Mozambique (19o08'S; 33o08'E) is a large (116 km2) impoundment in the headwaters of the Buzi River system that was invaded by Oreochromis niloticus in 1996 from a small (< 0.3 km2) upstream reservoir.

2. Experimental and artisanal catch data showed no O. niloticus until January 1996 whereafter O. niloticus was recorded in up to 83 % of experimental seine net catches; 33 % of experimental gill net catches; 43 % of boat angling and 23 % of shore angling catches and in 48 % of artisanal gill net catches.

3. During the period January to March 1997, O. niloticus mean [upper, lower 95% Confidence Interval] yields in the artisanal fishery were 5.2 [3.6, 7.0] t/month.

4. The rapid invasion of this lake illustrates the significant invasion threat that small point-sources of this species pose to southern African freshwater systems.

5. The study recommends: (1) that this species should not be used for aquaculture or fisheries enhancement in catchments that have not been invaded, and (2) that the eradication of potential point sources of O. niloticus in non-invaded catchment systems should be considered.

Keywords Buzi River, Mozambique, freshwater, alien, invasion, Oreochromis niloticus

Reprints are available from me on direct request from me or from the publisher: http://www3.interscience.wiley.com/journal/116320988/abstract.

Kind regards Olaf

-------------------------------- Dr Olaf Weyl Senior lecturer Department of Ichthyology and Fisheries Science Rhodes University, PO Box 94, Grahamstown 6140 SOUTH AFRICA Tel: (+27) (0)46 6038415/6; Mobile (+27) 83 46154 34 Fax: (+27) (0)46 6224827 Email: o.weyl@ru.ac.za

...

-----Original Message----- From: sarnissa-african-aquaculture-bounces@lists.stir.ac.uk [mailto:sarnissa-african-aquaculture-bounces@lists.stir.ac.uk] On Behalf Of william.leschen@stir.ac.uk Sent: 11 November 2008 12:04 PM To: Sarnissa-african-aquaculture@lists.stir.ac.uk Subject: [Sarnissa-african-aquaculture] Introduction of niloticus

Introduction of niloticus There's currently an equally interesting discussion ongoing on the french forum - please see below translated text and links to publications - thanks Lionel

Best wishes Will

To accompany the current discussion on niloticus, following an article in French online ( also in English). There is also an abundant literature in English on this subject (analysis of the risks associated with the introduction of niloticus, effect on others etc.).. If you know equivalent documents in French, do not hesitate to send them here!

J. Moreau, J. Arrignon and R. A. Jubb. The introductions of alien species in inland waters in Africa. Interest and limits.

Minilien <http://minilien.com/?CnC1LHBCzi>

Direct link <http://horizon.documentation.ird.fr/exl-doc/pleins_textes/ pleins_textes_5/pt5/travaux_d/26406.pdf>

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Reply Link: <http://www.sarnissa.org//tiki- view_forum_thread.php?forumId=2&comments_parentId=116#form>

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