Conservation genetics of tilapias: Seeking to define appropriate units for management

Tilapias comprise 112 species and subspecies of cichlid fishes of the genera Oreochromis, Sarotherodon, and Tilapia. Because of the importance of tilapias to aquatic ecosystem structure and function, fisheries, and aquaculture, it is critically important to conserve wild genetic resources. Of the 112 recognized tilapia taxa, 41 are regarded as imperiled. In order to manage adaptively important genetic variation in tilapias, we need to recognize and define biologically appropriate units of conservation. An evolutionary significant unit (ESU) can be defined as a population or group of populations that merits priority for conservation and separate management because of high genetic and ecological distinctiveness from other such units. Management units (MUs) are defined as populations that are demographically independent of one another; that is, their population dynamics depend mostly on local birth and death rates, and not on genetically effective migration. Identification of MUs – similar to “stocks” widely referred to in fisheries management – is useful for short-term management, such as managing habitat, delineating fishing areas, setting harvest rates, and monitoring population status. Against this background, the goal of our review and synthesis was to summarize knowledge and recommend critical work yet to be done regarding conservation of tilapias, approaching this task in a species-by-species manner. With the exceptions of Oreochromis niloticus, O. mossambicus, and Sarotherodon melanotheron, the body of existing work regarding genetic and adaptive differentiation of populations of tilapias is insufficient to inform identification of evolutionary significant units, a knowledge gap that should be addressed by targeted research. We note that competition and interspecific hybridization caused by introductions of tilapias for aquaculture purposes jeopardizes the genetic resources of certain native tilapia species. We hope that our review and synthesis spark critical discussion of tilapia conservation within the tilapia aquaculture, management, and conservation communities. The IJA appears exclusively as a peer-reviewed on-line open-access journal at http://www.siamb. org.il. To read papers free of charge, please register online at registration form. Sale of IJA papers is strictly forbidden. 3 Conservation genetics of tilapias: Seeking to define appropriate units for management Introduction Need for conservation of tilapias. The tilapias comprise 112 species and subspecies of cichlid fishes of the genera Oreochromis, Sarotherodon, and Tilapia (FishBase, 2013, Table 1). Table 1. Valid species and subspecies of tilapias (Fishbase 2013), with conservation status as assessed by the International Union for the Conservation of Nature (IUCN 2013). IUCN’s levels of imperilment from least to greatest are: LC (LC), Near Threatened (NT), V (V), Endangered (E), Critically Endangered (CE), Extinct in the Wild (Ex.W), and Extinct (Ex). The table also includes Not endangered (NE) Latin Name Conservation Threat(s) Status Oreochromis amphimelas E Growth overfishing, effects of drought on shallow lakes Oreochromis andersonii V Competition with introduced O. niloticus, overfishing with no refuge areas Oreochromis angolensis LC Oreochromis aureus NE Oreochromis chungruruensis CE Restricted to small crater lake subject to fluctuation in water level, siltation Oreochromis esculentus CE Competition with introduced O. niloticus, predation by introduced Nile perch, overfishing, siltation, pollution, changes in plant community Oreochromis hunteri CE Siltation, seasonal drought, weed-infestation Oreochromis ismailiaensis Data deficient