Entering unchartered waters: distributional shifts across international boundaries and subsequent hybridization of commercially harvested species produce an entirely new management dilemma

Prof WHH Sauer1, WM Potts1, MC Parkinson1, B Pringle1, A-R Childs1, MI Duncan1, R Henriques2, S Mafilwa3, M Wilhelm3

1Department of Ichthyology and Fisheries Science, Rhodes University, Grahamstown, South Africa, 2Technical University of Denmark, National Institute of Aquatic Resources, Kongens Lyngby, Denmark, 3Department of Fisheries and Aquatic Sciences, University of Namibia,  Windhoek, Namibia

Movement across international boundaries and subsequent hybridization of congeners can cause     a management nightmare for species harvested commercially. One first global example is a recent climate-driven poleward shift in the distribution of a commercially important telesost, the west coast dusky kob, Argyrosomus coronus and its subsequent hybridization with is congeneric silver kob, A. inodorus is likely to have significant consequences for coastal fisheries in Angola and Namibia. Understanding the severity and consequences of this event is critical for the development of sustainable management plans. In a first attempt to understand some of the consequences, we combined in-situ temperature and acoustic telemetry data to gain an understanding of the thermal range of A. coronus in Angola, and used that information along with the data from a high-resolution ocean model to predict the extent of the southward distributional shift of the species. These findings were compared predictions based on with thermal physiology data (aerobic scope) of the two congeneric species (and their hybrids) in Namibia provided an estimation of the likely extent and consequences of the hybridization.  Future management is going to be challenging. A precautionary and simple management approach, such as the use of a traffic light framework, is likely to be the most suitable approach, able to be easily adapted as the consequences of changing life history characteristics and distribution become clearer.

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