There is serious concern over the fluctuations in Nile perch catches in Lake Victoria. However, a recent study suggests that fishing does not drive trends in Nile perch population size-structure; this result has lead to the hypothesis that eutrophication drives observed trends. Indeed, over the past decades, Lake Victoria’s food web has been greatly restructured and changed; there have been visible signs of eutrophication while human pressure on its resources has also increased. We aim, here, to unravel how Nile perch population growth and size-structures respond to changes in resources and fishing mortality. We build a population-growth model for Nile perch that accounts for known changes in length at ontogenetic diet shift and observed fluctuations in resource abundance.
Results/Conclusions
With this model we estimate existence boundaries for Nile perch in the resource-mortality space. We analyse the influence of ontogenetic diet shifts and relative resource abundances on these boundaries and describe the populations on either side of these boundaries. Our results show that ignoring ontogeny can lead to over-estimating the maximum sustainable mortality of a Nile perch population. Also, while size distributions can be good indicators of processes driving Nile perch population dynamics, we find they are poor indicators of the sustainability of exploitation. Nile perch populations are most resilient to over-fishing when fish rather than invertebrates dominate their diets. We therefore suggest that the ecosystem — instead of populations — should be used as an indicator of the state of its components.