This article is strange. I am not sure that I understand where the data - the figures presented - are being taken from. The GERD, as far as I understand, is not going to alter any flow from the Blue Nile river. It is an impoundment dam that holds back water in a reservoir to generate hydroelectricity. It is not a diversion dam. In order to generate electricity, water needs to be released. Nor is it possible, in my honest opinion, to hold back so much water to reduce the flow to Egypt and Sudan as stated.
Also, if this really were the case, this is a technical matter that can be resolved by water managers in the three countries. Why the fear? If the Blue Nile is to have dams developed, like GERD, coordination between countries on these matters is of upmost importance.
The other troubling thing about this article is that it says nothing really about the authority of the woman who is quoted expert field: fisheries. Is this person making a political statement or one that holds concern for her field of expertise? If the GERD is going to impact fisheries downstream, tell us how and where. Perhaps this information is needed to make further discussions possible, to bring up issues that may have been missed to date. It seems the only concrete thing that is raised is a loss in hydropower production. But are the dams in question running at full capacity now?
I am not sure that this article holds any real information worth publishing, but rather fear. But that, in itself, is an important issue to consider.
More is "understood" or "known" about the GERD through perception than through reality.