Linyati River - In a last-ditch attempt to save at least 100 hippos stranded in mud ponds in northern Namibia, authorities are drilling three boreholes to try to increase the water levels, a newspaper reported on Thursday.The hippos were first reported trapped in fast-drying ponds in a tributary of the Linyanti River last month, sparking widespread concern. Hippos need to spend about 16 hours a day in water. There are also a number of crocodiles in the ponds.Five hippos are already reported to have died. Namibia's environment ministry insists this is not because of the low water levels, but because of fights between hippo bulls. The ministry announced in a statement earlier this month that it would have to cull up to five bulls "to reduce fighting". The New Era newspaper suggested on Thursday that two had already been culled. Their deaths appear to be on top of the five which had already died.Unusually low water levels in the river this year are being blamed on drought. The Kwando River did not flow into the Linyanti catchment this year, officials say. And there was no overflow of water from Lake Liambezi, as normally happens.There had been calls for the animals to be urgently relocated (and fears the hippos might have to be shot by trophy hunters). But now, water is being pumped from two of three new boreholes into the drying ponds and hopes are high the animals can be saved, reports the New Era.Environment Minister Pohamba Shifeta told the paper: "We started pumping water into the ponds. One borehole will be completed this week. One has been rehabilitated. We will have all ponds filled up with water to save the animals."The minister said the hippos were "fine" and fighting between the bulls "is also calmer".A number of hippos and crocodiles also got stuck in the mud in a stream of the Linyanti River in 2003.