Effects of river regulations on fjord dynamics and retention of coastal cod eggs

High annual precipitation and steep mountains make Norway especially suited for the extraction of potential energy from waterfalls and is the country with largest production of hydroelectric power in Europe. The power production may affect both the position of river outlets and the seasonal cycle of freshwater discharge. To study the impact of river regulation on fjord dynamics and the transport of cod eggs, a numerical model is applied for the year 2009. We investigate two cases; the first case with only the natural seasonal cycle of freshwater discharge, whereas the second case includes all river regulations in the fjord system. The results show stronger surface outflow in the main part of the fjord when applying river regulations compared with natural run-off. The transport of cod eggs out of the fjord system increased with regulated run-off compared with the natural run-off, which correspondingly caused a reduced local retention of eggs within the various fjord branches. Changes in the seasonal cycle of freshwater discharge due to hydroelectric power production cannot be neglected as a contributing factor to the observed decline in coastal and fjord cod subpopulations.